How can I control Japanese Knotweed on my new property?
Question:
I am a new homeowner and have found an infestation of Japanese Knotweed on my 1/2 acre lot. It is in the front and back lawn. lt has also spread over the fence to my neighbors yard. It is coming up in other places that should be mowed periodically over the summer. I am hesitant to mow over the JKW as I am told that the mower can carry bits of the plant or maybe throw pieces of the plant around and thus spread the infestation further. Is this correct? I plan to cut the plants in June and apply glysophate in the fall as recommended. Do you have any advise as to mowing and how to treat the plants in the grassy areas. Do you have any advice on mowing or eradication? For what situations is foliar application best and what situations would you want to inject glysophate?
Answer:
I’m sorry to hear about the Japanese knotweed, but it it’s any consolation at all, you aren’t alone in fighting it. I’m including our standard reply on the topic below, including some excellent university and state resources, but I’ll briefly address your specific questions first.
Re: mowing. This is still recommended as an important tool in combating the infestation. Depending on how large the patch is, you can either bag the debris or rake it up afterward (disposing of it in the trash, not the compost pile, in either case).
Re: foliar application versus injection. The foliar application is typically done (usually by a licensed pesticide applicator) if a large patch exists and it’s impractical to treat individual stems, but keep in mind glyphosate is a non-selective herbicide and it will kill any plants in that area (e.g., the grassy areas you mention, but also many native/beneficial species that you might otherwise want to keep). If the patch is smaller and more confined, injecting or painting the glyphosate onto severed stems is a more targeted, less destructive approach.
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Japanese Knotweed
By Jonathan Foster, UMaine Cooperative Extension Horticulture Outreach Professional
Japanese Knotweed (Fallopia japonica) is a headache for home- and landowners all over Maine, so you certainly aren’t alone in this struggle!
First, let me provide a few resources for you:
State of ME Natural Areas Program fact page on Japanese knotweed (quick read)
State of NH Dept of Agriculture booklet on Japanese knotweed (far more in-depth)
Maine Public Piece on Japanese knotweed
The short version is that once established, Japanese knotweed is very difficult to control and will out-compete just about everything in its ecological niche. Regrettably, our short growing season and long winter don’t do much to depress the species’ growth. Frequent mowing or burning, along with the careful application of glyphosate (as always, reading instructions carefully and following them conscientiously–more is not better) are the primary methods of intervention, and for a large stand, it’s a long term project.
White Japanese knotweed is a growing problem, many gardeners and landowners have persevered and triumphed. Happily, there are very few plants that can survive indefinitely without access to the photosynthetic machinery of their foliage, so you can outlast this pernicious invasive species, but it will require diligence and effort. Be careful about tilling or physical removal of the plant, as broken rhizomes (the spreading underground stems) will simply resprout as new plants.
We’re very sorry not to have more positive news/advice for you, but Cooperative Extension experts are working on the problem. We wish you good luck and happy gardening, nonetheless.