Learn how to identify invasive knotweed, understand how it damages native plant communities and streambanks, and find out how to control it using manual methods and injecting or spraying chemical herbicides.

*Click here* if you're interested in borrowing a knotweed injector

(must reside in King County)

Introduction to knotweed

About the plant, how to identify it, and how it damages river banks.

Resources:

• Learn more about knotweed identification, biology and impacts.
• Take a free class on knotweed control

How to control knotweed without chemicals

Learn about digging out this invasive weed, cutting and starving roots, and cutting, covering and shading roots as control methods.

Resources:

• Learn more about shovels or hand-cutting tools or plastic mulching materials (external links)
  

How to control knotweed by using herbicide injection

Learn how to inject chemical herbicide into knotweed stems using a stem injector. View the herbicide injection album on Vimeo. Injectors are available at a variety of sites online. If you're a King County resident interested in borrowing an injector for free from the King County Noxious Weed Control Program, please see next section (below).

How-to Borrow an Injector (Available July - October to King County Residents)

 The King County Noxious Weeds Control Program is piloting a new process to lend out knotweed injectors to the public. Please be patient as we test out this new system. There will be an option to provide feedback on the survey. Please note that the window to control knotweed with injectors is mid-July to early October. Injectors can be borrowed in 10-day increments.

STEP 1: Watch all the above videos on how to control knotweed by using herbicide injection

STEP 2: Take this assessment.

• At the end of the assessment there will be a link to the booking form. Please save this link. If you lose the booking form, you can take the quiz again or email us at knotweed.injectors@kingcounty.gov

BOOKING FORM (steps 3-5)

• Tip: staff members are available on Tuesdays and Thursdays, plan to schedule steps 3-5 for one of those days.
• If you have no way to meet on either of those days email us at knotweed.injectors@kingcounty.gov and we can try to accommodate your schedule as able. 

STEP 3: Reserve an injector (10-day reservation window, can drop off sooner)

STEP 4: Schedule a time to pick up the injector at King Street Center.

STEP 5: Schedule a time to return the injector (any time before the end of your 10-day reservation)

STEP 6: After using the injector, be sure to return it during at the time you chose, and please fill out the survey in your confirmation email! Doing so allows us to keep this service free to the public and helps us keep the injectors maintained and ready for use.

STEP 7: You're done! Thanks for reducing the impact of knotweed in King County.

More resources:

• Purchase a stem injection gun/tool (external link)
• Read about herbicides labeled for use with stem injection method (external link)

How to control knotweed using herbicide spray

Learn how to apply chemical herbicide on knotweed using a backpack sprayer.

More resources:
Get more information on using herbicides to control knotweed
Learn about choosing and calibrating a backpack sprayer (external link)
Find out about pesticide license requirements in Washington State (external link)
Apply for a permit to apply aquatic herbicides in Washington State (external link)

Related information

• Invasive knotweed
• Noxious weed best management practices
• Water and Land Resources Division multimedia
• Agriculture in King County, Washington
• Northwest yard and garden
• Animals, plants and habitat

Related agencies

• Dept. of Natural Resources and Parks
• Water and Land Resources Division

Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact a staff member at the King County Noxious Weed Control Program, please call 206-477-9333 or by reach them by email.