Experts and BHG readers answer.
Help with an invasive!
How do i kill Japanese knot weed? Help!
Thanks for writing. Persistence is key with plants like Japanese knotweed. One way to go about it is to spray with an herbicide such as Roundup. The knotweed will bounce back; as soon as you see new growth develop, spray again. Repeat until, after four or five applications, you don't see it grow back any more.
An organic solution that takes longer is to cut all the knotweed back, run over the area with a tiller, water it in, and weigh down clear plastic over the ground. Leave that all summer long; the heat that builds up should be enough to kill the knotweed.
---Justin, Better Homes and Gardens