Japanese Knotweed Brochure

DRWA has produced, with the help of the Massachusetts Environmental Trust and CopyCat Print Shop of Greenfield, a brochure (in PDF format) that explains the identification and ecology of Japanese knotweed and the impacts of the plant on the environment. The brochure also outlines some of the methods that can be used to control knotweed and where to go for more information. The brochure will be revised and updated as additional information becomes available on control methods and other aspects of knotweed ecology. 

Japanese Knotweed: History and Suggested Methods of Control

It is our hope that readers will use this site as a place to start learning about invasive plants, especially Japanese knotweed. The information on this page was gleaned from several sources, including websites (see list below) and The Japanese Knotweed Manual by Lois Child and Max Wade (2000). The information may be used for educational purposes but not for commercial use. The DRWA is not responsible for the efficacy of any of the methods outlined on this site. In the future, we will include information on the control of other invasive plants in addition to knotweed. We will continue to update information on controlling invasives as new methods become available.

History and Ecology of Japanese Knotweed

Japanese knotweed ( Polygonum cuspidatum ) is an herbaceous, perennial plant that was originally from Asia. It was introduced to Great Britain around 1825 and was naturalized by 1886. In the United States it was introduced for horticultural purposes and became naturalized between the late 1800s and early 1900s. Knotweed is now found growing in a wide range of habitats, including abandoned lots, highway rights-of-way, roadway edges, streambanks, and wetland edges.

Japanese knotweed can grow up to 10 feet tall by late summer and has large, green, heart-shaped leaves with clusters of white or cream-colored flowers.

 Japanese knotweed patch growing in a flat, sunny area next to the Green River (Sept. 2004).

A close-up view of the large, heart-shaped leaves and flower clusters.

A large infestation of knotweed growing on the banks of the Green River. Note how the plants have taken up the entire streambank and are hanging over the water.

A small patch of knotweed growing in a sandy area next to the Green River.

Dead canes or stalks left over from the previous growing season. Note the rusty red color of the plants.

Japanese knotweed is an extremely difficult plant to eradicate because of its ability to spread by its roots or rhizomes, which can grow to a depth of more than six feet. Roots can also grow horizontally as far as 23 feet from the original plant. A tiny piece of root can develop into a plant, and even pieces of the stem can form new plants. Knotweed is spread throughout watersheds when pieces of the roots and stems are transported in piles of dirt or fill, or are swept downstream during high water events. In the past, researchers thought that knotweed did not reproduce by seeds in North America, but results of recent studies suggest that this may not be the case. Once plants are established at a site, whether by seeds or vegetatively, they continue to grow by sending out roots, resulting in larger and larger patches.

Controlling Japanese Knotweed

Successful eradication of Japanese knotweed can be achieved through a three-part process of removal, disposal, and re-vegetation. However, eradication becomes more difficult as the size of the area covered by plants increases. And no matter which method is used, the process will take a considerable amount of time and effort. In areas where the patches are large and dense, preventing the spread of knotweed may make more sense than trying to control it.

Methods that have been used to successfully control knotweed include: mechanical means (cutting, pulling by hand, mowing, grazing), using herbicides, or a combination of both. I won't be discussing the use of herbicides here because of the strict guidelines for their use in wetlands, which includes stream banks, and many people do not like to use herbicides at all.

The particular method chosen should depend on the size of the infestation or patch, ease of access to the site, presence of rare or sensitive plants or communities, cost, and the preference of the landowner. Landowners should be aware that removal of any type of knotweed or other plants that occurs in or near a wetland (and this includes stream or river banks) needs to be cleared through the local Conservation Commission. Regardless of the method used, it pays to be pro-active: if you have a patch of knotweed on your property take care of it right away.

Mechanical Control Methods

Knotweed plants can be cut using a variety of tools: a brush cutter, lawnmower, machete, or loppers. Keep in mind; however, that even small pieces of the plant have the potential to cause a new infestation so be careful not to spread it when cutting.

Landowners using mechanical methods of control should keep the following in mind:

• Cutting knotweed plants eventually kills the plant by starving the roots
• Use several different methods to eradicate the plants
• It will likely take several years to get rid of knotweed completely
• Extreme vigilance is necessary to make sure that the plants do not re-infest a site

A. Cutting or Mowing :

Whether cutting or mowing, it should be done at least four times a year between April and September. Some land managers recommend cutting plants every 2-3 weeks between April and September 1 st and less frequently thereafter (until plants die back in fall). Make the last cut when the plant is about to go dormant (e.g., has stopped growing).

Knotweed plants should be cut at ground-level. Make sure to check the surrounding areas (up to 20 feet away) for re-sprouts. At a site owned by the Conte National Fish and Wildlife Refuge, biologists found that cutting a patch of knotweed approximately 300 ft by 300 ft in size, 2-3 times a year during the growing season was enough to control the knotweed after 3 years. They were careful not to cut the native vegetation growing in adjacent areas because encouraging re-vegetation helps prevent knotweed from dominating the site.

Some land managers do not recommend mowing the plant as it can still spread by the small pieces that are generated by this method. Do not “chip” knotweed for the same reason – small pieces can regenerate.

B. Grazing:

If you have grazing animals, Japanese knotweed can be safely eaten by sheep, cattle, horses, and goats. They prefer the new shoots, though. Grazing will not completely remove the plant from an area, but will prevent it from spreading into adjacent areas.

C. Pulling or Grubbing:

You can also try pulling out small plants, which can be successful because they don't have long roots. Pulling the entire plant out of the ground is most effective when the infestation is new, plants are small, and you can get the entire plant. If any parts of the roots are left behind, these have the potential to re-sprout. At a site in Great Britain, a small patch was eradicated using this method after three years. Plants were pulled out during the growing season. Initially, this patch was only about 6 sq ft in size.

D. Covering:

Covering plants with plastic sheeting after cutting to ground level has received mixed results. Because knotweed can grow through plastic, it can be difficult to achieve good control. The New York State's Greene County Soil & Water Conservation District Stream Stewardship Program describes how to use plastic and the pros and cons of this method. See their "Management Without Herbicide” website on this and more control methods.

Disposal of Plants

You're probably wondering what to do with the knotweed once it's been cut. This step is one of the most crucial in terms of preventing new infestations. Improper disposal of knotweed can lead to new infestations in areas that were previously knotweed-free.

Japanese knotweed can be burned or buried, but it cannot be composted while “green”. You can pile it up and make sure it dries out thoroughly but you must monitor the pile to make sure it doesn't re-sprout or get blown into new areas or washed into a stream or pond. Placing plant remains on plastic or some other impermeable surface is recommended. If the plants are buried, make sure they are buried at least 10 feet deep! Please do not cut or pull plants and throw them in the river or stream, or on someone else's property . Plants can also be burned if it's the proper time of year.

Site Re-vegetation and Restoration

As knotweed is removed from a site, you will want to encourage the growth and establishment of native plants. If you are working in your yard or garden, choose either native species or non-natives that do not have invasive properties. In some situations native plants will readily re-establish themselves without any help. However, you may want to replant the area to jumpstart the process of re-vegetation. Regardless of whether you have removed knotweed in your yard or at a natural area it is essential that the site is monitored to make sure that re-infestation does not occur.

Do's and Don'ts for Mechanical Control

Do make sure to clean all cutting equipment to prevent the spread of knotweed to new areas

• Do use protective clothing and/or glasses with brush cutters
• Do not burn actively growing plants
• Do not dig out large stands of knotweed – this will result in an increase in stem density from the fragmented root pieces
• Do make sure that knotweed is disposed of correctly

Websites :

I have included several websites that provide accurate and easy-to-understand information on the identification of knotweed, control methods, and why we should be concerned about this plant. Two of the sites also give general information on other species of invasive plants and animals.

(1) Invasive Plant Atlas of New England (IPANE): This site is good for photographs of invasive plants that are found in New England, as well as information on early detection of invasives: plants that are not yet found in our area but are likely candidates to show up here. If you click on IPANE species you can look up individual invasive plants and get the description, ecology, and history of the plant in our area.

(2) Cornwall, England website: This site is particularly good for how to identify the different parts of the plant and for general advice on knotweed control. It has a discussion of how serious the problem is in parts of Great Britain and why it's important to eradicate it when it's first found and not wait. Note: The expression “fly-tip or tipping” refers to disposing of trash or littering.

(3) US government website: There is a lot of information here, but for knotweed-specific information go to Species Profiles, click on plants, and scroll down to Japanese knotweed. This site has data on all invasive species, plant and non-plant. The information ranges from articles to images to recent news. For a discussion of management options, click on Manager's Toolkit on the right side, then go to “Control” and “Management Plans” by species. From there you'll find “Plants” and then a specific article on controlling knotweed ( Controlling knotweed in the Pacific Northwest ) written by ecologists at The Nature Conservancy.

(4) In My Garden is Sue Sweeney's private website: She has a lot of good information on invasive plants, including some very clear photographs of the various stages of knotweed, from seedling to adult, and how the appearance of the plants change during the different seasons. Scroll down to Eco-gardening – invasive plants and alley weeds - on the left side and click on Japanese knotweed.

(5) The Global Invasive Species Initiative: This is The Nature Conservancy's website on invasive plants and animals and it has a wealth of information. It's fairly easy to get around in, too.

Partners

The Deerfield River Watershed Association would like to acknowledge the organizations and businesses that are making valuable contributions to the Association's project to manage Japanese knotweed in the Deerfield River watershed: