Knotweed, also known as Japanese knotweed, is an invasive, perennial plant that can grow up to 10 feet tall. This weed is not only an annoying pest, but can actually be dangerous to native plants. The roots of the knotweed plant can become so extensive that they damage sidewalks and building foundations. Knotweed can be controlled by using a combination of cutting and chemical herbicide treatment.
Fill a spray bottle with the herbicide, diluted as recommended on package instructions. Add enough food coloring to thoroughly dye the solution so that you will be able to see where you are spraying it. This should be done in early spring when rain is not expected for at least one hour.
Cut the knotweed back to within 2 to 3 inches of the soil using the lopping shears. Make the cuts as smooth as possible to ensure that the herbicide is easily absorbed by the stems.
Spray well around the top of the stem with the herbicide. It is important that you spray the stems immediately after cutting because the plant quickly begins to develop a protective seal after being wounded.
Wait at least seven days before you attempt to mow over the remaining stems. This will give the herbicide a chance to penetrate to the roots.
Re-treat in the same manner in early fall.