Japanese knotweed can be effectively controlled with herbicides but timing is key.

Chemical applications have been most effective after flowering, and up until the first killing frost. When using herbicide, make sure there is no rain expected for 24 hours.

  1. Cut down your knotweed plants to a 2 -3” stalks.
  2. If the stalks are not by the lake or a stream, then paint the cut rim of the stalk with a herbicide (small foam brush works well).  Using Concentrated Roundup at full strenth is effective. Before applying any herbicide product, make sure to thoroughly read and follow the label instructions. WEAR PROTECTIVE GLOVES.  HIGHLY TOXIC.

While not all the knotweed will die on the first round, this is the fastest way to kill off the knotweed roots and get rid of it for good.

Non-chemical techniques have proven effective.

  1. Cut back knotweed growth and dig up small knotweed growths weekly. Plant fast growing native grasses – such as American beach grass – to overshadow the knotweed stumps. This process will take 2 – 3 years. But like the lilly pads, you will see progress!
  2. Smothering is most effective in early summer once the plants have put on a significant amount of growth and have exhausted some of the reserves in their rhizomes. In the beginning of June, cut stems close to the ground and cover them with heavy 7-mil black plastic or non-woven landscape fabric. You can put mulch on top of the smothering material to improve its appearance. Smothering can take upwards of five years.

Managing Japanese Knotweed

Japanese Knotweed: Growing in YOUR backyard?