Japanese Knotweed
Plant Analysis
Japanese knotweed (Fallopia japonica) is a highly invasive plant species originating from East Asia that has become a significant problem in many parts of the world, including Massachusetts. Introduced as an ornamental plant in the 19th century, its rapid growth and ability to spread through its underground rhizomes make it difficult to control and eradicate. It can grow up to 4 inches per day during the growing season, quickly taking over large areas. Japanese knotweed's dense growth can damage building foundations, roads, and natural ecosystems by outcompeting native vegetation. Its presence can decrease property values and lead to costly removal efforts. Systemic herbicide treatments over multiple seasons are the only effective method for its long term control
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Management
Japanese knotweed is an extremely persistent plant. Its extensive root system including dormant rhizomes makes its management a multiple season effort. We expect any knotweed management plan to require between 3-5 seasons of treatments. Key to effective control is diligence and not skipping any seasons of treatment to prevent any lingering growth from reestablishing a population of knotweed.
A strategic cut back during the month of June followed by a foliar herbicide treatment using glyphosate during the Pre Flower, Flowering, and Post Flower window of August/September is the most effective method for managing invasive Japanese knotweed. The June cutback serves to reset its mid season height, facilitating easier (and safer) herbicide applications to shorter plant material during its treatment window. Glyphosate foliar applications are performed during the plant's pre flower, flowering, post flower window, when it most effectively absorbs and translocates the herbicide throughout its roots. The timing of the cutback and subsequent herbicide application is crucial for maximizing the treatment’s efficacy on the knotweed.
To minimize exposure to pollinators on flowers, we recommend herbicide applications during the early August time frame just before flower, or in the mid September time frame after flowers have started to fade.
Keys to Success
Knotweed’s tall stature and aggressive growth can make the plant quite imposing and overwhelming. Here are some notes to beware of before taking action against its growth in your landscape.
• Avoid cutting down knotweed without plans for herbicide treatment. Cutting back knotweed alone will only encourage its spread while doing absolutely nothing to hinder its long term growth.
• Avoid moving/removing any material off site or into sensitive areas of your landscape. Freshly cut knotweed stems can reroot if allowed to come in contact with soil. Stem fragments can also produce new plant material. It is imperative that absolutely no plant material, while still green, is attempted to be composted or placed in the landscape waste stream. Material can be safety composted or removed off site only after it has completely dried (generally 2 months after cut back is a safe window). When we perform our cut backs of knotweed we leave the cut material in place so that during our return herbicide treatments we can also treat any material that might have rooted.
• Diligence is needed to keep this plant at bay. Plan on a 3-5 year treatment schedule to make sure you have a strong handle on the problem
Home Owner Guide to Controlling Knotweed
Unfortunately with the scope of the problem and the limited window of treatment options, we are not able to manage all our clients requests to treat knotweed on their property. For smaller patches or homeowners who like a challenge we have put together a how to guide on managing Knotweed in your landscape on your own.