Japanese knotweed

Asked April 29, 2013, 10:59 AM EDT

How can I kill Japanese knotweed in my back yard where I would like to build a raised garden. I have dug out some of the roots last year; I have laid black plastic down in the beginning of April in the area where I want my raised garden. It looks like the plant is still growing underneath. Will it stop or if is there any chemical to be used?

Luzerne County Pennsylvania master gardener program weeds horticulture

1 Response

There are anecdotal reports of successful control

of small patches of plants
using a combination of cutting, hand pulling, and/or tilling, followed by covering. After
cutting the plants down to ground level, cover the
area with several layers of black plastic
or several layers of cardboard. Extend the area of
coverage to at least 20 feet or more
around the outside of the plant and check at interv
als to make sure that shoots are not
coming up outside of the cover or through the cover
. Knotweeds have been known to
grow through asphalt! The cover needs to be left in
place for at least one full year and
probably longer. Inspect the site on a frequent bas
is to locate new growth or seedlings
and remove immediately to maintain major management
gains.