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.
Do not let knotweed move into rivers and streams. Stems and roots carried downstream can sprout in new locations.
Do not compost root parts. Instead, discard with the trash or take to a transfer station for disposal. Be sure stems are fully dried before composting.
Do not spread contaminated soil. Any soil gathered from within 20 feet of a knotweed patch may contain root fragments, which can grow into new plants. If you are using fill dirt, check to see if there is knotweed nearby or ask your supplier.