Native Plants
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Ask Mr. Smarty Plants
Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.
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Monday - July 28, 2008
From: Washington, CT
Region: Northeast
Topic: Invasive Plants, Problem Plants
Title: How to get rid of Japanese knotweed (Polygonum cuspidatum)
Answered by: Nan Hampton
QUESTION:
We have an enormous stand of japanese polygonum that we are trying to get rid of. What soil type is the most inhospitable to this aggressive and highly invasive species? Were thinking of planting some native white pines in the area to grow over them as well as manually digging them out. What would you suggest? Thanks.ANSWER:
Polygonum cuspidatum (Japanese knotweed) is on the Least Wanted list of the Plant Conservation Alliance's Alien Plant Working Group. It appears from their description that nothing is inhospitable to this plant. As their web site states: "Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity, and drought." Planting Pinus strobus (eastern white pine) over the area where the Japanese knotweed grows is a very nice idea, but it won't affect the status of the knotweed. Digging them out is the most environmentally friendly way to get rid of them; and, depending on the size of your population, it might be the best way to eliminate your plants. The Plant Conservation Alliance also gives specific instructions on chemical treatment for large or difficult populations. The Nature Conservancy and Clark County, Washington also have information on control programs and their success.
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