Threat Assessment for Japanese Knotweed

Focus Area

Invasive Species/Vegetation Management


Natural Resources







Research Idea Scope

We wish to ascertain the impact of Japanese knotweed on the stability and lifespan of roads, bridges, and other infrastructure near waterways. Japanese knotweed is one of the world’s worst invasives, and it is accepted knowledge among a wide range of invasive species experts that Japanese knotweed contributes to erosion and flood damage. However, there is very little data to support this assertion. Work in Vermont following Tropical Storm Irene (TSI) found that 70% of new plants established in the wake of the storm originated from underground rhizomes. As Japanese knotweed propagates vegetatively, rather than by seed, this result suggests the spread of rhizomes due to erosion appears to be a major cause of its invasiveness. Additional data were collected on a limited number of sites over the course of eight weeks to further investigate these observations. Analyses of the results suggested forested streambanks were significantly less eroded than bare soil or knotweed invaded streambanks. The bare soil and knotweed invaded streambanks were nearly significantly different (Mann–Whitney U = 2051.00, n1 = n2 = 70, P =0.096, two-tailed), with mean total erosion, and erosion rates, higher on knotweed banks. Since this work was conducted, at least two papers have been published that suggest Japanese knotweed increases erosion rates along waterways, in some cases by a factor of 10. Looking at these results, we see this plant as posing a significant and unacknowledged threat to streambank stability. The resulting impact of this plant on transportation infrastructure is hypothesized to include the sudden loss of supporting soils along waterways, leading to the swift failure of any nearby infrastructure. Japanese knotweed would cause this by amplifying and exacerbating erosive forces only at locations infested by this plant, causing bank failures that otherwise would not take place. This proposal seeks to generate the information required to assess the threat this plant poses to our infrastructure, especially in the context of severe weather and flooding predicted to become more common. The objective of this research would be to generate a data set capable of informing a wide range of planning and engineering related decisions, by determining the scope and magnitude of Japanese knotweed’s impact on erosion rates. Several different research methodologies could be utilized. What they would have in common however, is many riparian and roadside sites for data collection. Roughly 30 would be an appropriate number. With so many variables to account for, including: soil type, waterway type, size of knotweed stand, storm size and duration, and local land cover, among many other likely variables, a large number of sites would reduce the impact of any statistical anomalies on the final data analysis. Additionally, sites will need to be spread over a wide area to increase the likelihood of generating data during both local and regional floods. The combination of data from disparate localized floods, across varying cartographies, would ensure that the results would be useful to a wide range of transportation agencies and partners. The required data may take many months to acquire, as the nature of flooding, and extreme weather, in general, is unpredictable. To generate the statistically valid results required for a proper analysis, the aforementioned 30 field sites would need to be selected and established. Each field sites would consist of at least one control location, and one knotweed infested location. Each location should be monitored with 30 erosion pins, again to ensure statistical validity. An erosion pin generally consists of a length of material, such as rebar, which can be sunk into the streambank and left in place for the duration of the study. It then acts as a constant against which to measure changes in the streambank. Data collection would be conducted in person, by hand, at regular intervals. However, a major downfall of this data collection method is that it only provides information when data collection takes place. The cause and/or exact time of any recorded erosion would remain a mystery. As the hypothesis this work would be investigating connects large storm events such as TSI with sudden bank failures in specific locations, this project would require some form of erosion monitoring device that can track erosion in real time and link observed erosion to high water events. Each site would, therefore, require a PEEP – a Photo Electronic Erosion Pin, or similar device. PEEPs measure how much of the pin is exposed to light and can therefore track erosion, if sufficient light is available, and store the data for later recovery. During a storm such a device would allow for a direct link between how differently vegetated banks eroded as waters rose and fell. Each field site established would therefore require potentially two types of data collection. Downloadable data stored on a PEEP or similar device, and manually collected erosion data, at both control and knotweed infested locations at each site.

Urgency and Payoff

Japanese knotweed is one of the worst invasive species in the world and has been documented at this point in 43 states, according to the EDDMaps database. The infestation is worst in ME, NH, VT, NY, MA, RI, CT, NJ, PA, WV, DC, MI, WI, IN, & WA. Other states with notable infestations include CA, OR, MT, ID, VA, NC, IL, OH, & MN. As an invasive plant with a worldwide reputation for disrupting all kinds of natural and constructed environments, the risks this plant poses are well documented and diverse. The United States has only begun to feel the impacts of this species. As mentioned, one accepted and unsupported impact of this plant is the speeding of erosive processes along waterways. This would lead to the loss of support for infrastructure near rivers, which represents an unacknowledged threat to our transportation infrastructure. As this plant spreads, it is expected that more sudden failures of roads, bridges, and other transportation infrastructure will take place during severe flooding than might otherwise be predicted. The lack of data associated with this problem makes it difficult for planners and engineers to account for this threat and adapt accordingly. The severity of this problem should also increase very quickly, as invasive species related problems usually do. As this problem is also related to extreme weather, which is expected to get worse in most of the locations impacted by this plant, the urgency to address this lack of information, which is already high, will only grow. The anticipated benefits of the work would be significant. An unknown cause of infrastructure destruction would become known. Expectations and plans could be developed to take this new knowledge into account. The savings created by managing this plant, and preventing infrastructure failures, would be significant. British Columbia’s Ministry of Transportation & Infrastructure estimates they spend roughly six thousand dollars to control an acre of Japanese knotweed. Most knotweed patches that threaten roadways are significantly smaller. Control costs therefore represent far less than 1% of any repair that could be imagined for failed roads, bridges, or other infrastructure. By preventing transportation network problems during severe weather, first responders would also have fewer road closures to work around, with implications for health emergencies. Outside of severe weather, non-point source pollution prevention programs are also likely to benefit from the insights this work generates. Beyond the resources saved and insights gained, understanding this threat would allow for better budgetary expectations at all levels of government, especially in the realm of emergency expenditures. It also provides an opportunity for transportation officials at all levels of government to develop effective partnerships with groups concerned with invasive species and the ecological well-being of rivers and their surroundings. By partnering to take on a common threat, opportunities for relationship building, trust building, and positive news stories to result are limited only by the imagination of those involved. Should the impact of this plant on the transportation network be as hypothesized, these benefits and others will accrue in a classic triple bottom line manner. An environmental harm removed, fiscal resiliency increased, and all to the benefit of the citizenry.

Suggested By

Brian Colleran Ecological Land Management 9783581423

[email protected]