Borrow a Knotweed Stem-Injector

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.


  1. DO
  2. DON'T
  • Check your property. If you have knotweed, contact us.
  • Avoid spreading knotweed. Be careful when working around knotweed, particularly if you are mowing or doing yard work. Small fragments can spread and form new colonies.
  • Encourage natives. Plant suitable native species around your property so that knotweed cannot gain a foothold.
  • Tell others about knotweed! Knotweed control is most effective when neighbors take action together.