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Originally Posted by Mercadante  Don't cut it! Killing it that way, although possible, takes many years, and you risk spreading it, as Dan says.
Spray it with glyphosate (Round-up). I think the very best time to do that is the autumn after it's flowered, because you can poison a lot of leaves that way, just as the plant is sucking down the goodness preparing for winter. But in the spring is good too. You will need to do this for at least three years, probably. I'm on year 2, and the evil weed is certainly looking nervous now. 
See the Japanese Knotweed Alliance website for more information. |
I worked in a garden and used to spray each stem as it was coming up in the spring, I did this for 4 years and eradicated it. You need to be vigilant.
Never move it from the site in which it grows this is one of the reasons it spreads.