Contact Snohomish County to reserve a knotweed stem-injector to control small knotweed patches in an upland area (see sidebar).
Be patient and wait until the knotweed is large enough; is producing flowers, or after it has bloomed. This could involve resisting the urge to cut or mow knotweed. The best timing is between August to first frost, as long as the leaves are not yellow.
Knotweed stems need to be at least ½ inch wide. You need to provide your own herbicide. Use undiluted herbicide which has glyphosate as an active ingredient. It is widely sold under different brand names.
Contact us for advice if knotweed is growing near water, or is in a wetland.
1111 Washington St.
P.O. Box 42560
Olympia, WA 98504