KNOCKOUT KNOTWEED

Japanese Knotweed is an invasive species that was introduced from eastern Asia. 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. 

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Map of the layout of our Riverwalk Park site in Montgomery. 

During this past summer, the Missisquoi River Basin Association (MRBA) 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 experiments took place at three different locations around our watershed: behind the MRBA office, Big Falls in North Troy, and at Riverwalk Park in Montgomery. Within each location, there was five 8'x10' treatment plots and a sixth plot used as control. MRBA staff collected data weekly at the three locations. By taking time-lapse photos, weights, and stem counts, we have been able to monitor the level of knotweed growth depending on the method of treatment. We are planning on using our knowledge gained from this summer's field season to apply to next year in order to continue to try to find the most cost effective method of non-chemical removal of Japanese Knotweed. 

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Japanese knotweed treatments plots:

Control Plot

Control plots were only be cut at the end of the growing season. We measured the overall fresh weight where initially cut, as well as the dry weight a week afterward. The control plot weights are compared to the cumulative weights of the other treatment plots to determine the effectiveness of each treatment. 

Control plot in North Troy plot at week 1 of experiment

Control plot at the MRBA office site at week 4 of 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. 

Smother Methods:

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 plan to not continue using this method for next year. 

Example of a girdled stalk found at our Riverwalk Park site

Woodchip plot in North Troy with a shoot appearing on week 2

Metal mesh treatment at week 2 in the MRBA office plot

Woodchip plot in North Troy with a multiple new shoots appearing on week 4

Pond Liner

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. 

Pond liner plot at North Troy during week 1

Pond liner plot in North Troy in week 8

Cutting methods:

Weekly

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, 

Cut weekly plot at Riverwalk Park at week 1

Cut weekly plot at the MRBA office location at week 2

Monthly

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 monthly plot at MRBA office location at week 2

Cut monthly plot at MRBA office at week 4

Conclusion

Our overall conclusion from this past summer 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. 

Helpful Knotweed Documents:

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

Connect with us via phone or email if you have questions about our project, or knotweed in general!

How Lindsey feels about knotweed!

Our thanks to the Lake Champlain Basin Program for the Aquatic Invasive Species Spread Prevention grant that is supporting this project!

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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.