Experts and BHG readers answer.
rid my property of japanese knotweed
Thanks for writing. You may need to use a brush-killing herbicide, such as Roundup, on the plants. Then reapply as soon as you see any growth; reapplication may be necessary three or four times since the plants have such strong root systems. If there are desirable plants nearby, you can spot-treat by apply the brush killer with a paintbrush.
---Justin, Senior Garden Editor, BHG.com