Knotweed buds, early spring
Photo credit: Jeff Parsons
Knotweed shoots, spring
Photo credit: MRBA
Flowering knotweed, fall
Photo credit: Jeff Parsons
Knotweed stalks, late fall
Photo credit: MRBA
Over the past three years, MRBA has been battling the incredibly pervasive invasive, Japanese knotweed.
Knotweed is an invasive species that was introduced from eastern Asia to the United States in the 19th century. This rapid-spreading and dense vegetation shades out native plants, and its shallow root system creates large areas vulnerable to streambank erosion. Additionally, knotweed can reproduce and spread from even small fragments of the plant, which makes removal and treatment difficult.
The Knockout Knotweed project strives to identify effective
non-chemical treatment methods to prevent and combat knotweed invasions, with the goal of providing ways local landowners and residents can treat knotweed patches on their property. As we progress with this project, we will include updates to our findings on our website and encourage you to join us at our annual Knockout Knotweed Bonfire in the late fall.
Check out our events page for our most up to date details on upcoming events by clicking below.
Plot of knotweed at Riverwalk Park in Montgomery
Knotweed invasion on embankment along
Missisquoi River in East Berkshire
Flowering knotweed at Riverwalk Park
Unstable bank with knotweed stalks along the Missisquoi River in Troy
Knotweed leaves at our pond liner testing plot at Riverwalk Park
JAPANESE KNOTWEED: WHAT IS IT AND WHY IS IT A PROBLEM?
Japanese knotweed is an invasive species that is commonly found along river banks and along disturbed land like road sides or old agricultural fields. Japanese knotweed can be found in 42 of the 50 U.S States, and covers miles of river front property in Vermont, throughout the Missisquoi River Basin and beyond.
Knotweed can eventually suffocate native species, in part due to its ability to rapidly grow and shade out large areas. Knotweed has shallow roots which lead to river bank destabilization because of the lack of support from native vegetation and trees that were once there. During large rain events, the weakened river banks often break down and fall into the river, causing sediment to flow downstream, through our rivers, into Lake Champlain. Knotweed is incredibly resilient and eradication requires multiple years of treatment to remove, even when treating with herbicide. Overall, knotweed decreases water quality of our rivers and is an increasing issue of concern within our river basin.
HOW TO IDENTIFY JAPANESE KNOTWEED
Knotweed is a hollow herbaceous plant that can grow up to 13+ feet tall at the peak of its growing season during the summer months. The stems are mostly green during the growing season, and turn reddish dark brown in the winter months. Knotweed has oval shaped leaves that end in a pointed tip. While emerging in the spring, Japanese knotweed resembles thick shoots like asparagus. As the knotweed continues to grow, it begins to resemble bamboo. In the late summer and early fall, knotweed will bloom with white flowers along the stem of the plant. During the late fall, - usually after the first few frosts - the knotweed plant tends to "die back" for the winter and the leaves will fall off.
Knotweed stalks in the early spring
Knotweed stalks in the late fall.
WHERE IN OUR REGION IS THERE JAPANESE KNOTWEED?
In 2020, Arrowhead Environmental was contracted to map the knotweed within our region. On the left is an interactive map of the knotweed found during that summer survey. Red circles refer to knotweed found on the right bank, and green circles are knotweed on the left bank.
This map allows us to explore the prevalence of knotweed in our area, as well as areas that we can still protect through proper management strategies.
WHAT IS THE KNOCKOUT KNOTWEED PROJECT?
During the past two summers, the Missisquoi River Basin Association conducted experiments to assess various ways to control this invasive species through mechanical means of smothering, cutting, and wire mesh to choke the plant. The inaugural year experiments occurred at three different locations around our watershed: behind the MRBA office in East Berkshire, downstream of Big Falls in Troy, and at Riverwalk Park in Montgomery. For the second summer, we focused on our more successful Troy and Montgomery plots.
Initially, five 8'x10' treatment plots and a sixth plot, used as a control comparison, were laid out at each location. MRBA staff collected data weekly at the three locations, taking photos for a time-lapse comparison, weights, and stem counts, and thereby monitoring the level of knotweed growth for each of the methods of treatment.
During year one, we used five treatment methods:
smothering knotweed with pond liner, smothering with cardboard and woodchips, covering with a metal mesh, cutting weekly, and cutting monthly.
As we refined our project for 2022, we determined that the three most successful methods have been: smothering with pond liner, cover with metal mesh, and cutting monthly (no significant difference was found between monthly and weekly cutting). These three mechanical methods have so far proven to be the most effective combat techniques with the least amount of maintenance labor required.
This project continues: we will use the knowledge gained from the 2021 and 2022 field seasons during 2023 to continue seeking and refining the most cost- and labor-effective method of non-chemical removal of Japanese Knotweed. When we feel we have learned what we can, and begin to close out the treatment plots, we will plant native vegetation over the previously infested Japanese knotweed area, and will continue to monitor them.
Map of the layout of our 2021 Riverwalk Park site in Montgomery.
MRBA Staff preparing experiment plots in 2021.
All 2021 collected, bagged, and weighed knotweed before proper disposal during our 2021 Knockout Knotweed Bonfire Event.
WHAT ARE THE OUR TREATMENT PLOTS AND METHODS?
Control plots were left untouched during the field seasons, and only cut at the end of the growing season. After cutting, we measured the overall fresh weight. These control plot weights can then be compared to the cumulative weights of the other treatment plots, to help determine the effectiveness of each treatment type.
Control plot at Riverwalk Park at week 1 of 2021 experiment
Control plot at Riverwalk Park at week 1 of 2022 experiment
Metal Mesh Method:
1/2 inch hardware cloth was staked down over a knotweed patch 5-10 cm above the ground, crushing any stems present. The idea of this method is that Japanese knotweed stems would then grow through the grid eventually getting large enough causing the stems to girdle or choke themselves in the mesh, killing the stem. The continually growth would eventually weaken the rhizome structure cause then death of the plant. In practice, this method has been met with mixed results. Some plots have shown girdling of the stems, however, they still appear to sometimes continue to grow regardless of the breakage of stem. We plan to continue using this method for the next field season to monitor the success.
Woodchips and Cardboard
A thick layer of cardboard was laid down over a knotweed patch (crushing any stems present), and a 4-inch thick layer of wood chips was layered on top of this. During our summer experiment, we found this method to not work well as shoots were easily able to sprout up as early as two weeks after smothering at one site.
We will not continue using this method after 2021.
At a second smother plot thick (45mil) pond liner was staked over a knotweed patch, crushing any stems present. We did not see any shoots make it through any of the pond liner on any of our plots, however, there still was growth of shoots underneath the plastic layer as well as on the sides. We plan to continue to use this method and leave the liner covered over the plots for as long as we are able to see how many seasons it takes for plant death to occur.
Example of a girdled stalk found at Riverwalk Park in 2021.
Metal mesh plot at Riverwalk Park in August 2022
Woodchip plot in North Troy with a shoot appearing on week 2, 2021.
Woodchip plot in North Troy with a multiple new shoots appearing on week 4 2021.
Pond liner plot at North Troy during week 1, 2021.
Pond liner plot in North Troy in August 2022.
We cut all stems within one plot down to the ground weekly, weighing the cuttings each week. This method was very labor intensive, especially in the earlier months of the year before the plant began to dieback. By the end of the season, there was little growth within the plot. We recommend to try this method if you have the labor, time, and a small enough plot to manage. However, we found no significant difference between cutting monthly and weekly, so have not continued this more labor-intensive method for our treatment experiments, focusing instead on the cutting monthly method.
We will not continue using this method after 2021.
Instead of weekly cutting, we also had a plot that was strictly cut once a month. This method was much less labor intensive than the weekly cutting, and seemed to have similar results. We also recommend this method if you have a small enough plot and are willing to put in the manual effort to maintain monthly.
Cut weekly plot at Riverwalk Park at week 1, 2021.
Cut weekly plot at the MRBA office location at week 2. 2021
Cut monthly plot at Riverwalk Park location in June, 2021
Cut monthly plot at Riverwalk Park in August 2022.
HOW TO DISPOSE OF JAPANESE KNOTWEED
Under Vermont law, it is illegal to dispose of Japanese knotweed by throwing any plant material in the trash or to take knotweed to the dump. Proper disposal technique is to contain the knotweed in a place where it can fully dry, but cannot blow away (we use onion produce bags, but have also seen methods involving stacking it between pallets). Leave the knotweed to dry for a minimum of a month, though larger root balls may require multiple months of drying. Rhizomes of the plant (white under soil structures) are the most dangerous part of the plant for spreading. Ensure they are dried completely before disposal, which may take multiple months. Completely dried knotweed can be disposed of like any compostable material - or can be burned in a controlled setting: a fun way to celebrate the end of a summer battle!
Join us for our annual Knockout Knotweed Bonfire in the late fall at one of our experiment plots at Riverwalk Park in Montgomery, VT. Learn more about our efforts to control, manage, and prevent knotweed invasions, as well as learn how to properly dispose of knotweed with our disposal of collected knotweed. Snacks, fire, and good company will be provided! Check us out in 2023 for our next bonfire event!
Find out more by checking out our upcoming events page by clicking the button to the right.
2021 Knockout Knotweed Bonfire
2022 Knockout Knotweed Bonfire
Our overall conclusion from these past two summers of knotweed management is that Japanese Knotweed is in incredibly persuasive and tough plant that requires intensive removal efforts. The invasion of knotweed on our riverbanks is a continual problem that we will see the long-term effects of for years to come as climate change factors continue to worsen for our planet. We hope to continue our project into next year to further our knowledge of this intrusive and difficult invasive plant.
In the 2023 summer we intend to continue our experiments, and begin native plantings at our experiment plots. Check back for updates, and reach out via email or phone for any burning knotweed questions!
Our 2022 MRBA LEAP Interns and MRBA Project Manager planting native vegetation along a knotweed plot.
Native plants where knotweed used to be!
At the end of each experiment year, we summarize our findings and share them during our knotweed bonfire in November. Below are downloads for our first year (2021) and second year (2022) summary sheets.
Other Knotweed treatment efforts:
Our partners at the Upper Missisquoi and Trout Rivers (UMATR) Wild & Scenic Committee are offering small grants to landowners who are committed to using mechanical methods to treat a patch of knotweed on their property. Learn more here: https://www.umatrwildandscenic.org/knotweed
Our thanks to the Lake Champlain Basin Program for the Aquatic Invasive Species Spread Prevention grant that is supporting this project!
This project has been funded wholly or in part by the United States Environmental Protection Agency (EPA) under assistance agreement (LC - 00A006950) to the New England Interstate Water Pollution Control Commission (NEIWPCC) in partnership with the Lake Champlain Basin Program (LCBP). NEIWPCC manages LCBP’s personnel, contract, grant, and budget tasks and provides input on the program’s activities through a partnership with the LCBP. The contents of this document do not necessarily reflect the views and policies of NEIWPCC, the LCBP, or the EPA, nor does NEIWPCC, the LCBP or the EPA endorse trade names or recommend the use of commercial products mentioned in this document.
How we feel about knotweed!
Did you know that you can EAT Japanese knotweed? Check out some of these intriguing recipes - and feel free to ask us about; we have tried them all, but are working our way through them!
We are always interested to learn new ways to combat this plant - whether it's through consumption, or some other method!