The Q&A Archives: What is this plant and how can I get rid of it?

Question: It looks similar to a Japanese Lilac but the small white flowers are not in a group or cluster. They are all along the branches of the plant and in bunches of 3's that come out of the stems. The flower stems itself are 4-5 inches long and the leaves are 3-7 inches long and 2-4 inches across. One plant itself it about 8-10 ft. high when you lift it up in the air.
They are inbetween my fence line and garage and I want to get rid of them to put in a flower bed and some grass. I CAN'T GET RID OF THEM!
I tried Roundup and even pulling by hand and tilling them up and then picking out the roots, but they take right over and start growing again.
Can you help me? What are they for one and how can I get rid of this stuff?
Thanks-a-million Laurie Peotter

Answer: I think you are describing Polygonum cuspidatum, or knotweed. It is truly a horror and is able to regenerate from even small broken bits of stem or root. So tilling and simple pulling will not work. Studies seem to indicate it can be controlled by repeated cutting combined with an August application of glyphosate (the active ingredient in Roundup), or possibly cutting and covering with black plastic to exclude light -- although with the smothering in my experience it will come back later if you try to plant the area. I would suggest you work with your county extension to get a specific identification of the plant and based on knowing that, their control recommendations. It is considered a serious invasive exotic weed and so they may have some additional control suggestions. Good luck!

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