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Chapter 9
Roadside Vegetation Management
9.6. Roadside Vegetation Control Methods and Resources

A successful control strategy for an invasive often begins with checking on-line and other data sources about management options for the target species. Successful methods used under similar conditions, i.e., in similar habitats and climates, are preferred. Since invasions and the effectiveness of control methods vary with local conditions, general statements about suitable control methods should be approached with caution. Experience-based reports of methods used to control certain species and their effectiveness under specific environmental factors are essential for invasives management. DOTs may benefit from using shared databases or having common points of contact on the internet, to share and benefit from each other's experience and learning. Invasives management can greatly benefit by use of the best practices available and dissemination of experience and information.

9.6.1 General vs. Species Specific Control Methods
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The most successful invasive species control has been achieved with species-specific methods, which also have the least impact on non-target species. In some instances, such as highly degraded habitats without any native species left, a more general method is acceptable. In these cases a broad-spectrum herbicide, or bulldozing the ground, has limited negative effects on native biodiversity. However, in less disturbed areas, in particular nature reserves; for example, the use of a species-specific method is highly recommended.

Though DOTs strive to avoid reaching unacceptable levels of unintended consequences, impacts on non-target species may be expected while carrying out such control measures. When control or eradication is successful, the reduced impact of the alien species on the native biodiversity normally outweighs the cost; native species populations that suffer losses during the control efforts typically rebound following the removal of the invasive species.
In choosing a management strategy, DOTs usually consider:

  • Legal requirements related to management of invasive species and particular regulations on herbicide usage, including those in health and safety legislation.
  • Best methods that have been used for this target species.
  • The types of herbicides, baits and equipment that are readily available and the ways by which further supplies can be obtained.

Plant control may involve: manual methods (e.g. hand-pulling, cutting, mowing, bulldozing, girdling); herbicides; release of biological control agents; controlled use of grazing or browsing animals; prescribed fires; flooding; planting competitive native species and other land management practices. Land invertebrate control may involve traps (e.g. light traps, pitfall traps, pheromone traps), mechanical/physical means (e.g. handpicking, removal and destruction of host species), insecticides, biological control (e.g. fungi, other insects), and other specialized means (e.g. mass release of sterile males). Land vertebrate control may involve trapping, shooting, baiting, biological control, contraceptives or sterilization. Control of pathogens often focuses on hosts rather than measures directly orientated against the pathogen species. In some cases the hosts are eliminated - this is a preferred choice when the hosts are non-indigenous as well - in others, including diseases of humans and domesticated animals, the hosts are vaccinated. Resistance of the host can also be induced or intensified. If vectors are a part of the pathogen's lifecycle, vector management should be considered. Control of marine bioinvasions is more difficult, though pathogens, pesticides, and hand-picking have been used. Prevention is generally considered the principle defense against marine invasive species, which are distributed by one principal pathway, the ship. Organisms in freshwater habitats can be controlled with mechanical, chemical, and biological measures and habitat management. Aquatic weeds can be harvested when floating on the surface, pulled out when rooted, or sprayed with herbicides. Biological control has been particularly effective against several water weeds in different parts of the world. For example, weevils have been used to control water lettuce in the U.S. and other countries. Fish-specific poison has been used in the eradication of several fish invasions. Another control option for fish is recreational or industrial fishing. Mosquito larvae and pathogens vectored, and by extrapolation other freshwater insects, can be controlled by spraying chemicals or biological pesticides onto the infested water. The community of a freshwater system can be influenced by changes in the water quality and quantity in favor of native species. [N]

Biological, cultural, physical, and chemical control methods can be used to contain and eradicate invasives.

  • Biological weed control includes the use of insects or pathogens.
  • Physical control of invasive species includes tilling, mowing, and burning areas to control invasives.
  • Mechanical methods include handpulling and mowing, which can be effective in conjunction with other control methods.
  • Chemical control of invasive species relies or herbicides and pesticides.
  • Cultural control of invasives includes planting native grasses or competing plant species to force out invasive species.

No individual method will control invasive species in a single treatment; diligence and persistence are often required over a number of years to bring infestations under control. In most cases the best practice to manage an invasive species may involve a system of integrated management tailored for the species and the location. Thus, it is important to accumulate the available information, assess all potential methods, and use the best method or combination of methods to achieve the target level of control. These methods are usually combined in strategic planning to control invasives or in IVM or IRM planning, as discussed in the previous chapter. All methods should include follow-up monitoring, treatments, and revegetation to prevent new infestations or resurgence of the target weed.

All control methods, with the exception of classical biological control, which is self-sustaining, need long-term funding and commitment. If the funding ceases, the population and the corresponding negative impacts will normally increase, in some cases leading to irreversible damage. Successful control may be easiest to achieve in areas of lower density of the invasive species. The degree of success will vary with different organisms, the ecosystem, the duration of effort, the restoration effort, etc. and the optimum management strategies are location specific and must be tested and fine-tuned for different areas.

Control of non-indigenous invasive species aims for the long-term reduction in density and abundance to below a pre-set acceptable threshold. If prevention methods have failed and eradication is not feasible, managers will have to live with the introduced species and can only try to mitigate the negative impacts on biodiversity and ecosystems. Since, in the short-term, control seems to be a cheaper option than eradication, it is often the preferred method. Funding and commitment do not need to be at such high levels as for eradication programs, and funding can be varied between the years depending on the perceived importance of the problem, political pressure, and public awareness.

The DOT makes a decision to consider harm caused by the species under this threshold as acceptable with regard to maintenance objectives, costs, damage to biodiversity and economy. Suppression of the invasive population below that threshold can tip the balance in favor of native competing species. The weakened state of the invasive species allows native species to regain ground and even further diminish the abundance of the alien species. In rare cases this might even lead to extinction of the non-indigenous species (especially combined with habitat restoration efforts to support native species and put intact natural systems back in place), but this is not the principle goal of control efforts. [N]

A summary of DOTs' use of various control methods and BMPs is included in the table below. Most of these methods are discussed in greater detail later in this section.

Control Methods and Practices 1
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9.6.2 Tailoring Treatments to DOT Functions and Development Stages
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New York State DOT has tailored invasive species control procedures and methods to DOT functional areas and stages in the transportation planning, development, construction, and management processes. A number of these practices are described in the previous section on Prevention. For example, NYSDOT capital projects, and appropriate maintenance activities and roadside operations consider and address, as practical, the potential environmental effects of invasive species. This process and analysis includes, at a minimum: 1) an inventory of the project area; 2) consideration of potential environmental impacts; and 3) incorporation of appropriate preventive measures and control practices into project documents and activities.


9.6.3 Mechanical Controls
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Mechanical control methods involve directly removing individual plant or animal invasives, either as a means for eradication in small areas or as a means for controlling species density and abundance. It encompasses tractor mowing and can involve anything from complete tillage for reseeding to hand scythes, shovels, string trimmers, push mowers, pruning shears, etc. for weed control and desirable vegetation maintenance. Mechanical eradication methods can be effective when the population of the invader is still small and the population is limited to a small area. Weeds that grow vigorously from cut plant parts or multiply vegetatively are more difficult to control. The downside of mechanical methods is the labor requirements. Most manual work is expensive and has to be repeated. Plant parts of some species, left in contact with soil may survive and grow. For example Japanese Knotweed (Fallopia japonica), an invasive alien in Europe and North America, will regenerate from rhizome fragments of less than 1 gram.

In many cases introduced pests can be controlled or even eradicated in small-scale infestations by mechanical control, for example hand-pulling weeds or handpicking animals. An advanced method of mechanical control is the removal of plants by specifically designed tools and even machines, such as harvesting vehicles for water hyacinth infested lakes and rivers. In some cases of very persistent plants and depending on the area, e.g. on large open areas like pastures, bulldozing may be necessary. [N]

Invasive plants can be cut or hand-pulled (see for some simple tools); larger plants can be uprooted, with the aid of tools, such as winches, if necessary. The effectiveness of this technique will vary considerably depending on the response of the weed. If there is no information available about the plant's response to uprooting, some simple tests should be carried out to discover its effectiveness, and ways to treat the residues, e.g. composting or burning the uprooted material. Repeated cutting of a woody weed may eventually drain the resources stored in the root system and kill the plant. In many cases combined cutting of the plant and painting the stem with a systemic herbicide proves to be more efficient. Mowing of herbs and grasses may lead to the same result, when the plants are not adapted to heavy grazing. Annuals are especially susceptible if mown shortly before setting flowers, because they will have used up most of their root reserves to produce the buds. Girdling can kill trees; cutting with a knife through the cambium of a tree trunk and removing 5 cm of bark will interrupt the flow of nutrients and kill the plant. Girdling alone may not suffice for rapidly killing those species where the water and nutrient movement are not restricted to the outermost layer of the trunk, but an application of herbicide will speed up the process.

Regarding fauna, control of most insect species dependents on traps, which are more or less specific to insect groups or species-specific using pheromones. Sedentary species such as scale insects or mealybugs can be killed by destroying their food plants. Trapping and shooting can be considered the "mechanical" or "manual" way of dealing with invasive vertebrates. Recreational hunting of game can be effective in keeping populations down to an acceptable level and can be a money source for other management activities in the area. This is a rare case where control does not involve costs, but earns money. It does, however, give rise to the concern that the invasive species then becomes a valuable commodity that should be preserved in order to continue to generate this income. Furthermore, there are many instances where recreational hunting will not reduce the target population sufficiently. Similarly, recreational hunting can be counter productive due to amateur hunters creating a shy target population and not being skilled enough to reduce target species down to desired densities. Also, depending on the species, recreational hunters may select only mature trophy males as targets; this will have little or no impact on the reproductive capacity of the species. In order to reach the pre-determined target population level, it may be necessary to employ professional hunters. Using animals such as dogs, which can be specifically trained to target individual invasive species, can be extremely successful in combination with shooting and other forms of control. Perhaps the only mechanical control method against pathogens is to eradicate or control the vector or the host, e.g. felling of diseased trees.

Fencing is another option for containment of species, either fencing the species in a certain area or fencing off ecologically valuable land. One obviously needs to be sure that the invasive species is not present on both sides of the fence.


Maintenance roadside mowing, while essential for safety, operational, aesthetic and environmental purposes, can, has and does play a significant role in the introduction, spread and proliferation of invasive plants. Mowing can serve as a control method for certain invasives during certain periods of their reproductive cycles, but repeated mowing and attention to timing will be required. Mowing is most effectively used in combination with other vegetation management and invasive species control techniques. Few studies have been conducted on the effects of mowing plant communities and invasive species.

Just 30 percent of responding state DOTs say they are timing mowing to control invasive species. Slightly fewer (25% of respondents) say they will mow in order to minimize seed transfer, in some areas.

Reduced mowing practices were initiated as early as the 1950s. The Wisconsin DOT was a pioneer in the cutbacks. Many more DOTs reduced mowing in the 1970s, when high energy costs forced vegetation managers to mow less and spot spray, with the positive consequence of increased wildlife habitat, enhanced natural beauty, minimized herbicide use, reduced maintenance dollars, and public acceptance. [N] NCHRP 20-5, 33-04 indicates that state DOTs remain heavily dependent on mechanical control methods, with the bulk of states cutting over 90 percent of their ROW, a smaller set having reduced mowing to 50 to 90 percent, and only Florida and Washington indicating less than 50 percent was managed using mechanical methods. [N]

DOT Mowing Policies and Programs

Eleven (11) state DOTs have developed conservation mowing/spraying programs to protect native communities, minimize maintenance costs, and control invasives. [N] New York State DOT's conservation mowing program is among the most well known. North Carolina DOT has developed a Clear Zone Improvement Program (C-ZIP) initiative, which relies on native grasses and forbs. Fourteen state DOTs (35% of respondents) have employed reduced mowing widths (one-mower width) as a statewide standard, to control costs and preserve native communities. Texas DOT is among those that have marked many areas as "no mow" to facilitate their return to native vegetation.

A few states, such as Vermont and Alaska said their roadsides consist primarily of native vegetation. Alaska DOT&PF indicated the agency sometimes specifies just adding fertilizer and allowing the site to naturally revegetate. More information about DOT mowing practices may be found in this Guide to Environmental Stewardship Practices and Procedures. A selection of these, with invasive species control components, is included below.

NCDOT Mowing Program Modifications to Encourage Wildlife Native and Rare Plant Species

NCDOT implements their mowing program with an environmental perspective to encourage wildflowers, protect rare or endangered plants and protect or create wildlife nesting areas. Currently NCDOT protects over 35 populations of rare plant species growing along its roadsides. Most of these plants are listed as federally threatened or endangered by the U.S. Fish and Wildlife Service (USFWS). Endangered species such as Smooth Coneflower, Schweinitzii's Sunflower, Michaux's Sumac, Rough-leaved Loosestrife and Cooley's Meadowrue often occur along roadsides and in powerline rights-of-way, or in natural habitats that were once dominated by fire but are now mowed to mimic fire maintained ecosystems. Other species are listed as significantly rare in North Carolina. NCDOT works with USFWS, the North Carolina Natural Heritage Program and the North Carolina Plant Protection Program as well as different utility companies to protect these roadside and powerline populations of rare species. Endangered plant populations are marked with white-topped wooden stakes, an indication to mowers that the area is off limits during the growing season. These areas are managed on a site-by-site basis according to their individual needs. Management strategies to control invasive woody vegetation include mowing during the dormant season, hand pruning and prescribed fire. Effective communication among environmental biologists and horticulturists, environmental engineers, and roadside maintenance personnel is crucial to the success of this program. NCDOT cooperates with the North Carolina Wildlife Commission in posting and managing small game wildlife habitat areas in the ROW. Properly timed cleanup mowings enhance wildlife habitat. [N]

NYSDOT Stewardship Mowing Practices

General NYSDOT mowing guidelines are outlined in NYSDOT's Mowing Limits Manual, Highway Maintenance Subdivision Operational Guidelines, and Environmental Handbook for Transportation Operations. The Mowing Limits Manual addresses safety, water quality and erosion and sedimentation control, appearance and screening, landscape plantings and woody vegetation, and natural revegetation. NYSDOT's Environmental Handbook for Operations outlines the following stewardship practices and expectations for mowing, which go beyond mowing reduction policies to address other environmental features in the ROW: [N]

  • Ditches are mowed to control vegetation rather than mechanically cleaning ditches with heavy equipment because mowing causes less erosion of exposed soil and can result in improved water quality.
  • Wetland mitigation areas are not mowed. Permanent markers are installed around these areas.
  • Since many federally-protected ground nesting migratory songbirds and waterfowl nest prior to July 1, mowing of large relatively flat areas located outside of built up or developed areas (such as on the interstate system) is avoided or limited during nesting season. In advance of mowing likely nesting areas are identified and marked.
  • Areas that are managed for wildflowers are not mowed more than once a year and after the first hard frost or in the late fall.
  • A minimum 3 meter (10 feet) unmowed buffer strip is left along the edge of all streams and wetlands.
  • Mowing is avoided on days when ozone levels are expected to approach or exceed unhealthy levels and voluntary actions are needed to reduce emissions and formation of ozone.
  • Mowing is restricted or avoided in habitats for threatened or endangered species.

In addition, NYSDOT is implementing Conservation Alternative Mowing Plans (CAMPs) that preserve safety and aesthetics while reducing invasive species and enhancing native habitat. CAMPs involve identification of rich landscapes as part of the maintenance planning process, threshold values for several species and suitable habitat in the landscape, species or groups of species to be used as indicators, and the barrier effect of roads. [N] CAMPs have been successfully developed and implemented on Interstates, Expressways and Parkways at NYSDOT through a multi-disciplinary team approach, culminating in the following guidelines, using four zones that describe to the operators what management is expected in the different zones. A High Management Zone is comprised of an intensely managed area immediately adjacent to shoulder or curb; a Frequently Mowed Zone next to it is mowed multiple times per year; an Annually Mowed Zone provides a transition between the Frequently Mowed Zone and No-Mow Zone (or left to regenerate naturally); and a No-Mow Zone is left in natural state or left to regenerate naturally. The following stewardship practices are taken from NYSDOT's draft CAMP guidelines: [N]

  • Generally, the Frequently Mowed Zone will be 30 feet in width or will be set at the back side of the drainage ditch. The limits of this zone may be reduced or extend further depending on actual site conditions.
  • The limits of the Frequently Mowed Zone may require adjustment to preclude the development of annually mowed areas that are too narrow (less than 60 feet wide), too small (less than 2 acres), or too linear. Annually Mowed Zones will not be mowed until after August 1st. Annually mowed zones should be mowed no more than once per year, however to further increase the conservation benefits, these areas may be mowed less frequently but with due consideration of the desire to limit establishment of woody plants within periodically mowed zones.
  • The decision to include an annually mowed zone should consider many factors including, the adjacent land use and the width and length of the area. Since the annually mowed zone is important for ground nesting birds, these areas should be at least 60 feet in width and greater than 2 acre in size to reduce nest predation and allow a large enough nesting territory. Annually mowed areas will be most effective when located adjacent to or in close proximity to existing grassy fields and in these situations should extend to the limits of the ROW. When the adjacent land use is mature forest, the annually mowed zone should not be included or should be combined with a no-mow zone, as appropriate.
  • For consistency, Mowing Limit Markers should only be placed at the transition between the Annually Mowed Zone and the No-Mow Zone (the markers can be placed to create a natural appearing, meandering boundary.) Markers should be standard wooden or fiberglass stakes. Over time, the need for these markers should diminish as the demarcation of the zones becomes well established. These zones should also be captured electronically using GPS. Mowing Limit Markers should be placed with due consideration of sight distance. This is especially important at entrance/exit ramps. The No-Mow Zone shall not encroach into the sight distance cone and restrict visibility.
  • The No-Mow Zone can be left to regenerate naturally. This process is termed succession. The final stage of succession is a self perpetuating, sustainable, and interdependent community of plant and animal life. The establishment of No-Mow zones is intended, in part, to permanently reduce the amount of mowed area and to reduce the negative environmental effects of habitat fragmentation.
  • On narrow medians (less than 120 feet between the High Management Zones), new No-Mow zones may be established between the 30 foot minimum Frequently Mowed Zones with due consideration for maintenance structures such as drainage ditches, deer reflectors, living snowfences, etc.
  • New woody plantings can be included to the No-Mow Zone. The purpose of these plantings should be to increase diversity of the plant community.
  • Careful consideration should be given to any new planting of trees and/or shrubs in the Annually Mowed Zone. In addition, new plantings should be grouped in such a manner as to preclude the necessity of maintenance personnel to mow around individual plants. Supplemental (new) plantings can be included to correct existing conditions by "filling in" the spaces between the existing plants where mowing is difficult or not possible. Living snowfences, wildflowers or deer reflectors may also be included in this zone. All new planting in this zone should be carefully coordinated with NYSDOT Maintenance to insure that it can be maintained without undue effort. Any exceptions to this rule must be agreed to by NYSDOT Maintenance.
  • Any dead and/or dying trees should be carefully reviewed for potential hazard. If it is determined that the tree would not cause a hazard if it falls, consideration should be made to leave the tree standing. Dead trees provide cover, nesting cavities and perches for birds and small animals.
  • Vegetation management practices may be modified depending on the characteristics of the land use adjacent to the corridor (urban, suburban, and rural). Urban corridors may be expected to have a greater proportion of High Management and Frequently Mowed vs. Annually and No-Mow Zones. Additionally, Parkways and Expressways may require different management due to the nature of the different roads.

Alabama DOT Mowing Program to Reduce Congongrass

In Alabama, mowing is restricted in Cogongrass (Imperata cylindrical) areas during seed head production; mowing is to progress from areas without Cogongrass infestations into areas with Cogongrass infestations; and mowers are to be cleaned of all vegetative propagules before leaving Cogongrass infested areas.

Mowing Management in Southern Quebec, Canada

According to a study commissioned by the Ministère des Transports du Québec, traditional methods of controlling vegetation along the agency's 2000 km of highway corridors in southern Quebec (Canada) "result in a boring landscape, deteriorate the various wildlife habitats and impoverish wild plant life while generating high maintenance costs." Recently, the agency has pursued develop new maintenance methods, including elimination of multiple annual mowings, to improve the safety of the highway system's users, satisfy neighboring residents, beautify the landscape and consider the plant life and wildlife present along the highways. The new approach eliminates multiple mowings except on the first two meters from the pavement, which will be mowed more frequently to ensure highway safety (visibility) and better control of the allergen, ragweed. The agency and its researchers have also been monitoring the slowly increasing biodiversity in the unmowed area since 1998. [N]

Good Mowing Practices

Mowing should be carefully considered to ensure that the target species will not actually be spread by the practice. Because mowing cuts the tops of plants, more buds may grow, producing more stems than before; however, continuous mowing when plant reserves are low can gradually deplete root reserves. If flower-feeding biocontrols are used, weeds can still be mowed at times when natural enemies are not feeding. One of the greatest advantages of mowing is the ability to quickly reduce seed production.

Since drainage ditches, streams and wetlands can rapidly spread invasive plants through dispersal of seeds and, to a lesser degree, plant parts, it is strongly recommended to control (herbicide, excavate, etc.) priority invasive plant populations in and adjacent to drainage ditches and streams, prior to mowing. This also will provide operational benefits because invasive plant populations in the drainage system that are "only" mowed will immediately re-grow and spread and therefore require additional mowing. Isolated (upland) roadside populations of invasive plants can be mowed with less chance of dispersing the plant seeds and parts to new areas. Nevertheless, mowing should always be done prior to seed maturation, which typically occurs later in the last half of the summer.

Since mechanical mowing spreads invasive plant seeds and each segment of many invasive plants, including the rootstock, can vegetatively sprout a new plant, priority invasive plant species should be mowed with due consideration for the following factors: [N]

  • When mowing untreated or uncontrolled invasive plant populations, mow prior to seed maturation.
  • Plan on mowing invasive plant populations 2 - 3 times per year, prior to seed maturation, for successive years if mowing is the only control practice used. The "mowing only" option should be used in locations that are not in or adjacent to drainage ditches and are inaccessible or too large for other control options.
  • Control small invasive plant populations comprised of tender, young plants with herbicide early in the summer, prior to mowing, especially in and adjacent to drainage ditches.
  • If invasive plant populations consist of large, mature plants, mow the plants prior to seed maturation, allow the plants to re-grow to a height of 2 - 4 feet and then treat with foliar herbicide, especially in or adjacent to drainage ditches.
  • Physically remove flower or seed heads (cut and bag) of small invasive plant populations prior to mowing, especially in and adjacent to drainage ditches.
  • Physically remove rootstock (mechanically excavate) of small invasive plant populations prior to mowing, especially in and adjacent to drainage ditches.
  • Control large purple loosestrife plant populations with biocontrol beetles (Hylobias sp. or Galrucella sp.) prior to mowing. Beetle releases will take several years to significantly reduce purple loosestrife populations.
  • If mowing occurs after seed maturation, hand clean, with brush or broom, upper parts of contaminated mowing equipment prior to moving to new locations - especially uncontaminated locations. This is especially important for purple loosestrife as each mature plant is capable of producing up to 2.5 million viable seeds.
  • Take care to minimize scalping and rutting during mowing operations. These situations can be avoided by properly adjusting the equipment and avoiding operating equipment directly in wet areas or rough terrain. Any scalped or rutted areas should be immediately seeded and mulched. Any badly rutted areas should be repaired, seeded and mulched.
  • Use of optional mowing equipment, such as "over-the-rail" boom-type mowers may be necessary in some situations to reach invasive plants.

The Montana State University Extension Service has compiled a number of recommendations on mowing to manage invasive species, which are summarized in many of the sections below. [N]

Identify Targets and Communicate with Relevant Parties

When mowing to reduce invasive species, recommended practices include:

  • Identify invasive species that can and should be controlled by mowing (at least in part), identify location of patches, and thresholds when mowing should occur for these areas (such as when patch area exceeds a certain size).
  • Identify invasive species that should not be mowed or conditions under which they should not be mowed, to avoid inadvertently spreading these invasives.
  • Communicate with other maintenance staff to avoid mowing areas soon after or just before spraying, and to avoid mowing areas of leafy spurge (Euphorbia esula) and other species that spread with mowing.

Time Mowing to Suppress Invasive Species

Properly timed mowing can suppress some invasive species while favoring desired methods, if used in conjunction with other methods. Timing is based primarily on the growth stage of the plants to be mowed and secondarily on the growth stage of the desired plants. If desired vegetation cover is inadequate (usually less than 20 percent), revegetation is likely to be necessary.

Some desired plants, like grasses, have equal amounts of growth above and below ground. When grasses are defoliated during the growing season, this stress reduces vigor and competitive ability to maintain competitiveness and to allow grasses to produce seed for next year's stand, it is best to mow when these plants are dormant. Caution should be exercised, as mowing during the weeds' seeding period can facilitate seed dispersal.

Establish Mowing Frequency

Mowing frequency for invasive species control should depend upon precipitation and the mowing tolerances of the vegetational function of relative growth rates, leaf replacement potential and the plant's ability to increase photosynthesis after mowing to compensate for leaf loss. Particularly important are the number, location and source of growing points on plant stems. An effective mowing strategy minimizes the removal of growing points of desired plants and maximizes removal of growing points of weeds.

For annual, biennial and taprooted perennials, the frequency of mowing depends primarily on precipitation. A single midsummer mowing after flower production can reduce or eliminate seed production and shift the balance in favor of desired species in areas with little or no summer rain. In one study, 78 percent control of diffuse knapweed (Centaurea diffusa) was achieved after mowing to a two-inch height each month during the growing season (no mention of revegetation context). However, as summer rains increase, regrowth potential increases, and mowing may increase plant vigor and seed production similar to pruning. In this case, additional mowing is necessary.

Rhizomatous weeds usually require more frequent mowing. Repeated mowing is considered an effective control of rhizomatous weeds in alfalfa and many pastures. In one study, mowing alfalfa two times per year reduced Canada thistle (Cirsium arvense) 86 percent after one year and 100 percent after four years. In other studies, mowing three or four times per year nearly eliminated Canada thistle in three years. Other studies have suggested that mowing Canada thistle kept stands in check but did not eliminate the weed.

Mowing of large infestations is a long-term commitment. Some weeds, such as those that spread through rhizomesa prostrate stem growing beneath the soil surface have large energy storage capacities. During the first few years, mowing these weeds can stimulate shoot production from root buds and increase stem densities. However, over time, frequent mowing at each early flowering stage can affect underground reserves and eventually reduce stem densities. In addition to Canada thistle (Cirsium arvense), rhizomatous invasive species include Dalmatian toadflax (Linaria dalmatica), leafy spurge (Euphorbia esula), ox-eye daisy (Chrysanthemum leucanthemum), Russian knapweed (Acroptilon repens), whitetop or hoary cress (Cardaria draba).

Set Mower Height

Most grasses can tolerate short mowings once dormant. If the dominant vegetation has not yet shifted to invasive species and still contains adequate grass cover, mowing should generally be timed so the weeds are at the flowering stage and the grasses are dormant.

When the dominant vegetation is a noxious weed, MDT recommends mowing two inches high when the weed is at the flowering stage. However, in some cases, invasive species will reach the appropriate stage for mowing, but the grasses have not reached dormancy. If so, MDT recommends mowing the weeds at a height above the desired plants. Mowing above the height of actively growing grasses allows seed production and unrestricted growth; this maintains vigor needed to minimize reinvasion. Defoliating the weeds reduces seed production and vigor, increasing resources available for neighboring grasses.

Spotted Knapweed (Centaurea biebersteinii): A Case Study

Montana State University performed a study on the effectiveness and timing of 15 different mowing strategies for spotted knapweed (Centaurea biebersteinii), showing that mowing can greatly reduce or diminish seed production and adult spotted knapweed density. The mower was set to cut at 10 inches height to cut the bolted knapweed plants while passing over the grasses. The study found that the most effective time to mow spotted knapweed was during the flowering stage. Mowing at this time decreased adult density by about 85 percent. Seedling density also was slightly reduced. Grasses were only decreased by the most aggressive mowing treatments.

Integrating Mowing with Other Management Methods

While mowing will not eradicate invasive species, it can stress them, greatly reduce seed production, and help give native plants a competitive edge. The pervasiveness and complexity of invasive species, combined with their cost of control, makes an integrated management plan necessary, as an IVM strategy seeks to use the most economic, ecologic and environmentally effective combination of principles, practices, technologies and systems to meet noxious weed management goals and objectives. Although little research has been conducted on incorporating mowing into an integrated plan, experience provides some practical hints. Some evidence suggests combining mowing with herbicides can enhance perennial weed control. For example, mowing two or three times a year consistently enhanced Canada thistle (Cirsium arvense) control following applications of picloram, picloram + 2,4-D, clopyralid + 2,4-D and dicamba. [N] Mowing may also be combined with herbicides over several years. Most invasive species grow low to the ground after long-term repeated mowing. In these cases, periodic control through herbicides can remove plants that have acclimated to frequent mowing.

Manual Removal - Pulling Weeds and Seed Head Cutting

Pulling weeds by hand is practical and efficient on small, isolated patches of weeds. It has the advantages of low damage to other plants and minimal equipment cost. Hand pulling is most effective on annuals and biennials that do not resprout from root fragments. The labor requirements often make handpulling impractical, but since perennial weeds can resprout from extensive root systems, hand pulling has the potential to be ineffective as well.

  • Pull plants when soils are moist and before seeds are produced to prevent additional spread of the weed.
  • Try to remove the entire root system since many weeds can resprout from remaining root fragments. The success of hand pulling often depends on the thoroughness of the worker.
  • Try to keep soil disturbance to a minimum.
  • Ten DOTs (25% of respondents) said they utilized hand-picking of weeds in some areas. NYSDOT has done manual cutting of seed heads where further control is not possible. Bagging and disposal should accompany handpicking.

Tilling and Disking

Tilling and disking mechanically removes weeds from the soil, slices through roots, or buries weeds. It is fast and effective against a wide variety of weeds; however, it can only be used in crops, pastures, and some rangeland. It can also lead to loss of soil moisture, increased soil erosion, and mixing of seeds into soil, a danger as it is not usually followed by residual control. Tilling does not kill most perennial weeds, but deep tillage of taprooted plants in late fall can expose roots to deadly winter frosts and interfere with the translocation of nutrients to roots for storage. In many areas the disturbance caused by cultivation creates a niche for new weeds to become established.

Cutting/Brushing and Management of Woody Vegetation

Trees and shrubs are pruned to preserve their health, remove dead branches, protect utilities, maintain sight distances, preserve aesthetics and prevent property damage. In the name of safety, improved visibility and obstacle-free roadsides, roadside vegetation managers favor grasslands. Management of woody vegetation comprises a significant expense for many DOTs. For example, 60 percent of PENNDOT's roadside maintenance budget goes to brush removal, approximately $26 million annually for maintenance of approximately 250,000 acres of ROW land statewide; reducing encroachment of the roadway is a major focus and main roads receive brush control annually. [N] A 1994 New Jersey study implied that mowing once every 4-5 years would be enough to discourage forest invasion into the roadside recovery zone. [N]

Cutting is used primarily for woody plants. It minimizes soil disturbance and involves tools such as brush cutters, power saws, axes, machetes, hand pruning tools, loppers, and clippers. Smaller shrubs can be cut with power mowers, string cutters, machetes, scythes, or weed whips. When plants are cut, roots remain intact and are helpful in stabilizing soil on steep terrain. To minimize resprouting, cut stems close to the ground under maximum drought conditions.

The effectiveness of cutting largely depends on the plant species, stem diameter, time of cut, and the age of the plant. For example, scotch broom's ability to resprout declines with age. Cutting broom to the ground during dry months (usually after flowering) usually kills the plant, whereas cutting prior to flowering, although effective in preventing another seed crop, may result in resprouts and little mortality.

Cutting flowers of herbaceous plants can prevent seed production. Since purple loosestrife, (Lythrum salicaria), grows in sensitive areas that cannot sustain heavy equipment such as mowers, flowers are cut by hand, bagged, and properly disposed of.

Brush Control Guidelines

Brush control is designed to restore sight distance, control noxious weeds, prevent snow drifting, reduce ice (due to shading) and to control/prevent slope failure. These actions involve mechanical mowing, trimming, removal of brush and cleanup. This includes vegetation management around existing bridges. The primary purpose of bridge vegetation management is to maintain sight distance. Bridge vegetation management must also maintain access to the bridge structure for maintenance, fire safety and to maintain the integrity of the structure.
Minnesota DOT provides the following environmental stewardship practices for brush control in the ROW: [N]

  • Don't spray big brush; rather, chop it down. The extreme color change from spraying may cause public concern.
  • Spray when trees and shrubs are small (less than 6 feet tall), and preferably in the fall
  • Mow smaller brush before spraying.

The Montana Department of Transportation employs the following guidance to avoid and minimize environmental impacts from brush control. [N]

  • Leave cut brush in place in riparian areas, where doing so does not interfere with sight distance, create safety issues, cause fire hazards, involve noxious weeds or the proper functioning of highway features (e.g. drainage).
  • Limit mowing to no more than 8 to 10 feet off edge of pavement in significant resource areas defined by the Department of Environmental Quality as state water quality impaired segments, unless needed to maintain proper functioning of highway features (e.g. drainage or snow drift control).
  • Maintain shade trees along streams and rivers, unless those trees are danger trees, could potentially impact bridge structures, constitute a probable clear zone hazard, or could impact line of sight. If trees provide shade or bank stabilization within 50 feet of streams and are determined to be danger trees that must be removed, tree removal should be coordinated through the DOT botanist or other regulatory agency.
  • Only remove brush necessary to perform the activity.
  • Only remove the brush within 20 feet (on either side) of and under all bridge structures for access and repair to the structure. (In some instances, road access under or adjacent to the structure will be outside the 20 foot buffer.) All other brush not within the DOT's clear zones should be left in its current condition, unless the brush interferes with sight distance, shades the structure, shades the highway, or the brush is a noxious weed. Mapping of sensitive resource areas such as listed state water quality impaired water bodies may lead to additional areas not being brushed.
  • On culverts that convey live streams, only remove 10 feet of brush on both sides of the culvert on the upstream end of the culvert and 10 feet on both sides on the downstream end, unless the brush around the culvert is a noxious weed.
  • When removing mature trees (over 12-inch (30cm) diameter at breast height (dbh)) in riparian areas, Maintenance will coordinate with the MDT Botanist or District Biologist to determine appropriateness of replanting two seedling/cuttings for every tree removed. Coordinate on species and location of trees to be replanted within the same watershed and to ensure that the replanted trees will not pose a future threat to MDT structures.

Removal of Danger Trees

Maintenance identifies and removes "danger trees," often in coordination with the DOT botanist or environmental staff and/or the appropriate resource agencies. Trees may be removed from forested areas which are weighting unstable slide areas, or where the trees or slide have the potential to reach the highway. Maintenance also occasionally removes trees that threaten to fall and uproot large portions of bank area. The Montana Department of Transportation (MDT) utilizes the following practices for removal of danger trees. [N]

  • Where possible, attempt to maintain buffer strips along riparian areas: 100 feet in width for large rivers, 70 feet in width for medium rivers and no less than 50 feet in width for most streams (first-second order tributaries).
  • Maintain shade trees along streams or rivers unless those trees are "danger trees" as described above. If trees provide shade or bank stabilization, are within 50 feet of streams, and are determined to be danger trees that must be removed, tree removal should be performed in consultation with DOT environmental staff. Develop a replanting and erosion control plan for removal of many trees from streamside areas. Significant consideration will be given to retaining trees, which provide stream shading (e.g. within 50 feet of the active channel). When removing mature trees (over 12-inch (30cm) dbh) in riparian areas, coordinate with the DOT Botanist or District Biologist to determine the appropriateness of replanting two seedling/cuttings for every tree removed.
  • Pursue permanent solutions to chronically unstable areas through the project development process. Solutions could include artificial hillside drainage or permanent shoring.

Tree Care and Pruning Guidelines

  • Remove trees greater than 4 inches in diameter from zone 2.
  • When spraying, keep an adequate distance from desirable woody plants.
  • Prune every two years on young trees and every five years on trees in intensively managed areas.
    • Prune early in a tree's life so that pruning wounds are small and growth occurs at the best location.
    • Begin with a visual inspection at the top of the tree and work downward.
    • Identify the best leader and lateral branches before pruning, and remove defective parts before pruning for form.
    • Aside from protecting against oak wilt, pruning cuts need not be protected if they are done properly. For aesthetics, you may feel better painting larger wounds with neutral color tree paint, but evidence shows that it does not prevent or reduce decay.
    • Keep tools sharp. One-hand bypass or scissors cut (not anvil-type) pruning shears with curved blades work best on young trees.
    • Make safety a number one priority. For high branches, use a pole pruner. Some, like the one shown in have both a saw and a shears on the same tool.
    • When you prune back to the trunk for a larger limb, branches too small to have formed a collar (the swollen area at the base) should be cut close. (Note in the figure of the pruning shears that the cutting blade is cutting upward for less effort and a close cut.) Otherwise, follow the rules of good pruning of larger limbs by cutting just outside the branch ridge and collar, at a slight down and outward angle, so as not to injure the collar. Do not leave a protruding stub.
    • When simply shortening a small branch, make the cut at a lateral bud or another lateral branch. Favor a bud that will produce a branch that will grow in a desired direction (usually outward). The cut should be sharp and clean, and made at a slight angle, about 1/4 inch beyond the bud.
  • Don't ignore the mid-size tree.
  • Follow safety and OSHA standards.

Further information on correct pruning methods can be found on-line.

Compost and Shredded Brush on ROWs

Compost consists of mixtures of peat moss, bark, processed wood chips, lawn grass chippings, manure, and other materials which interact to produce a healthy growing ecosystem, using debris that might normally be landfilled or burned. Over 50 percent of municipal sold waste compost is recycled. [N] An important trend in tree, brush, and wood waste management is the fact that in most states, this material can no longer be burned or buried. As a result, more and more material is being processed on site or is being recycled in central locations as compost. Chippers and grinders are a cost-effective way to recycle wood waste into useable mulch. [N]

Cleaning Mowing, Cutting, and Removal Equipment

Equipment used for invasive species control, whether hand tools or power driven, should be cleaned prior to entering a new site and prior to leaving the site, in order to reduce transport of plant propagules and reduce the potential for new invasive introductions. This is particularly important for some invasives, which produce millions of seeds.


9.6.4 Physical Methods
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Physical methods of invasive species control include covering, burning, grazing, and changing abiotic factors.


Covering plants deprives them of the sunlight needed for growth and hastens plant decomposition by contact with soil microbes. Less cover is used (mulching), when trying to foster new germination.

  • Materials used to cover plants should block all light. Common materials include thick black plastic, black geotextile fabric, and mulches. Black plastic can be found in many hardware stores and black geotextile mats are often used under highways, as landfill lining, and in landscaping projects.
  • Before covering, cut, burn, or mow the weed close to the ground to reduce biomass and to put stress on the plants. Treatment sites require regular monitoring to detect and repair torn fabric.

Although covering can be very useful, it has several drawbacks. Unless the material is biodegradable, the cover must be removed after the treatment. In addition, the amount of time needed to kill weeds varies and will need to be experimentally determined.

Caltrans is testing two types of solid mats - Durotrim, a black matting of 1 inch thick made of recycled rubber tires, and Weedender, a light green product made of recycled plastic bottles that is about 1/4" thick - and a liquid soil sealer called Polypavement, which binds to the soil making it impervious to plant growth and erosion. The product from recycled plastic bottles is so light that it requires workers to secure it to guardrails. Though Caltrans has used all the products before, the test is the first time the agency has applied the weed control products simultaneously in one area for a side-by-side comparison of effectiveness with no herbicide use. Caltrans District Five is experimenting with the use of native grasses and organic materials, such as wood chip mulch and corn gluten meal, to test their effectiveness in mitigating the growth of invasive weed species. The tests are also assessing community response to the look of the products, ease of application and cost relative to herbicide application. According to Caltrans project management, other potential benefits that may be realized by the test include: [N]

  • Minimizing traffic congestion by eliminating lane closures associated with more traditional vegetation control activities.
  • Enhancing worker safety by reducing the frequency of on-site work on the highways.
  • Minimizing fire concerns by eliminating vegetation overgrowth.
  • Improving drainage by removing weeds around drainage facilities.
  • Promoting visibility of traffic, highway structures, and wildlife.
  • Reducing the need for herbicides.

The total cost of the pilot project is $200,000. Installation work involves removing existing weeds and rocks, cultivating and leveling off the soil, and then applying the weed barrier materials. Caltrans plans to monitor the treated sites for a three-year period before reaching a conclusion. [N] More information on Minor Concrete (Weed Barrier), Herbicidal Fabric, Asphalt Concrete, Stamped Asphalt, Weed Control Mat (Fiber). Weed Control Mat (Rubber), Minor Concrete (Weed Barrier), and Gravel Mulch are available as information sheets in Caltrans' on-line Roadside Management Toolbox.

Controlled Burns

Eight percent of DOTs are utilizing controlled burns to control invasive species and foster native revegetation. [N] Controlled burning or prescribed fire is a carefully planned and controlled fire conducted to manage natural areas such as prairie, oak savanna, wetlands and oak woodlands. Prescribed or controlled burns have been used by land managers for over 25 years in modern history and for over hundreds of years by Native American tribes. Fire kills the above ground parts of shrubs and small trees.

In certain environments the practice of prescribed burning can change the vegetation cover in favor of native plant species, thereby decreasing population levels of weeds. Prescribed burning is particularly appropriate for restoring or maintaining fire-adapted or fire-dependent species and natural communities. Many invasive plants are not adapted to fire and ecological burning may be an effective tool for controlling these species.

Fire has been used quite frequently to manage invasive alien species in the U.S. and to stimulate natural vegetation in areas adapted to fire. For example, The Nature Conservancy (TNC) has used controlled burns to eradicate Australian pine (Casuarina equisetifolia) in pine forests and other fire-tolerant communities in the U.S., but less frequently elsewhere. Spot treatment is also possible, for example, early in the growing season baby's-breath (Gypsophila paniculata) can be burned with a hand-held propane torch. Prairie plants grow more vigorously when built-up plant materials and shade are removed. Spring fire uncovers the soil, warming it sooner and extending the growing season. Roadside areas across the United States are the site of important remnant native grassland habitats, many of which can be enhanced by management by fire. Many invasive plants are not adapted to fire; thus, ecological burning may be an effective tool for controlling these species. However, land managers must first determine if fire is a natural component in the plant community in question and if prescribed fire can be expected to help meet site goals.

The context should be carefully evaluated to avoid notable failures and so that desirable species may be promoted rather than invasive species. For example, an Australian study of roadside burning noted that non-native plants were spread into adjacent woodland and that native species decreased. [N] It should be remembered that growth of some invasive alien plants, such as garlic mustard (Alliaria petiolata) in woodland of North-East U.S. are stimulated by fire. In other areas, fires can cause disturbance and create establishment sites for new weed infestations.

Only trained and experienced people should undertake prescribed burning due to the many health and safety risks involved. Smaller infestations can be controlled with the aid of a flame-thrower. The risks of a large-scale fire limit the use of these tools, especially in dry climates. Given these ecological and logistical challenges, prescribed burning may not be an appropriate method if considered for invasive species control only. It is best suited to a site where restoration and maintenance of fire-dependent or fire-tolerant communities are primary conservation goals.

Controlled Burn Practices and Consideration

Fire intensity, continuity, and duration are factors to consider in the success of a treatment. Controlled burning offers the following benefits: [N]

  • Control invasives and woody invaders
  • Stimulate the growth of many native prairie plants
  • Remove thatch
  • Recycle nutrients
  • Warm the soil and give warm-season plants an earlier start.

After two growing seasons, planted prairies need to be burned annually for the next several years to become well established (mature prairies with no serious weed problems may need burning only once every two to four years). [N] Recommended practices include the following:

  • Always use caution when burning.
  • Check local fire and air quality regulations and obtain permits.
  • Try to burn or mow only one-third of the prairie area each year to preserve over-wintering insects, their eggs and pupae.
  • Always plan fire safety into plantings, even if you are not going to use burn management. Prairie fires intentionally or accidentally set during fall or spring dormancy can burn very rapidly.
  • Use any existing features such as roads, driveways, streams, lakes, or mowed lawns as fire breaks.
  • In addition to paths through a prairie, also include a wide path around the perimeter.
  • A mowed lawn buffer 20 feet in width between buildings and prairie is advised.
  • An alternative to burning is to mow in late fall after seeds set or preferably in early spring (late March to mid-April). Sites that are too wet in spring need fall mowing when soil is dry.
  • If burning does not occur periodically, cuttings need to be removed to avoid a thatch layer buildup.
  • Do not cut and then burn large quantities of plant material (creating thick piles) or you will sterilize the soil beneath.
  • Revegetation after a fire is important in reducing bare ground and preventing the establishment of other weeds.

Before undertaking a controlled burn, staff must be properly trained and plans developed. Planning considerations should include:

  • Traffic safety. Any burning plan must include smoke management provisions for safety purposes.
  • Weather conditions.
  • Equipment.
  • Staffing.
  • Timing. Burning is most beneficial from mid-April to early May for warm-season grasses. As with spraying growing invasives, burning earlier is better for wildflowers, and waiting does more harm than good.

Controlled burning is practically explained by Wayne Pauley in his How to Manage Small Prairie Fires. [N] The Missouri Department of Conservation recommends the following practices for controlled burns, drawing on Pauley's work: While fire management requires training and knowledgeable individuals, it takes "as little as a few hundred dollars in equipment, including drip torches, rakes, and safety clothing…Roadside prescribed burns are easy. The road is one fire break and the others can be a mowed field of harvested hay or lawns." [N] Staff undertaking burns should be forewarned that corn stubble and older fence posts smolder.

More detailed information on controlled burns for invasive species management may be found in the Center for Invasive Plant Management's on-line resource: Fire As a Tool for Controlling Non-Native Invasive Plants. The review "focuses on the intentional use of fire, alone or integrated with other methods, to control exotic plants in North America." Approximately 235 citations are noted in the 51-page report that is divided into four sections: Managers' Objectives, Limitations on Tactics, Detailed Case Studies, and References. Additional information regarding proper burning procedures can be obtained from the Fire Management and Research Program at The Nature Conservancy (850-668-0827) or your state resource agency. The Texas Parks and Wildlife Department (TPWD) has on-line burn information for controlled burns on state property and provides a Sample Burn Plan. They also have information on prescribed burning.

Controlled Burning or Hay Removal as Roadside Grassland Management Alternatives to Mowing

Mn/DOT and the University of Minnesota have been exploring whether mowing can be as effective as yearly burning at encouraging native prairie grasses and discouraging botanical invaders. A research team investigated the impacts of burning and mowing on three separate test areas, examining above-ground vegetation and below-ground fungal communities, as well as measuring changes in various soil parameters. Findings and recommendations were as follows: [N]

  • Prescribed burning has the strongest effects on plant community composition and was the most effective method to increase aboveground plant biomass in a restored tallgrass prairie. Burning especially favors warm season grasses and legume species, though it also favors certain annual species. Also, when immediate grass cover is desired, burning is the best maintenance technique available to increase grassland productivity.
  • When burning is not an option, haying may be the next best alternative. The addition of lime may be important to consider on restorations of former agricultural lands.
  • Adding lime to hayed prairie may help benefit the cool-season plants, native and exotic.
  • Spring haying is an acceptable alternative to spring burning, though its effects are less dramatic than the burn. In particular, haying does not favor warm season grasses as extensively and may not damage cool-season species as thoroughly as burning. Spring haying did not control exotic species.
  • Burning and haying provided the greatest increase in arbuscular mycorrhizal fungal structures, which may correspond to the increases in plant growth on these treatments. In prairie restoration, the addition of arbuscular mycorrhizal inoculum appears to provide long-term benefits.
  • Mowing the prairie in the spring has a similar affect on the plant community as no management. It is only useful for the control of woody species. Mowing may decrease nitrogen mineralization rates temporarily. This may help to prevent invasive species but is not likely to do so if mowed annually.
  • Frequent burning or haying should be performed in order to prevent the accumulation of inorganic soil nitrogen, which may favor many weedy species. If haying is used instead of burning, soil pH should be tested periodically to detect acidic soil. Although this did not become apparent on this experiment, it may occur on long-term hayed grasslands. Acidification may lead to decreases in certain plant populations or losses in productivity.
  • The process of removing litter seems to be the most important cause of the ecosystem response to prescribed burning. Hayed plots are the most similar to burned plots in terms of soil moisture, temperature, and litter quantity. Hence, litter removal by haying will likely be a sufficient practice to replace prescribed burning at many sites.

DOT Examples of Controlled Burning for Invasive Species Management

A number of Midwestern state DOTs use controlled burns. Illinois DOT attempts to use burns if the timing is right. The agency mows if burning is not an option and uses chemicals as necessary.

In California, five acres of highway ROW were targeted to learn more about prescribed burns as a management tool. The Bear Creek Botanical Management Area, one of the last examples of Upland Wildflower Fields in California, contained a plant community remnant with more than 200 native California plant species. After careful planning, Caltrans District 3 forces coordinated the safe passage of vehicles and the California Department of Forestry and Fire Protection (CDF) conducted the burn. The key target was yellow star thistle (Centaurea solstitialis) which had invaded half the site within a short time. Observations following the fire have shown the prescribed burn to be more effective than the preceding years of mowing, spot spraying, and hand pulling of star thistle; however the state is attending to air quality issues. [N]

Infrared Treatments for Managing Invasive Species

Infrared technology uses radiant energy (heat) to kill unwanted vegetation. Intense heat generated by liquid propane coagulates plant proteins and bursts cell walls, killing seedling plants and destroying the tops of established vegetation. Repeated treatments at regular intervals deplete the root reserves of established plants and lead to their decline and eradication. In a study by Oregon DOT, infrared treatments were applied at three rates (8, 6 and 4 treatments/year) along Oregon highways and compared to shoulders treated with herbicides and to unmanaged control sites. Results suggested that infrared technology can keep vegetation under control on roadway shoulders, but timing of treatments to plant growth cycles, weather, and fire conditions is critical. [N]


Grazing is a biological alternative to mowing and has been considered a physical method, a biocontrol method, or a habitat management method. Continual grazing of the tops of young plants can retard plant development and seed formation and can gradually deplete root reserves. Since animals might prefer to eat nearby grasses in lieu of the target weed, they may be enclosed in a fenced-off, weedy area.

This invasive species control method works best where the plants that are to be preserved are adapted to grazing, i.e., they are either adapted to high populations of large herbivorous mammals or prevalent in human-made habitats such as pastures and heathland. On the other hand unmanaged grazing often favors alien plants, as grazing can preferentially remove native vegetation leaving alien plants, especially toxic species, to grow under reduced competition, leading to a monotypic stand of an alien plant.

Four classes of grazing management plans that can be used for weeds in general:

  • Seed removal involves grazing the weed in the spring to remove the bracts and flowers, and perhaps again in late summer. This type of grazing prevents seed-set but does little to reduce the root system.
  • Multiple pasture rotational system is a properly-timed, rotational grazing system that continuously defoliates the target weed throughout the growing season. This eliminates seed production and causes limited stress on the root system. It is useful when large infestations occur over many acres.
  • Intensive rotational grazing involves grazing the plant in the spring until it is completely defoliated, and then rotating the animals to the next pasture. In late summer each pasture is grazed a second time. This method places optimum stress on the plant, which decreases plant vigor and carbohydrate reserves.
  • Continuous grazing, for about four months, allows goats or sheep to graze throughout the growing season, thus preventing weeds from recovering and maximizing stress on the root system and its reserves.
  • Permit animals to graze weeds only before they flower and set seed. If this is impossible, contain animals for 7 to 14 days in a holding area before moving them to non-infested areas.

Habitat management with grazing mammals can be a suitable option to obtain the desired plant cover. Goats and sheep are economical and they do not pose the environmental dangers of applying chemicals. In addition to their value for weed control, sheep can be used for income from the sale of their wool. If confined, Angora and Spanish goats will trample or browse virtually any vegetation within a fenced area. Desirable trees or shrubs can be protected with light-weight flexible fencing. [N]

Maintenance managers have "hired" goats for control of broadleafed invasive species control in Montana, New Mexico, Nevada, and California, among other places. Caltrans' Technology Transfer newsletter described several examples, as follows. [N] In 2000, the Billings Public Utilities Department maintenance mechanic Fred Charette hired on a pair of angora goats to munch on a particularly abundant yellow-flowered noxious weed which grows along the banks of the Yellowstone River. Charette paid about $30 each for his goats and built a pen for them. The goats showed a preference for the top part of the leafy green perennial where the buds bloom into flowers, preventing blooming and subsequent spread. The goats continue to be utilized to clear around drainage facilities in Billings. Charette observed that the goats prefer to graze close by each other. He often tethers his two goats, 4-H club retirees in the weed abatement area, and finds with just the two that there is no need for fencing. He has one of his staff check on them about every 90 minutes. He has become a keen supporter of the goat method and has put lots of extra energy into educating the wider community about utilizing goats for yard clean-up. He notes that goats will happily chomp on whitetop, Canada thistle, spotted knapweed, leafy spurge and bindweed. The Billings Public Utilities Department has also used the goats as part of an integrated bio control management program in conjunction with introduction of the Lacertosa or flea beetle, which attacks the weeds at their root systems.

In Albuquerque, about a thousand goats were brought in to clean up weeds along the Rio Grande. The district biologist, Sterling Grogan, says that "tightly managed and limited use of goats is a really good and ecologically sound way to manage vegetation without having to use herbicides or fossil fuel for mowers" and leave the native grasses to flourish. The state of New Mexico plans to clear more than 1,200 miles of canals, drains and ditches, which would normally be cleared using costly electrical equipment, herbicides and manual (human) labor. The goats provide a way to do this that is more cost-effective, saves energy, and safeguards the environment. Community members phoned in support, saying that the goat plan was the most intelligent thing they had heard of a government agency doing in a long time. [N] Denver's naturalist attempted something similar, but encountered problems stemming from the sheer logistics of trying to mobilize 50 to 275 goats in an urban environment as well as unleashed dogs which chased the goats. Nevada uses a combination of grazing with goats, careful herbicide application, mowing, and seeding with annual plants. Along the Truckee River, goats, which were put out to graze on whitetop weeds infesting the floodplain, ate about 75 percent of the young, tender re-growth. Such efforts have done much to control the growth and spread of weeds. In Montana the Bureau of Land Management found that grazing sheep controlled about 90 percent of the leafy spurge weed over four years. Without sheep, the Bureau was spending $40-$50 per acre on herbicides to control the weed; today, with the sheep assisting them, there is only a need for minor spot applications of herbicides. In Wyoming the DOT estimates that sheep grazing the land cost $18.80 an acre to maintain, compared with $185-$310 an acre to control growth using herbicide, and $350 an acre to control growth through hand-cutting and mowing. [N]

In Rackerby, California, Goats Unlimited raises small New Zealand "Kiko" goats just for controlling weed growth. Their goats are serviced out to assist with rejuvenation, erosion control, and restoration projects, as well as creating firebreaks and clearing ditches. Goats can help an agency reduce its dependence on fossil fuels and because goats, unlike mowers, do not start brush fires with sparks from their motors, they have been used extensively since the fires of 1990 in the Oakland-Berkeley Hills to safely manage the growth of undesirable vegetation by clearing dense undergrowth, including the highly flammable manzanita. Numerous agencies in the Bay Area now employ the goats for vegetation control, including the East Bay Park District, the City of Oakland and the San Francisco International Airport. In the Sacramento area, 350 goats are clearing an acre a day. In San Luis Obispo, flood control managers have used goats to clear drainage channels along the Arroyo Grande Creek. Goats, traditionally mountaindwellers, do well on steep terrain - which poses a challenge for workers with power mowers. And the goats will contentedly "work" any time of day or any day of the week. For large groups of goats, goat herders are sometimes required. [N]

Changing Abiotic Factors

Most invasions of non-indigenous species are caused or at least favored by human disturbance of the ecosystems. In these cases a mitigation of negative impacts by the invasive species could be achieved by changes in the human behavior that has led to the invasion. An example would be a change in the quantity of nutrients and/or water available for plants, which would alter the plant community. In some cases invasive aquatic organisms can be controlled by improving the water quality, addressing eutrophication and pollution problems, or even changing the quantity of water, e.g. draining or a water level regime adverse for the invasive species.

Hunting and Other Use of Non-Indigenous Species

Continuous hunting can be used to control exotic species, such as deer, originally introduced for hunting purposes. There are two approaches: commercial hunting principally for meat and recreational hunting. Both approaches can generate income for the landowner and/or the state. Some exotic species are both comparatively easy to hunt and are favored species for hunters, and so should be straightforward to manage by hunting, but conversely more wary species or those less preferred by hunters are less likely to be effectively managed in this manner.

Problems encountered trying to control an alien species through hunting usually relate to land ownership and the distribution of the invasive species. Some species spread into suburban areas where hunting is not allowed. Significant groups of the human population, particularly in developed countries, find hunting morally unacceptable, and so it may decrease in popularity, thus allowing alien species formerly controlled by hunting to explode in numbers.

Many other invasive species can be eaten or have edible fruits, which can be exploited for human consumption or as fodder for domesticated animals. In many parts of the world with high human density invasive plants are esteemed also for their production of highly valued firewood or other uses. A high percentage of introduced fish and crustacean species are fished either recreationally or on an industrial scale.

A significant drawback of this approach as a control method is the promotion of an alien species as a food resource and the promotion of new industries depending on what otherwise might be better to eradicate. The market may provide an incentive for individuals to spread the alien species to as yet uninfested areas, or breed them in captivity, from where they may eventually escape. Thus the costs and benefits must be evaluated on a case-by-case basis.


9.6.5 Chemical Methods
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Herbicides adversely affect the physiological activity and development of plants and are used to control vegetation by causing death or suppressing growth. These compounds, as active ingredients, are incorporated into a wide variety of commercial herbicide formulations for application to plants and to surrounding soil or water as sprays, granules, and liquid concentrates. Herbicides can be grouped on the basis of their chemical structure and physiological action, or on the timing and method of their application. Chemicals with similar structure usually produce the same type of physiological reaction in plants, and control similar species. Nonselective herbicides control all vegetation because they affect physiological processes common to all plant species. Selective herbicides will damage only those groups of plants that carry the biological pathways they target.

Herbicides are the major method of invasive species control in non-crop situations as well as in agriculture because they are relatively effective, reliable, cost-efficient, safe and easy to use. In addition, some areas virtually require treatment by herbicides; eradicating vegetation that grows around and under standard current guardrail is very difficult without the use of herbicides. Nevertheless, human health concerns have encouraged public agencies in particular to examine and encourage alternatives to herbicide use. In the past, extensively used broad-spectrum herbicides such as DDT had massive detrimental impacts to the environment as well as human health, but today these are banned in most countries, and there are more specific products on the market with fewer negative non-target effects. Some insecticides, such as those based on chemical structures similar to insect hormones, can also be specific to target groups of insects.

Other major drawbacks are the high costs, the necessity of repeating an application, and the impacts on non-target species. Repeated use of pesticides also provides the selective pressure which enables many target species to evolve increasingly effective resistance to these chemicals. In response either the dose has to be increased or a different group of pesticides has to be used, usually further increasing the control costs. There is also the possibility that indigenous peoples will oppose the use of toxins on their land, for example where toxins may accumulate in sub-lethal levels in non-target species that may be an important food source for indigenous peoples. This latter concern is mainly true of persistent pesticides such as modern anticoagulants and the now largely obsolete organo-chlorine compounds.

Selection of a pesticide to control an invasive species begins with a determination of effectiveness against the target and all appropriate non-target species that might come in contact with the chemical, either directly or through secondary sources. Additionally, the environmental half-life, method of delivery, means of reducing non-target species contact, demonstration of efficacy, and collection of data to ensure compliance with environmentally safe use (as set out by the regulatory bodies in the country where it will be used) must be evaluated. Most countries require pesticides to be registered for specific uses. Once identified, tested, and registered, a pesticide can allow the rapid control of a target species over large areas, and as a result reduce the need for personnel and costs for the more traditional methods such as traps and barriers.

Widely used application methods for herbicides include treatments of the bark of young trees or applying herbicide into the wounds created by girdling or cutting. This cut-stump application method, mentioned already in the section on mechanical control, is very effective against many woody plants. Herbicide can also be applied directly to the leaves of the invasive species by using a sponge, wick, or a syringe, but a less specific and more common method is spraying of infested areas. As a general rule, foliar herbicides are applied to young, tender, actively growing plants prior to flowering. If the plants are too mature for effective herbicide application, a common practice is to mow an invasive plant infestation, prior to seed maturation, allow the plants to re-grow to a height of 2 - 4 feet and then apply the appropriate foliar herbicide. This process can weaken the plant and prevent spread by seed, and maximize the effectiveness of the herbicide application. Due to the vigorous nature of many invasive plants, especially knotweed and Phragmites, this process may need to be repeated 2 or 3 times over consecutive years. Stem "cut and treat" herbicide applications involve the cutting and removal of the growing plant stems and then the spot application of herbicide to the freshly cut surface of the remaining rooted portion of the stem. The herbicide will then be translocated down into the plants root system. The "spot" application can be accomplished with a swab or hand sprayer and should occur as soon as possible but not more than one hour from the time the stem is cut (within 15 minutes is preferred for best results). A common practice is to have one person cut the stems while a second person follows and applies the herbicide to the recently cut stems. It is important to mark which stems have been treated - using a marker dye is a common technique. This technique is recommended in situations where foliar treatments are not possible or effective, such as with Japanese Knotweed. If stands of invasive plants extend beyond the ROW, a written release may be sought from the adjacent landowner to implement controls. All sites where herbicides are applied should be inspected 3 - 4 weeks post application to assess success and to determine if natural re-vegetation by native or non-invasive plants is adequate or if additional restoration, e.g. seeding and mulching, is required. [N]

Insecticides can be sprayed selectively on infested plants or plant parts or indiscriminately over a large area. Application should always be as focused as possible on the pest, e.g. spraying of the attacked plant part, at the most susceptible time for the target, and limiting the use to the efficient dose, in order to minimize side-effects on other species.

Pesticides are used against vertebrates mainly in baits, e.g. bait stations for rats. Before using bait, small-scale experiments and observations can be carried out to determine which non-target species might take the bait. With some ingenuity, it may be possible to develop bait stations to give easy access to the target species but prevent, as far as possible, other species from entering it. Naturally, a more target specific bait station is easier to design for an ecosystem with no species similar to the target species.

Chemical substances are used to mitigate diseases in humans and animals. Disinfection of water and surfaces capable and suspected of disease transmission are treated with disinfectants to kill pathogens before entering their hosts.

Chemical treatment offers one of the few options for control of marine invasive species, although its potential is limited. Herbicides (e.g. glyphosate and 2,4-D) have been used extensively around the world as a quick and effective means of controlling weeds in freshwater environments. However, since they are non-selective and more difficult to apply directly to the target plant in water, they are more likely to cause harm to non-target species. The fish poison rotenone is frequently used to control fish species in ponds and other small water bodies. This method is efficient for the eradication of species, but the non-selective character limits its use for large-scale infestations.

DOTs are required to follow appropriate laws and mandates when using herbicides or pesticides. DOTs are required to use all herbicides in accordance with label instructions, state and federal law (including adjacent landowner notification requirements), and by or under the supervision of certified applicators.

Herbicide Use as Part of an IVM Program

Judicious use of herbicides is an important tool in invasive plant control efforts. IVM stresses the need for selectivity, restraint and proper training and protections whenever herbicides must be used. On the shoulder and in other zones, too, invasive species must be controlled to protect against undesirable succession of plant communities, not only for the sake of the roadside zone itself, but also to prevent the roadside from becoming a refuge for invasive species and source of further spreading. Chemical vegetation controls are used to protect preferred vegetation, to provide fire protection and to improve roadside appearance.

In IVM programs, herbicides are considered transition tools that enable the manager to suppress weeds and replace them with desirable, competitive vegetation. Thus, it is important to select the least-toxic, low-residual herbicide that is effective against the target weed, and to apply it in a judicious manner. Consult the Cooperative Extension Service, State Agricultural Experiment Stations, or County Agriculture Commissioner for specific herbicide recommendations and information on their use in particular localities.

Responding to Public Concerns of Herbicide with IRVM and Improved Stewardship Practice

DOTs find themselves in the public spotlight over herbicide use as well as other matters. In integrating all the IRVM tools, FHWA has outlined the following recommended steps prior to herbicide use: [N]

  • Evaluating each site to determine if invasives really present a problem.
  • Spot mowing to prevent annual weed seed production.
  • Removing a [non-native species] and allowing desirable species to reclaim the area.
  • Prescribing burning of prairie communities to promote healthy vegetation.
  • Using biological controls as alternatives.
  • Frequent roadside management equipment cleaning to help reduce seed transfer.

DOTs have also responded to public concerns with herbicide reduction and/or notification efforts: [N]

  • Caltrans pledged to decrease herbicide use by 50% between 1992 and 2000, and by 80% by 2012. District 1 governments can opt for no herbicide spraying.
  • Caltrans and UC-Davis are undertaking research to develop an improved Intelligent Herbicide Application System (IHAS) to assist the Caltrans in reducing the amount of herbicide applied for roadside vegetation management. The system selectively applies post-emergent herbicide to weeds at the edge of the roadway and not to bare soil, reducing the amount of herbicide required for weed control. [N]
  • Iowa DOT controls invasives with herbicides only if mowing or other controls are not practical. Forty-one out of 99 counties in the state participate in IRVM programs.
  • New York State DOT set up a toll free number for members of the public to find out about spraying plans.
  • North Carolina, Oregon and Washington State DOTs use integrated vegetation management and offer no-spray agreements to adjacent landowners. Washington State DOT tracks and reports on reductions in herbicide usage.
  • The New Mexico State Highway and Transportation Department has undertaken internal research to minimize the use of herbicides while successfully controlling noxious weeds. As part of the study NMSHTD is conducting a review of methods or combinations of methods that are available for the control of noxious weeds and it finds existing policies and procedures in other states for minimizing the use of herbicides. [N]
  • The Alabama DOT has implemented herbicide delivery systems that will ensure better control of herbicides ordered for application on state roadways, not only cutting purchase cost, but greatly reducing disposal cost for hazardous herbicides. [N]

New Equipment to Focus and Minimize Herbicide Application

Herbicide spraying helps eliminate unwanted vegetation, but typically, 80 to 90 percent of the sprayed chemical misses its mark and is wasted, according to horticultural scientists at NC State University. A number of DOTs have been active in developing the best technology for roadside vegetation control. Recent research and development work by Purdue University led to commercial production of an equipment system that can electronically identify individual invasives within its path and deliver a prescribed targeted application of herbicide, in a single pass over the roadside. Treatment of various invasives often requires the use of different herbicides. Sprayers must have the capacity to spray more than one chemical at a time, negotiate rough terrains, and apply herbicides safely and in a way that preserves the environment. Purdue's technology and the other following examples reduce the amount of herbicide needed and DOT costs: [N]

  • Iowa's Clinton County uses a commercial roadway management system that uses a sprayer-mounted computer connected to a GPS receiver to record the placement of multiple herbicides. The spraying equipment was purchased in 1995 for approximately $23,000. The county's objective is to integrate data from years of mapping in order to target necessary spraying and avoid sensitive areas. The County upgraded their GIS system, at a cost of about $10,000 in 1999 and purchased a GPS realtime mapping on spray system for $17,000 the following year. [N]
  • Small sensors manufactured by Patchen, Inc. in Ukiah, California can be used on trucks or other equipment to pinpoint the location of an undesirable plant and then target and spray the weed with herbicide. Each sensor views a 12-inch-wide area. When it finds weeds, it signals a spray nozzle to deliver a precise amount of herbicide. Called the WeedSeeker, the unit will spray only weeds and not bare ground. Several California Department of Transportation districts have mounted the sensors onto equipment. A side-mounted strip of sensors at the rear of a vehicle lets the unit target and spray roadside weeds at 10 miles per hour. Using the sensors cuts herbicide amounts and costs by 50 to 80 percent compared to broadcast or manual spot spraying. The sensors have their own light source so they can be used at night when traffic is light. Units with the sensors need only the driver, reducing work hours required as well.
  • At North Carolina State University, Drs. Jim Burton and Walter Skroch came up with an herbicide applicator that can be attached to weed mowers. The unit applies a film of chemical to the weed stem as the plant is cut by the mower. From 70 to 90% of the herbicide is absorbed into the plant to prevent future growth. The applicator, called the Burch Wet Blade, is designed for use on rotary roadside weed cutters. A reservoir mounted on the cutter's deck holds the premixed chemical solution. A pump regulates an adjustable flow of the chemical through the spindle shaft and out along the blade to an opening on the cutting edge. "When you cut the plant, its vascular system sucks in the herbicide and sends it plantwide," Burton says. The system is a closed one that never sprays into the open air, which is safer for the operator.
  • Mn/DOT tested four sprayer designs, each of which saved money compared to traditional sprayer use, according to a research report by Claudius Toussaint, Minnesota DOT Office of Maintenance Operations Research. Savings of up to $65,000/sprayer were cited. [N]
  • The B & B Ditch Sprayer 300 prototype contains a 300-gallon over-shaped plastic tank and a sprayer equipped with two spray booms, each consisting of drop nozzles and providing a 60-foot spray pattern. The spray boom has an innovative spring-loaded feature. Even though this system had the lowest net savings, $23,255, of the four tested sprayers, it offered a rate of return of 360%.
  • The Wanner Innovative Sprayer system includes an 850-gal. stainless-steel tank, with spray material dispensed from the tank and delivered to the spray nozzles via a pumping system. Spray booms consist of drop nozzles and end-mounted boom-buster roadside nozzles. Additionally, there is a single straight stream nozzle mounted on the end of the boom. The system is also equipped with drop nozzles, which are used for spraying road shoulders. All spray nozzles are remotely controlled from the truck cab. The Wanner Innovative Sprayer was mounted on a Class-33 truck. The Wanner Sprayer is more economical for use in large districts or in areas with extensive road miles, due to its capability for applying herbicides at high speeds and accurate amounts. The large site of the supply tank is also a factor in the sprayer's ability to service large areas. The rate of return was measured at 76%.
  • The Micro-Track Spray System, a multiple-injection spray system with console monitors, can be connected to any size of supply tank. The system is capable of spraying up to five different herbicides simultaneously, and the supply tank, chemical tanks, pumps, monitors, and nozzles are all mounted on one unit. No trailers are needed. The Micro-Track sprayer had the highest rate of return, 1,142%. It was able to reduce costs by about $64,942 annually.
  • The SCS 750, which was developed by Raven Industries Inc. of Sioux Falls, South Dakota can independently control the liquid, the granular dispensing systems, and the overall hydraulic system at the same time. If spot spraying is needed, each chemical can be controlled by a manual switch. Hand spraying attachments are also available for spot spraying. The spray rate is controlled by a wheel-driven speed sensor. Visual and audible alarms alert the operator to any deviation from the programmed rate of application. Measurements can be calculated using the metric or U.S. measurement systems. Calibrations and weather information are stored in the 10-year memory capacity. A compact printer is plugged into the 750 console to obtain a readout of the daily activities. The SCS 750 Chemical Injection System can be equipped with up to 10 spray booms. Each boom has down spray nozzles and end-boom buster nozzles. The booms are controlled via a boom control box. The SCS 750 Spray System uses a 300-gal. elliptical tank that is placed on a trailer. The SCS 750 Sprayer had the highest net annual savings, $65,812, along with the advantage of being able to monitor, tabulate, and print all vital statistics (areas, dosages, weather conditions, application rates, and so on).

NCHRP 20-5, 33-04 on Integrated Vegetation Management also reviewed some of the equipment on the market today to focus and reduce herbicide applications. [N]

  • Commercial GPS/GIS systems can provide or record information, such as environmentally sensitive site locations, for use in planning or implementing integrated vegetation management programs. Tracking and record keeping systems that are linked to today's compatible high tech injection sprayers or roadside mowers are being marketed.
  • Computer controlled spray equipment is on the market today. Injection type sprayers that measure and inject multiple ingredients used in herbicide applications make it possible for equipment operators to use computers to change materials and/or rates of materials as they move along the right‑of‑way. The total volume of mixed spray is controlled, allowing the equipment operators to vary their travel speed and area of coverage while moving. Onboard computers can generate the required pesticide application record information for downloading or storage in databases. Today's injection systems can be coupled with the use of packaged pesticide concentrates that are totally ‘closed', minimizing the potential for spills and/or operator exposure associated with traditional material transfer from packaged materials to spray tanks.
  • Nozzles and materials that reduce the potential for off‑target drift of sprayed materials are available. Nozzles that reduce or eliminate the generation of spray particles that are under 200 microns in size reduce the potential for significant off‑target movement of liquid applied herbicides. Nozzles have been developed which enable applications to be made to targets at the outside edge of many right‑of‑ways, improving the efficiency of roadside vegetation management applications without increasing the risk of off‑target placement.
  • Spray mix additives and/or special mixing equipment have led to roadside invert-emulsion spray equipment that can deliver large droplet, oil-rich, herbicide applications to target plants with minimal risks of drift, and with improved herbicide absorption by the target plant. This invert emulsion technology and equipment has been around for several decades but has recently been reintroduced to roadside vegetation management programs. A roadside vegetation management research report by the Commonwealth of Pennsylvania and Pennsylvania State University [N] contains information about invert emulsion sprays, and other roadside vegetation management items.
  • Recent research and development work by Purdue University has led to commercial production of an equipment system [N] , which can electronically identify individual weeds within its path and deliver a prescribed targeted application of herbicide, in a single pass over the roadside. This innovation has the potential to reduce the amount of herbicide needed to treat an acre of roadside, reducing costs, and minimize the amount of herbicide introduced into the roadside environment.
  • Mowing equipment with herbicide application nozzles incorporated within the cutting head cowling has helped produce little or no `brownout' beyond that associated with the mowing operation.

Performance Indicators for Herbicide Use

MoDOT Mowing and Herbicide Costs are included in the DOT's Dashboard Annual Report of high level indicators, under the strategic goal of improving maintenance of the state's highway system. MoDOT's has determined that herbicide use is more efficient than mowing and thus has designed a system to show reduced mowing costs and stable or increasing herbicide costs as a positive indicator, in the effort to reduce mowing. The agency acknowledges that the maximum amount of herbicide expense vs. mowing expense needed to reach the highest level of cost efficiency on roadside maintenance is unknown at the time the measure was drafted, but will be revised as the agency moves forward. In contrast, WSDOT is aiming for and tracking reduction in herbicide usage, as part of its IRVM program, though usage has increased in various years as noxious weed treatments have increased.

On-line Sources of Information

The following is a guide to the use of various herbicides for the management of noxious and nuisance plant species.

The TAG is composed of federal agencies as well as representatives from Canada and Mexico to propose and evaluate biological control agents. APHIS responds to the recommendations of the TAG before a biological control agent is approved.


9.6.6 Biological Control of Invasive Species
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Biological control is the intentional use of populations of upper trophic level organisms commonly referred to as natural enemies against pest species to suppress pest populations. It involves introducing parasites, predators, or pathogenic microorganisms in order to suppress a target plant or animal pest. Biocontrol agents are living organisms that have specific requirements for growing and thriving. Understanding the life cycle, habitat requirements, and mode of attack assists in finding a hospitable release site and allowing the user to integrate the biocontrols into other control efforts.

According to a survey by the author, almost a quarter of DOTs (13 state DOTs, 33% of respondents) are using biological controls to some extent. [N] Biocontrols also have played a particular role in some DOTs in wetlands or inaccessible areas.
Several DOTs have become very active in use of biological controls. NCHRP 20-5, 33-04 on IRVM reports that biological control practices using herbivorous organisms such as beetles and seed flies are in use on ½ to 2 percent of the ROW in Florida, Illinois, Kentucky, Maryland, Utah, and Washington. [N] Other examples include:

  • Caltrans has research underway on biological controls for Yellow Star thistle (Centaurea solstitialis) and Tumbleweed (Russian Thistle; Salsola tragus). [N]
  • NYSDOT and other DOTs use Hylobius sp. and Galrucella sp. Beetles on large, dense stands of purple loosestrife (1/2 acre or larger). Cornell University conducted extensive research prior to the selection of this particular species and prepared a Generic Environmental Impact Statement for their release throughout New York State. These beetles feed exclusively on purple loosestrife (they will starve rather than eat any other plants), will reproduce after release and can be harvested from prior release sites for use in other locations. Research is currently well underway by Cornell University to identify and test an effective biological control(s) for Common Reed (Phragmites). Unfortunately, no acceptable biocontrols currently exist for Japanese Knotweed;
  • In 1995, Mn/DOT launched their first school partnership in beetle rearing for roadside use, an educational and public awareness success story, after two beetle species released at a site in southern Ontario effectively reduced purple loosestrife (Lythrum salicaria) infestation by over 90 percent over five years, allowing native plant populations to extend their reach. [N] The Mn/DOT Office of Environmental Services uses beetles to control leafy spurge (Euphorbia esula). [N]
  • Between 1997 and 1999, NHDOT and the Department of Agriculture (NHDA) monitored a beetle release at a mitigation site infested with purple loosestrife (Lythrum salicaria); by 2000 all loosestrife within and adjacent to the site was either dead or extremely stressed and dying and none of the remaining live plants appeared to develop flowers and therefore seed. Self-sustaining populations of beetles still were found among the remaining loosestrife plants. Indigenous vegetation, likely from seed in the existing soil bank, filled the void and restored diversity. NHDOT and NHDA released beetles at 12 additional sites the following year. [N]
  • The Michigan State University's lab produces 150,000 beetles per year, which Michigan DNR has been releasing on state game areas infested with purple loose strife (Lythrum salicaria) since 1994. The lab has trained local groups around the State to rear the beetles, release and monitor their affect, leading to an expected 80 percent reduction in density in 10-20 years.
  • The Vermont Agency of Natural Resources and VTrans have mapped purple loosestrife (Lythrum salicaria) populations and VTrans is testing three approaches: 1) release of beetles without mowing or spraying; 2) mowing right after flowering begins for easy identification yet not be mature enough to disperse seed; and 3) spraying. [N]
  • Spotted and diffuse knap invasives can be controlled using one of 12 insect species cleared by the USDA for use in the United States. In Tennessee, the DOT was able to reduce musk thistle (Carduus nutans) infestations by 95 percent with a biocontrol beetle. [N]

Different Types of Biological Controls

Biological control does not aim to eradicate targets, but to keep them at low, manageable levels. It is important to match the insect or pathogen to the invasive species site. While biological control is highly recommended to control an established population of an invasive alien species, the theory of natural population regulation underlying the principle of biological control does not anticipate eradication with this method. In a successful biological control program, the invasive species' population will be reduced to an acceptable level, but the populations of prey/host and predator/parasitoid will remain present in a dynamic balance. Biological control is particularly appropriate for use in nature reserves and other conservation areas because of its environmental-friendly nature and the prohibition of pesticide use in many such areas.

As used today biocontrols are a relatively inexpensive and safe alternative to chemical or mechanical control. Some of the introductions made over 100 years ago were of generalist predators, including vertebrates such as mongooses and cane toads, and these did have severe adverse affects on non-target populations, including species of conservation importance. Such species would not be used today in biological control, and some of them are good examples of invasive alien species causing serious problems. However, today the safety standards of biological control are very rigorous. It is a normal requirement (e.g. IPPC Code of Conduct) to assess the specificity of all agents proposed for introduction. This involves extensive laboratory and field screening tests. An informed decision can then be made by the appropriate national authority taking into consideration the potential for any effects on non-target organisms. The U.S. Department of Agriculture (USDA) conducts a biological control program that involves importing, propagating, and distributing invasives' natural enemies. In general, insects are best used in areas of large infestation. Smaller infestations are better treated with herbicides. [N]

Several broad types of biocontrol approaches are recognized. Biological control can be split up in several approaches grouped under two headings: those that are self-sustaining and those that are not. Methods that are not self-sustaining include:

  • Mass release of sterile males to swamp the population with males which copulate with the females without producing any offspring in the next generation.
  • Inducing host resistance against the pest. This approach is particularly relevant to agriculture where plant breeders select (or create) varieties resistant to diseases and insects.
  • Biological chemicals, i.e., chemicals synthesised by living organisms. This category overlaps with chemical control and whether to list a particular method in one or the other category is a question of definition, e.g. while applying living Bacillus thuringiensis (BT) is without doubt a biological control option, to which group the use of the toxins stored in BT belong could be debatable. Other examples of chemicals in this group are rotenone, neem and pyrethrum, extracted from plants.
  • Inundative biological control using pathogens, parasitoids or predators that will not reproduce and survive effectively in the ecosystem. Large-scale or mass releases of natural enemies are made to react quickly to control a pest population.

Self-sustaining biological control includes:

  • Classical biological control. At its simplest, this is the introduction of natural enemies from the original range of the target species into new areas where the pest is invasive. Invasive alien species are often controlled in their indigenous range by their natural enemies, but are usually introduced into new environments without these natural enemies. Freed of their parasitoids, parasites and predators alien species often grow and/or reproduce more vigorously in the country of introduction. Natural enemies for introduction are selected on the basis of their host specificity to minimize or eliminate any risk of effects on non-target species. The aim is not the eradication of the invasive alien, but to reduce its competitiveness with native species, hence reducing its density, and its impact on the environment.
  • Augmentation of enemies under pest outbreak conditions for an immediate control, when the enemy can reproduce in the new environment. The control agent is reared or cultured in large numbers and released.
  • Habitat management can enhance populations of native predators and parasitoids, e.g. release/replant of native alternate hosts and food resources.

The most important of these for management of invasive alien species is classical biological control. Conservation managers are coming to realize that this method, if used following modern protocols such as the International Plant Protection Convention's Code of Conduct for the Import and Release of Exotic Biological Control Agents, provides the safest and most cost efficient approach to solve many invasive alien species problems.

Biocontrol is typically a long-term, environmentally acceptable approach for the control of a target plant species. In comparison with other methods, classical biological control is, when successful, highly cost-effective, permanent and self-sustaining. It is ecologically safe due to the high specificity of the agents used. The main disadvantages are the lack of certainty about the level of control that will be achieved, and the delays until the established agents achieve their full impact. Biocontrol agents can take five to 10 years to become established and increase to numbers large enough to cause damage. However, with a potentially very positive cost-benefit ratio, the benefits of classical biological control normally outweigh the drawbacks and it represents the cheapest and safest option to date. Such a long-term control method is best used in low-priority areas, at sites where the use of other control strategies would be cost prohibitive, or in conjunction with control methods with shorter effect times. However, in many cases it may not be feasible to wait such an extended period to affect control. Also, there are many noxious plant species for which biocontrol agents are not available at the present time.

As described by the Global Invasive Species Programme, biological controls may be categorized as follows: [N]

  • Pheromone traps, based on chemicals produced by the target species to attract other members of the same species, are species- or genus-specific in most cases and allow the selective collection of the target species. Occasionally species may be controlled effectively by using high densities of traps, particularly in a small or restricted area. Thus, if the pheromone is readily and cheaply available in large amounts, the release of high doses of the pheromone can interfere with mate location and mating. If the air is filled with the pheromone the insects are not able to detect and find a partner. This method is only feasible for small infestations. Generally pheromone traps are more effective when used to monitor the presence or abundance of a species. For instance, traps can be used for early detection of high-risk species. This may enable a rapid response action to attempt eradication or containment. Traps can also be used to monitor the density of pest species, so that when the catches reach a certain threshold other control measures are triggered. The progress of an eradication program can also be followed by monitoring the density (and later the lack) of the target species.
  • Biopesticides are biological pesticides based on beneficial insect and weed pathogens and entomopathogenic (i.e., insect-killing) nematodes. Pathogens used as biopesticides include fungi, bacteria, viruses and protozoa. Produced, formulated and applied in appropriate ways, such biopesticides can provide ecological and effective solutions to pest problems. Most product development to date has been directed towards control of pests having direct economic impact, particularly for the control of pests of agriculture, forestry and horticulture (caterpillars, locusts, various beetles, weeds), medical and nuisance pests (mosquitoes, blackflies and flies). Most types of biopesticides are relatively specific to their target pests, and many are very specific. It is this specificity which makes their use attractive compared to broad-spectrum chemical pesticides. The most widely available and used biopesticides are various formulations of Bacillus thuringiensis (known as ‘Bt'), which can be used to control the larval stages of Lepidoptera (caterpillars), and selected Coleoptera (beetles) and Diptera (e.g. mosquitoes and flies).
  • Entomopathogenic nematodes are increasingly available in specialized niche markets, such as horticulture and are used to kill selected invertebrate pest targets.
  • Fungi for control of specific weeds ("mycoherbicides" or "bioherbicides") have been available for some time, and the development of new ones is increasingly routine (see e.g. International Bioherbicide Group These products are usually host specific either due to the physiology of the fungus, or because of the way they are used. This makes their use attractive in many situations, but also means that the market is small, making them commercially less attractive than traditional herbicides. Nevertheless, a niche market exists, and could be developed to address specific conservation needs to control invasive alien plants, as part of a management program. For example, the development and use of mycoherbicide products to be used for stump painting in the control of plants such as Rhododendron ponticum in Europe is under consideration. Fungi for control of insects is also a relatively new research area, but products are now coming onto the market, notably Green Muscle, a formulation of Metarhizium anisopliae for control of locusts and acridid grasshoppers. [N]
  • Pathogens for control of vertebrates - Not only can pathogens be used as biopesticides but there are also opportunities to use them against vertebrates, e.g. against the brown tree snake playing havoc with Guam's ecology or the release of myxoma virus (myxomatosis) and calicivirus (rabbit haemorrhagic disease) against rabbits in Australia. Snakes differ markedly from birds and mammals in susceptibility to various diseases. Viral or bacterial pathogens capable of killing or weakening only the brown tree snake (and thereby reducing its population) are an attractive objective. Unlike more traditional interventionist techniques, a disease might spread with little human assistance and remain effective for years. Potential pathogens must be carefully screened for risks to other animals and humans. Thus, pathogens, like chemical insecticides require significant preliminary testing and verification prior to use, although these costs might easily be offset by rapid and widespread distribution in the brown tree snake population once released. Controlled and extensive laboratory experiments involving virologists, ecologists, and pathologists are required to test pathogens. Work is underway at the Guam National Zoological Park to determine the susceptibility of brown tree snake to a viral pathogen from zoo disease outbreaks and other sources.
  • Biological control of freshwater and marine targets The opportunity to use biological control against plants, invertebrates and vertebrates are described above. Classical biological control against water weeds has been particularly promising and has produced several success stories. No biological control project has being attempted against a marine invader to date, though studies on the suitability of several parasites against different organisms are underway, e.g. specific parasitic castrators of crabs.
  • Biological control of plant diseases. Biological control of plant diseases is still a young science. Many plant pathogens colonize parts of the plant that are initially free of microorganisms. Successful biological control in such circumstances depends on rapidly colonising these plant areas with non-pathogenic antagonists competing for the space. The principal antagonists used are saprotrophic fungi and antibiotic-producing bacteria. The biological control agent will ideally outcompete the pathogen. This concept is altogether a rather different approach than the biological control projects against weeds, invertebrates and vertebrates. In some cases less virulent strains of the same pathogen species can be used to replace the virulent strain physically or by transmission of the traits of the less virulent strain to the virulent one.

Biocontrol Resources on the Web

Information on biocontrols for various weeds can be found in Biological Control of Weeds in the West - Bibliography or at commercial weed biocontrol insectaries. Cornell's Invasive Plant website on Biological Control of Non-Indigenous Species is dedicated to promoting and educating people about the biological control of non-indigenous plant species. NYSDOT has been very supportive of Cornell's efforts.

The Global Invasive Species Program suggests the following sites:

Other resources include the Corps of Engineers Noxious and Nuisance Plant Management Information System, which covers:


9.6.7 Cultural Controls
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Cultural control methods introduce and manage desirable plants and covers to control invasive species and other undesirable plants. Many native plants are poor competitors in their early stages of growth, but once established they crowd out most other plants with minimum management. Prevention measures may also be considered cultural controls. Nine DOTs (23% of respondents) say they are using cultural control methods, but the percentage may be considered higher when altered DOT processes and procedures are considered. [N]

Cultural controls include land management practices that inhibit weed growth and prevent conditions that lead to weed establishment. For instance, planting and maintenance activities can be modified to reduce weed infestations. Attention to suitable seedbed preparation and proper fertilization can help prevent colonization by weeds. Planting in narrow rows reduces bare ground and increases the shaded area between rows, further decreasing potential weed habitat. Many agencies are using native grasses to control invasive species, since their dense, deep root systems inhibit weed growth. Both Minnesota and Iowa DOTs have found success in controlling invasive species, and Canada thistle (Cirsium arvense) in particular in Minnesota, using native grass stands. [N]

Review and Pretreatment of Construction and Materials Sites

Pre-construction planning and treatment to reduce infestations of invasives is on the upswing. Nine DOTs (23% of respondents) said that all construction sites are reviewed for invasive species before opening. [N] Wyoming DOT is among five that are working at pretreating construction zones prior to soil disturbance. [N]

WYDOT is trying to develop a more aggressive pit assessment and treatment program with local Weed & Pest districts. These sources have been identified as having the greatest potential for spreading invasives. At Nevada DOT a staff biologist surveys material source sites for invasives. When invasives are found, a management plan is developed in cooperation with the local BLM weed coordinator or botanist. Environmental Services is developing standard noxious weed control BMP specifications for weed management on material sites and project sites. Eight other DOTs (23% of respondents) also ensure dirt and gravel sources are evaluated. [N]

In addition: [N]

  • Over a third of DOTs (17 total) specify weed-free mulches on all projects. Inert mulch products such as straw or wood fiber are used in sensitive areas by 11 DOTs (28% of respondents).
  • Twenty-one state DOTs (53% of respondents) specify on project plans and bid contracts that seed and sod sources must be free of invasive species and/or weeds.

Possible solutions to the problem of weeds that are introduced by animal feed or mulches contaminated with weed seed are discussed in Certified Weed Free Forage: An Emerging Program for Western States, by UC-Davis.

Protection of Native Populations

Intact communities of native species both suppress invasives and shelter rare species. Thirty-eight percent (38%) of responding DOTs (15 states) are now screening for the presence of rare plant communities in the work zone or ROW. [N] In 13 states, areas in need of special management are identified by resource agencies or state Natural Heritage Program. North Carolina DOT, like many others, works cooperatively with their state Department of Environment and Natural Resources, in-house Botanists, and others to identify and protect state and federally endangered plant and animal species found on our rights-of-way. Oregon DOT has special management areas marked with signs that indicated the types and timing of maintenance techniques that are to be performed. Since Wisconsin DOT (WisDOT) hires County Highway Departments to perform highway maintenance, WisDOT has let several contracts to manage/restore high quality remnant plant communities discovered on the rights-of-way. Management plans are often developed to inventory and ensure appropriate management of special areas. Ten DOTs (25% of respondents) have taken it upon themselves to map and track protected communities on the DOT ROW, statewide. [N]

Staff at 14 DOTs (35% of respondents) identify special management areas, which are managed accordingly by maintenance forces. [N] As Illinois DOT described, commitments are added to plans and DOT staff are trained on how to maintain these areas through mowing, herbicide applications and burning. Protected species are identified and restricted maintenance practices are incorporated to protect the endangered species; construction practices are also altered to minimize disturbances of native plant communities whenever possible (NV). In several states, from Wisconsin to New York, special vegetation management programs have been developed to protect Karner Blue Butterfly habitat, including mowing date restrictions and native blue lupine and butterfly weed restoration planting and seeding. The Louisiana Department of Transportation Development (LADOTD) has taken high value remnant strips in several districts and moved them just beyond the ROW, with the consent of the owners of that property and discussions regarding proper management. If moving them is not deemed an option, LADOTD marks the site to prevent herbicide applications or mowing at the wrong time of year.

In most cases, DOTs are taking these conservation measures without knowing the total acreage of high quality forest, wetland, or native grassland remnants they have or are protecting in the ROW. Just 15 percent of those responding (6 states) could provide such an estimate, if asked. More than a third of DOTs (43% of respondents - 17 states) identify native/rare plant communities in EAs and EISs. [N] In fact, Hawaii DOT indicated that most of their protected areas have been identified as a result of EAs, EISs, and Special Management Areas.

Vehicle Cleaning Practices

Vehicle and equipment cleaning procedures and practices are typically used to minimize or eliminate the discharge of pollutants from vehicle and equipment cleaning operations to storm drain systems or watercourses, and to minimize transport of invasive species. Twenty-three percent of responding DOTs (9 DOTs) say they ensure vehicles are washed before and after use, to control the spread of invasive species. [N] Caltrans is among the DOTs that have developed an extensive set of construction vehicle cleaning environmental stewardship practices. [N] These are described in Chapter 4 of the Compendium with Construction Practices.

Soil Management on Construction Sites

Just 12 state DOTs (24 percent) say they stockpile desirable/uncontaminated topsoil to facilitate revegetation. Topsoil has become a commodity that contractors often strip and sell. Soils management on construction sites is an important factor in invasive species control. In addition to removing native cover that may effectively compete with invasive species and opening new pathways for infestation, the change in soil characteristics that may occur with loss of topsoil can also bolster invasives. One Florida study on roadside soils and invasive species suggested that disturbance alone may not increase the presence invasive species, but that a change in soil characteristics through the addition of clay and limerock from road construction may enhance invasion. [N]

Claassen et al. have performed a large percentage of the studies on topsoil usage and compost, with funding from FHWA and Caltrans. Among their many helpful recommendations are for DOTs to test nitrogen content in soils and ensure slow-release of amendments over three years; to use compost over use of commercial fertilizer; and to ensure that compost amendments have adequately decomposed. Where topsoil is not available other amendments can be used, the quantity and quality of the Nitrogen (N) materials applied is critical. The N release should be slow enough to keep plant-available N at modest levels, but the total amount of N amended should be high enough so that the site does not run out of N before the plant community is well established. The N amendment should be able to support three to five years' plant growth, for example. Controlled release of N is important because excessive N availability promotes weedy annual grass growth, drying out the site and crowding slow growing perennials. While the maintenance of moderate, sustained nitrogen levels may be achieved from commercial, slow release fertilizer sources, the inclusion of organic matter in the amendment is also important to improve the hard setting and poor water holding capacity of low organic content materials. [N] Biomass associated with compost has been more effective than N amendments that were evenly disturbed throughout the profile (0-30 cm) or applied deeply within the profile (20-30 cm). [N] Studies of plant communities established on "problem soils" amended with commercial fertilizers have shown vigorous initial growth, but that vegetative cover often becomes sparse or nonexistent within several years. [N] In addition to transportation related studies, those of fertilized mine reclamation spoil observed that revegetated areas tended to be highly productive for two to five years followed by a sharp decline in plant growth and nutrient availability. [N] Reapplication of topsoil to subsurface materials enhanced reestablishment of vegetation by increasing nutrient availability, water holding capacity, and microbial activity. [N] Compost can be used to replace the organic matter and nutrients and can act as a surface mulch to protect against erosion, extreme temperatures, and droughtiness. [N]

Long-term nitrogen release rates from most yard waste compost materials approached the N release rates of moderately fertile soils. Composts were shown to be able to regenerate the N availability characteristics of low-nutrient substrates that have been stripped of topsoil organic matter. Well-cured composts and co-composts (biosolids blends) approached the N release rates of highly fertile soils. Compost application provides longer N release duration compared to chemical fertilizer and also provides organic materials for improved infiltration and microbial activity. Potential compost sources and soils at the site should be analyzed before amendment, as compost products and the soils that are to be revegetated vary in fertility and water availability.

Shoulder Grading

DOTs are teaching maintenance staff that shoulder grading is not acceptable as a method of vegetation or invasive species control. Shoulder grading is only promoted as a means of refining lateral support for the road.


9.6.8 Revegetation through Restoration of Native Species
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According to NCHRP 20-5, 33-04 on IRVM, on average, 58 percent of DOTs' newly planted acreage requires no significant maintenance work on a perpetual basis; 23 percent indicted that less than 20 percent of the newly planted acreage requires significant maintenance work on a perpetual basis. Around a quarter of responding state DOTs were aiming for 90 to 100 percent of planted acreage requiring no significant maintenance work on a perpetual basis. [N]

Most weeds are opportunistic and become established following a disturbance or opening in the canopy. Thus, it is very important to cover the soil with dense, vigorous vegetation to prevent the establishment of invasive species. Since weed treatments often disturb the soil, open the canopy, or leave bare soil, revegetation after treatments is crucial in suppressing weed seedlings and preventing weed infestations from re-occurring. Although restoration efforts have certain elements in common, each invasion and area is unique. Revegetation/restoration projects need to be based both on general principles and site-specific considerations and analysis. Monitoring programs track the success of control and restoration efforts and to ensure that the area is not reinvaded.

Restoration is an integral component of comprehensive prevention and control programs for invasive species that may keep invasive species from causing greater environmental disturbances. Revegetation with native species provides better adaptation to the site and more natural appearance than introduced species, which would have the potential to escape into the natural environment. An earlier Federal Executive Memorandum E.M.on beneficial landscaping became effective in April 1994, encouraging the use of native plants as much as practicable on all federal lands and in all federally-funded projects. In 2000, this E.M. was incorporated into Executive Order 13148, on the Greening of Government. That E.O. also required agencies to purchase "environmentally preferable and recycled content products, including compost and mulch that contribute to environmentally and economically beneficial practices." [N]

Revegetation with native species is strongly encouraged federally as well. Federal agencies are directed or strongly encouraged to use native species by various Executive and Administrative Orders. These orders do not, as yet, specify sources; however, species collected near a disturbance tend to be more biologically suited for revegetating the site. Perhaps surprisingly, NCHRP 20-5, 33-04, reports that DOTs are averaging only 45 percent of use of native grasses for revegetation on projects, though this ranges to a high of 90-100 percent in a few states. [N]

Revegetation with native species provides the following advantages:

  • They are better adapted and appear more natural than introduced species.
  • Introduced species have the potential to escape into the natural environment.

Some DOTs are undertaking research to identify what vegetation establishment methods work best in their states and ecoregions. Studies on compost usage are occurring in the west and mid-west, from Texas in the south to Idaho in the north. The Nebraska Department of Roads has research results due in late 2006, which will be used to develop technical guidelines for vegetation establishment on roadway shoulders. The project is examining the interaction effects of seed priming, type of mulch, and level of irrigation on soil movement and establishment of the short grass mixture on the foreslope of roadway shoulders; and the interaction effects of composted manure applications and a 6-12 inch compacted buffer strip between the paved shoulder and the seedbed on soil movement and establishment of the short grass mixture on the foreslope of the roadway. Delaware DOT is publishing a vegetation management manual, in conjunction with the agency's tree preservation policy for a Livable Delaware. [N]

DelDOT-Livable Delaware Program to Revegetate with Natives

DelDOT's Roadside Environment program has undertaken to improve the appearance of Delaware roadsides by using landscape enhancements that include native plants that are adapted to the region, displace invasive species that are highly competitive and detrimental to most plant species, and to do so in a manner that is cost effective and does not result in more maintenance but will require the same level of maintenance or will reduce the frequency of maintenance operations.

While DelDOT made a commitment to improve the aesthetics of the roadsides of Delaware under the leadership of their former Secretary of Transportation and Roadside Environmental Administrator, DelDOT discovered that colorful mass plantings of annual wildflowers can be costly for labor and soil preparation and are sometimes lost due to highly competitive weed situations. As a result, DelDOT undertook a study, "Enhancing Delaware Highways" which recommended roadside trials of native plants to evaluate aesthetics, compatibility with existing vegetation, costs associated with installation and maintenance. DelDOT funded a 5-year initiative through 2006 with the University of Delaware Transportation Institute to determine how native vegetation alone and in combination with existing plant communities on DelDOT rights-of-way can provide aesthetically pleasing surroundings in a cost effective manner for motorists traveling the highways of the First State.

The effort is unique among DOTs in the extent to which it is examining use of trees and tree preservation within the right-of-way, to supplement the use of native grasses, wildflowers, and shrubs. The cost of maintenance is being estimated for any landscape enhancement proposed by the project team for large-scale implementation, along with a determination whether those costs will be offset by lower frequency of maintenance over a five-year period.

Iowa DOT Revegetation Program

Iowa DOT and many Iowa counties have shifted from traditional roadside maintenance of a monoculture of exotic grass in favor of management regimes which restore native vegetation and reduce the use of herbicides and mowing. Iowa DOT has identified maintaining a healthy stand of native grasses as the best way to control invasive weeds. These grasses have extensive roots that offer the toughest competition to Canada thistle. In addition, plant diversity along the roadsides creates a strong plant community. Prairie plants can adapt to a wide range of soil types, moisture levels, and climactic conditions. Most prairie grasses and wildflowers grow best during hot, dry summer months, providing excellent erosion control during the fall and spring.

Iowa DOT recently extended their landmark IRVM program to revegetate approximately 5,200 acres of roadside annually with native grasses and forbs. Forty percent of that acreage is restoration unrelated to construction. [N] The program is documenting species diversity and wildlife benefits as well. Twelve roadside areas were surveyed for abundance and species richness of disturbance-tolerant and habitat-sensitive butterflies and compared with nearby roadside dominated by primarily nonnative legumes and/or grasses; species richness of habitat-sensitive butterflies showed a two-fold increase on restored roadsides compared with grassy and weedy roadsides. Abundance increased five-fold for native grass and forb habitat over nonnative. Tracking studies found butterflies were less likely to exit the restored roadsides, indicating mortality rates may be lower and offering preliminary evidence that roadsides have the potential to be used as butterfly corridors. [N]

Iowa DOT recently released a roadside management guide containing collections of plant profiles, characteristics, requirements, and how theses species are used in roadside management.

Illinois DOT Enhancement and Maintenance Projects Restore Prairie, Native Wildflowers

Illinois DOT established the "Wildflowers of Illinois" program utilizing existing roadside enhancement and maintenance funding to plant native wildflowers and prairie plants in place of manicured turf along roadsides. Plant materials and labor will be contributed to the program by the Illinois Department of Natural Resources and the Illinois Department of Corrections. Illinois and other vendors will supply the balance of materials needed for successful planting and establishment of the gateways, which will be funded by existing roadside maintenance budgets. As part of the Governor's overall environmental emphasis and with support from the state's first lady, Illinois DOT anticipates that the program will foster economic development and tourism, promote responsible stewardship, encourage environmental understanding and reduce roadside maintenance costs.

Wildflowers for Communities involved various communities throughout Illinois in 2004 and beyond. After signing an agreement with the department to participate in the program, each community will select locations along state highways within their communities, and develop a plan for the establishment of the wildflowers with the assistance and approval of department Landscape Architects. The communities will then install the plantings with contractors, their own employees, or community volunteers such as Master Gardeners. Watering, weeding and other similar cultural needs will be arranged by the communities, usually employing similar resources. The agreement provides for an initial grant from the department of up to $35,000 with a $5,000 local match per community. The local portion may be a cash outlay or in-kind services. The agreements provide for two years maintenance by the communities with a reimbursement of $5,000 per year for that work. [N]

TxDOT Pilot on Context Sensitive and Natural Landscape Design in the Highway Right-of-Way

The purpose of TxDOT's pilot on context sensitive and natural landscape design in the highway right-of-way was to recreate the visual character of the regional native landscape and develop self-sustaining vegetation community groups that recycle nutrients, conserve soil moisture, regenerate themselves, and provide habitat for nesting birds. Their process included 1) identifying the environmental impacts of the highway on this site, 2) identifying the appropriate natural systems processes most suitable to solving these problems, and 3) gaining input and support from the community in developing design alternatives. Management needs placed a heavy emphasis on the reduction of maintenance while developing a publicly acceptable landscape aesthetic, and interviews with maintenance personnel provided the basis for the design program. Maintenance staff identified three problem areas:

  • A large amount of hand maintenance was required around the guardrails, bridge columns and areas that equipment could not access.
  • Steep slopes in parts of the project were difficult to mow without causing damage that would lead to erosion.
  • Grass and weeds in the detention ponds were considered to be difficult to mow due to moist conditions in the ponds.

Based on these observations, the first three goals of the design plan were established as:

  • Eliminate need for hand maintenance wherever possible, especially near travel lanes;
  • Prevent erosion on slopes; and
  • Improve the appearance and maintainability of the detention ponds.

This approach offered a design solution meeting specific goals regarding water quality and habitat in an urban area and while demonstrating visual acceptance by the public. The project elevated habitat, native plants, water quality, erosion control, reduced herbicide usage, and reduced mowing in TxDOT's design approach for roadside improvement projects. [N]

TxDOT and Houston Green Ribbon Program

In the past four years the Houston District has improved and removed from TxDOT maintenance more than 100 acres of right-of-way through agreements with partners or landscape planting. Approximately 200,000 trees, shrubs, and vines have been installed on state right-of-way in the same period without increasing maintenance activities, as part of implementation of Houston's Green Ribbon Project Corridor Aesthetics and Landscape Master Plan, released in December 1999. Proudly Called the Bayou City, Houston is naturally laced with attractive green belts and waterways now obscured by highway overpasses. The goals of the Green Ribbon Project are to:

  • Establish a higher level of visual appeal along the corridors through landscape and architectural improvements (aesthetics);
  • Promote and enhance highway safety and maintain traffic flows (mobility);
  • Promote fiscal responsibility in capital investments and reduce maintenance costs by the use of sustainable plantings, including the use of native trees, shrubs and grasses (sustainability);
  • Reduce implementation and maintenance costs through the design of sustainable landscapes (sensibility);
  • Promote public/private partnerships for implementation and maintenance of improvements (partnerships);
  • Develop unifying themes through the use of art and neighborhood gateway markers to express the cultural uniqueness of adjacent neighborhoods (expression);
  • Develop functional and innovative design solutions for architectural elements, including bridge components, walls, railings, barriers, sign supports, and lighting (innovation); and
  • Integrate civic art of any material or medium that is permanent in nature and integral to the environment in which it is placed (artistic expression).

Since the establishment of the GRP, the impact to the freeways has been dramatic. The Green Ribbon Project routinely oversees the planting of literally thousands of native trees in intersections, hundreds of oleanders, crepe myrtles, and palm trees, as well as, the installation of irrigation systems. Over 1,200 plants were installed at one freeway intersection, including some 80 palm trees, to emphasize the freeways Gulf connections. The Houston District has moved to a 100 percent naturally derived non-chemical landscape development and is currently working to reduce our chemical usage for herbicide control, as well.

In 2001, the Texas Legislature added Rider 57 to TxDOT's appropriation, requiring TxDOT to expand concepts from the successful program to other areas of the state. The guiding concepts or principles for the program are:
Five design principles guide the Green Ribbon Project. These are:

  • Green First - make new plantings or the preservation of existing plantings the first priority in recommended improvements;
  • Integration - consider all improvements in context with each other and design solutions to emphasize the visual, as well as the physical, integration of all components;
  • Continuity - design all improvements to create a continuous appearance;
  • Freeways are Public Space - the freeway rights-of-way belong to the public and should provide a visually pleasing experience; and
  • Maintenance - the planning and implementation of all improvements should include long-term maintenance costs with respect to plantings, structures, surface treatment and other materials along roadways.

TxDOT and the state legislature tied GRP improvements to air quality and CMAQ funds when measures when funding for landscaping and other enhancement activities occur in districts that are non-attainment and near non-attainment counties for air quality degradation. TxDOT's Design Division (DES) - Landscape Design Section now oversees the Green Ribbon Landscape Improvement Program so the GRP will continue to make a positive impact on TxDOT. The GRP program manager has facilitated integration of GRP principles into the project development process. TxDOT would not have the financial resources to implement all of the proposed design concepts and in response, the GRP manager has facilitated successful public/private partnerships with local governments in the six-county area - in Baytown, La Porte, Clute, Freeport.

In 1999, the project was recognized with the Highest Honor Award, the American Planning Association, Houston District, for its strategic planning effort. The Texas Forest Service gave the Texas Community Forestry Award of Merit to the project in 1999. The Park People, a Houston civic group, awarded the project its Visionary Award for 2000. Trees for Houston, another civic group, awarded the project its Arbor Day 2000 Award and the American Society of Landscape Architects-Texas Chapter honored the project with its Merit Award for 2000. In 2001, the Green Ribbon Project won the National Arbor Day Foundation's Lady Bird Johnson Award. It is awarded by the Foundation for individuals and organizations whose work sets a worthy example for others to follow in roadside beautification. [N]

FHWA Recommendations for Designing with Native Vegetation

FHWA recommends the following considerations and specifications in designing with native vegetation. [N]

  • Use natural region maps commonly available from the State's Natural Heritage Program instead of cold hardiness zones when designing with native plants.Visit native plant preserves that can serve as references for plantings. The State Natural Heritage Program can recommend sites.
  • Use seed mixes specific to the different conditions on the site. Dry conditions may be present on sandy slopes or forest edges and wet conditions in ditches, requiring different or adjusted mixes in these areas.
  • Eradicate invasives from planting site before planting.
  • Consider a line item for the contractor to control invasives and clean equipment.
  • Plant as much diversity as possible, unless an adjacent native seed source exists.
  • Match site microclimates with distinct seed mixes as much as practical.
  • Specify a locally-grown or collected source if possible. Most native species will establish more easily if locally grown or collected.
  • Order native seed when the contract is let to prevent unwanted substitutions.
  • Limit bids to experienced contractors and approved vendors for these projects.
  • Separate the planting contract from the general contract for best timing.
  • Extend the establishment period to three years.
  • Learn appropriate seed test criteria and seeding rates to avoid waste.
  • Plan for seed collection and plant salvage if native remnants will be disturbed by the project
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Continue to Section 9.7 »
Table of Contents
Chapter 9
Roadside Vegetation Management
9.0 Introduction
9.1 Inventory of and Management for Rare Species and Sensitive Resources in the ROW
9.2 Growing Threats Drive Expansion of DOT Invasive Species Practice
9.3 Practices for Prevention of Roadside Infestations
9.4 Statewide Inventory of Invasive or Noxious Species in the ROW and Update of Databases
9.5 Planning for Invasives Control
9.6 Roadside Vegetation Control Methods and Resources
9.7 Management of Visual Quality of the Roadside
9.8 Staffing, Training, & Partnerships
Lists: Examples | Tables | Figures
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