Controlling Knotweed: An Expert Shares Advice
Published in the Newtown Bee, July 18, 2025
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We asked Hannah Hudson, founder of the Worldwide Japanese Knotweed Support Group, to answer commonly asked questions for battling Japanese Knotweed. Hannah started the Support Group in 2015, after discovering that accurate data and up-to-date guidance was lacking in the U.S. compared with the United Kingdom. Knotweed is atypical of other invasive weeds, requiring a long-term, science-based control method as outlined in the Springer study,
Optimizing Physiochemical Control of Invasive Japanese Knotweed Jones, D., Bruce, G., Fowler, M.S. et al., Biol Invasions 20:2091-2105 (2018). Will regular mowing control knotweed? Regular mowing worsens knotweed infestations by leaving the deep, extensive root systems intact and stimulating further underground spread. Effective control requires targeted herbicide treatment during the appropriate season. Mowing alone can also unintentionally spread the plant through viable plant fragments. Can you tarp knotweed? Tarping is largely ineffective against knotweed because it mimics a weak barrier compared to the plant’s native volcanic environment, where it evolved to survive under lava. Knotweed will simply grow underground in search of sunlight, often emerging beyond the tarp’s edges or even through thin material. What is the most effective way of treating knotweed? The 2018 Springer Report recommends 41% glyphosate, diluted to a 2-4% concentration. If the herbicide is too strong, it will burn the leaves before it can be drawn down into the root system. When is the best time to treat knotweed? Herbicide treatment in spring or early summer fails because knotweed is pushing energy outward, preventing the herbicide from reaching the roots. The most effective time for treatment is during the Flower to Fall Treatment window. This is from the first flowers to 2 weeks before the first frost when the plant draws nutrients inward, allowing glyphosate to reach and weaken the root system from within. [Note: To minimize impact on pollinating insects, NKI encourages homeowners to delay herbicide treatment until after flowers fade.] If I have a very tall infestation, can I cut it in early June for easier access? Cutting knotweed in June may ease access but can disrupt its natural cycle, causing it to push energy outward and reducing the effectiveness of fall herbicide treatment. You also have the issue of removing the plant material. Many prefer to wait and spray during the fall window without cutting, preserving the plant’s inward flow to maximize root absorption and minimize spread risk from cut material. What is the best way to dispose of cut stalks? Freshly cut knotweed stalks must be dried above ground or double-bagged and bagged as trash, as even small fragments can trigger new infestations; composting or improper disposal in landfill is unsafe. Herbicide-treated stalks ONLY can be safely cut in winter when brittle, but cuts should stay above the crown to avoid disturbing live roots capable of regrowth. Never pull them out of the ground. I have concerns about using glyphosate. While concerns about glyphosate are common, it is considered the least harmful herbicide option for treating knotweed and is even granted exceptions in some areas where it's otherwise banned due to its effectiveness and minimal environmental persistence. For more information, go to the Northeastern USA Knotweed Support Group on FB, Newtown Knotweed Initiative on FB or www.newtownknotweed.org. |
