The rights-of-way that border the nation’s roads include more than 12 million acres of land, and road maintenance crews are on the front line in managing the land and responding to and preventing invasive species infestations. Control of invasive plant species along America’s roadsides is an increasing concern for transportation agencies nationwide.

The Federal Highway Administration (FHWA) described the unique issues transportation agencies face in addressing invasive species in 1999 guidance:

“Nonnative flora and fauna can cause significant changes to ecosystems, upset the ecological balance, and cause economic harm to our Nation’s agricultural and recreational sectors. For example, introduced plants, such as Kudzu in the southeastern States and purple loosestrife throughout the country, have choked out native plant species and consequently have altered wildlife and fish habitat. Transportation systems can facilitate the spread of plant and animal species outside their natural range, both domestically and internationally. Those species that are likely to harm the environment, human health, or economy are of particular concern.”

“Highway corridors provide opportunities for the movement of invasive species through the landscape. Invasive plant or animal species can move on vehicles and in the loads they carry. Invasive plants can be moved from site to site during spraying and mowing operations. Weed seed can be inadvertently introduced into the corridor during construction on equipment and through the use of mulch, imported soil or gravel, and sod. Some invasive plant species might be deliberately planted in erosion control, landscape, or wildflower projects. Millions of miles of highway rights-of-ways traverse public and private lands. Many of these adjacent lands have weed problems and the highway rights-of-way provide corridors for further spread.”

Laws, Regulations and Policies

Federal and state transportation agencies have been working for years to implement numerous laws and policies aimed at effective vegetation management, promotion of native plants and wildflowers, and control of invasive species in the nation’s transportation corridors.

The Surface Transportation and Uniform Relocation Assistance Act (1987) established mandatory requirements that native wildflowers be planted as part of landscaping projects on the Federal-aid highway system.

The Executive Memorandum on Beneficial Landscaping (1994) encouraged use of native plants as much as possible on all Federal lands and in all federally funded projects.

Executive Order 13112 on Invasive Species (1999) was adopted to encourage government agencies to prevent and control invasive species and to plant native species. The order set up a National Invasive Species Council, which was charged with producing a National Invasive Species Management Plan. The plan, completed in 2001, provides a blueprint for Federal agencies’ implementation of the executive order. In 1999, FHWA’s Guidance Implementing Executive Order on Invasive Species provided further direction for transportation agencies. The order was amended in 2016.

The Noxious Weed Control and Eradication Act (2004) requires the Secretary of Agriculture to develop a program that will provide financial and technical assistance to weed management entities to control or eradicate noxious weeds on public and private lands. Transportation agencies partner with many of these organizations.

The Safe, Accountable, Flexible, Efficient Transportation Equity Act for the 21st Century (SAFETEA-LU, Pub. L. No. 109-42, 2005) was enacted, including provisions to address invasive species and noxious weeds in transportation projects. Specifically, Section 6006 of the law extends eligibility of funds under the National Highway System and the Surface Transportation Program to include control of noxious weeds noxious weeds and establishment of native species in transportation projects.

FHWA Guidance

On May 16, 2006, FHWA issued Guidance on 23 U.S.C. §329 on the Control of Noxious Weeds and Aquatic Noxious Weeds and Establishment of Native Species, to implement SAFETEA-LU Section 6006. This guidance describes the activities eligible under the establishment and management of plants categories in the law. Transportation funds can be spent on the following eligible activities:

    • Right-of-way (ROW) surveys to determine management requirements to control Federal or state noxious weeds:
    • Establishment of plants, whether native or nonnative, with a preference for native to the maximum extent possible;
    • Control or elimination of plants (noxious weeds);
    • Elimination of plants to create fuel breaks for the prevention and control of wildfires; and
    • Training.

FHWA also provided specific direction to transportation agencies in its 1999 Guidance Implementing Executive Order on Invasive Species.

The guidance requires National Environmental Policy Act (NEPA) documents to make a determination of the likelihood that projects may introduce or spread invasive species and then describe measures that will be taken to avoid and minimize potential harm from conception through construction. In addition, FHWA encourages right-of-way inventories of vegetation, which can be used to identify rare species and natural habitats to be preserved as well as infestations of invasive species that need to be eradicated, along with tailored vegetation management plans. FHWA also recommends that roadside maintenance programs be given the necessary support to control and prevent invasive species. FHWA also encourages the selection of construction and landscaping techniques and equipment that will contribute to accomplishing the intent of the Executive Order.

Pollinator Health

In 2014, a Presidential Memorandum, Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators, directed federal agencies to take additional steps to improve habitat for pollinators including honey bees, native bees, birds, bats, and butterflies. In response, FHWA has provided a range of resources to support state and local transportation agencies in managing roadsides in a manner that can create important habitat corridors to link potentially fragmented pollinator habitat.

The memorandum directs the U.S. Department of Transportation to identify opportunities to increase pollinator habitat along roadways, and to work with state DOTs and transportation practitioners to promote pollinator-friendly practices. In support of the memo, FHWA said it will continue to support native plantings and integrated vegetation management (IVM) practices to reduce pesticide use and mowing, and increase native plantings. According to FHWA, “many States already follow an IVM approach that combines multiple forms of control (mechanical, chemical, and biological). An IVM plan can result in reduced long-term cost and increased safety while promoting native plantings that encourage ecological diversity.”

More information and resources on pollinator health for transportation agencies is available on the FHWA Pollinators website.

Vegetation Management and Control of Invasives

E.O. 13112 mandated a risk-based approach, including consideration of the likelihood that an invasive species will establish and spread, as well as the degree of harm it could cause. To do this, it helps to start with a master list of the transportation agency’s work activities that may inadvertently promote the spread of invasive species; a risk assessment identifies the most significant issues and helps decide where to focus. Construction projects, transportation systems, spraying and mowing operations can inadvertently spread invasives. Some common mechanisms may include:

    • Use of forage mulches that have not been certified weed-seed free mulches and other erosion control products.
    • Planting of species now controlled as invasive for erosion control purposes, including aggressive sweet clovers, alfalfa, smooth brome, trefoil, and perennial rye.
    • Allowing invasive species to exist on projects’ rights-of-way. Failure to control an invasive species allows it to spread on and off the right-of-way.
    • Placement of spoil or importation of topsoil contaminated with invasive species, such as ragweed, thistles, and sweet clovers.
    • Ill-timed maintenance disturbances like blading, mowing, ditch dredging, and bare-grounding, which have been known to increase invasive species.
    • Indirect mechanisms such as drainage flows, wind, vehicles, people, and wildlife.
    • Movement of construction equipment from a weedy site to a non weedy site, which can transport undesirable seeds.

Best management practices (BMPs) are being used to reduce the introduction or spread of invasive species.

Risk Management, Prevention, and Inventory

The first line of defense and the most cost-effective strategy against invasive species is preventing them from invading and becoming established in the first place. Once an infestation becomes well-established, management is expensive and eradication is very difficult. Detection is one of the most important components of prevention of spreading invasive species. The second most important is eradication before they produce seed or develop an established root system. Transportation agencies and other agencies use a variety of risk management, business aspect identification, species and action prioritization, and mapping techniques to get ahead of invasive species and manage roadside vegetation as efficiently as possible.

Early detection and eradication are often performed where species can most easily gain a foothold (such as construction sites) or cause the most damage (intact natural areas).

Early detection consists of identifying and documenting the newly introduced invasive weed species in an area. Then, rapid response may be employed to eradicate new infestations and methods may be taken to prevent movement to non-infested areas. Early detection of incipient invasions and quick coordinated responses are needed to eradicate or contain invasive species before they become too widespread and control becomes technically and/or financially impossible. Populations that are not addressed early may require costly ongoing control efforts. For example, spotted knapweed (Centaurea biebersteinii) was introduced to Montana in the 1920s, and by 1988, had infested more than 4.7 million acres. The economic impact is approximately $42 million annually.

About a quarter of transportation agencies surveyed as part of NCHRP Synthesis 363: Control of Invasive Species have undertaken a statewide inventory of at least one invasive species in the ROW; however, nearly a third of responding DOTs say they do not plan to implement a statewide survey in the future due to concerns about cost. DOT roadside invasive species inventories are all used to identify and locate areas for treatment, invasion by new species, and to set priorities. DOTs also indicated that inventories are being used to:

    • Partner with other agencies in providing funding for control of specific species.
    • Estimate expansion of weed presence and monitor treatment results and acres infested.
    • Guide and evaluate invasive species control efforts.
    • Guide effort/budgeting to meet established goals.

Prevention involves attention to the most common means of transmission, including contaminated seed, mulch, or soils; movement of unlearned equipment or machinery from an invasive weed-contaminated area to a non-contaminated area; and lack of restoration or revegetation after construction. Prevention depends upon limiting the introduction of new weeds through:

    • Minimizing the disturbance of desirable plants and soils.
    • Maintaining desired plant communities through good management.
    • Monitoring high-risk areas such as transportation corridors and bare ground.
    • Revegetating disturbed sites with desired plants, such as plants from local natural areas.
    • Evaluating the effectiveness of prevention efforts and adapting plans for the following year.
    • Early detection and eradication of small patches of weeds through regulatory inventory and corrective action.

A risk assessment process assesses species based on their perceived risk and potential impact. The objective of assessment is to predict whether or not a species is likely to become established and be invasive and to generate a relative ranking of risk. Processes for setting priorities often incorporate risk assessment.

Monitoring the numbers of a pest species killed or removed is a measure of the work being done but is not a measure of invasive species control. Success of an invasive species control project can be measured by monitoring numbers of the pest species that remain, and ultimately the condition of the ecosystem they are in. Removing an invasive alien species from an ecosystem will not automatically lead to the return of the indigenous flora and fauna. While this is often the case, removal of one alien species may simply open the way for colonization by another. Monitoring of the impact of control actions needs to be put in place, preferably starting with small-scale activities to verify the impact of control operations, and if the results are not as expected, the management plan may need to be reconsidered and adapted. This may require additional flexibility in vegetation contracts.

Integrated Roadside Vegetation Management

Transportation agencies are taking a variety of steps to plan for vegetation management and share information across division areas and professional specialties, to address cross-cutting needs, and to take a more integrated approach to invasive species control. Information is often exchanged informally. Chief among the more formal approaches are development of organization-wide and district-specific integrated vegetation management plans. Geographic Information Systems enable the locations of weed patches to be stored digitally, and allow treatments to be tracked, automatically administered in some cases, and assessed over time.

Integrated Roadside Vegetation Management (IRVM) or Integrated Vegetation Management (IVM) encourages stable self-sustaining vegetation with limited use of mowing, herbicides, tree removal, and other methods as necessary. Because no single tactic can solve a current weed problem or prevent future infestations, IVM encourages managers to combine several treatment methods into an integrated weed management program tailored to the site and resources available. In addition, such a program fosters communication and cooperation among the many individuals and agencies involved in right-of-way, construction, and vegetation management. Integrated methods focus on the ultimate goal, which for transportation agencies may be preservation (or increase) of indigenous biodiversity as well as management of overall maintenance costs. Planning helps achieve these ends and considers the range of control options.

The process of integrated control can be complicated, involving several different tactics in combination or in sequence, or it may involve one method. For example, cutting is frequently combined with chemical control to the stump, for woody invaders. Control of purple loosestrife (Lythrum salicaria) may involve biological control, mechanical removal, and other methods. Consideration of the environmental impacts of control actions requires that environmentally sound methods be available and judiciously deployed, especially in highly vulnerable areas. IVM has been described as a decision-making and management process that uses knowledge from a broad base of expertise, a combination of treatment methods, and a monitoring and evaluation system to achieve vegetation management goals. Common steps in an integrated vegetation management plan are:

    • Understand pest and ecosystem dynamics.
    • Assess situation and management controls.
    • Identify the species of highest priority for a management program.
    • Set management objectives and tolerance levels.
    • Compile treatment options, including detailed information on methods for eradication, containment, control, and mitigation.
    • Account for economic and environmental effects of treatments.
    • Develop site-specific treatment plans.
    • Secure resources and implement training.
    • Monitor outcomes and revise and adapt management plans.

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. 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 such as bull-dozing or a broad-spectrum herbicide is acceptable. In less disturbed areas, in particular nature reserves, for example, the use of a species-specific method is highly recommended. Note that consideration needs to be given to pollinators because some species depend on specific pollinators for reproduction. Bulldozing could be a real problem for native ground-nesting bees for example.

In choosing a management strategy, transportation agencies 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.

More information on control methods and management strategies used by transportation agencies may be found in NCHRP Synthesis 363: Control of Invasive Species andAASHTO’s Compendium of Environmental Stewardship Practices, Policies, and Procedures.


Nearly 85 percent of transportation agencies (34 states) responding to a research survey performed as part of NCHRP Synthesis 363: Control of Invasive Species said they are working with others outside the agency to identify existing or emerging populations of invasives. Just 10 percent said they were not. State transportation agencies are working with Weed Management Areas, regional associations and councils, other federal and state land management agencies, and entities that may be able to provide technical support or concrete assistance like agriculture departments, the U.S. Geological Survey, and various non-governmental organizations (NGOs). The most common transportation agency partnerships for invasive species control are DOT participation on State Invasive Species Councils or state noxious weed committees. Some transportation agencies work with another agency to have them review and treat the ROW, often the state Department of Agriculture and local weed and pest districts.

Transportation agencies also turn to nongovernmental organizations when they need assistance. Links to Professional & Non-profit Organizations with an interest in invasive species control are available on-line. State Natural Heritage Programs are available through NatureServe’s website. A number of innovative partnerships have been developed. Some of the most well-known are those with The Nature Conservancy. New York State’s Adirondack Park Invasive Plant Prevention Program is one such example.

Additional examples may be found in NCHRP Synthesis 363: Control of Invasive Species.