Invasive knotweed control videos
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.
Borrow a knotweed injector for free! Available only to King County residents July through October.
Scroll down to the "How to borrow an herbicide injector" section to learn more.Introduction to knotweed
About the plant, how to identify it, and how it damages river banks.
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.
Control knotweed using herbicide spray
Learn how to apply chemical herbicide on knotweed using a backpack sprayer.
How to control knotweed by using herbicide injection
Learn how to inject chemical herbicide into knotweed stems using a stem injector. 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).
Watch this series of videos about herbicide injection
Watch the videos below (expand to full screen: click arrow at bottom right corner of video, select "expand" icon) or access the series on Vimeo here.
How to borrow an herbicide injector
Herbicide injectors are available to King County residents from July to October.
The King County Noxious Weed Control Program is piloting a new process to lend out knotweed injectors to the public. Thank you for your patience as we test out this new system.
The window to control knotweed with injectors is July to early October. Injectors can be borrowed in 10-day increments. Borrowers must purchase their own herbicide and are expected to pick up and drop off the injector from our office in Seattle.
If you are seeing this outside of our borrowing season (November to June) but wish to borrow an injector - email us at noxious.weeds@kingcounty.gov and we will add you to the list of folks we will email as soon as the program is back for the season in July.
How to borrow an herbicide injector (PDF version of this section)
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Watch videos
Watch the videos on this page about how to control knotweed by using herbicide injection.
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Take the assessment quiz
- At the end of the assessment there will be a link to the booking form. If not booking immediately, please save this link. If you lose the booking form, you can take the quiz again or email us at knotweed.injectors@kingcounty.gov.
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Reserve an injector & schedule times for pick up and return
You will schedule the three items below individually through one online booking form (you get the link to the form after taking assessment quiz in step 2 above). You should get a confirmation email for each item scheduled.
i.Reserve an injector (10-day reservation window)
ii. Schedule a time to pick up the injector*
iii. Schedule a time to return the injector**The location for injector pick-ups and returns is the Noxious Weed Control Program office located at King Street Center (201 South Jackson Street, Seattle, WA 98104). -
Get some herbicide (active ingredient: 41% glyphosate)
We recommend securing your herbicide product well before your pick-up date (limited supply in stores, anticipate ordering online or to store).
Herbicide Guidance and Sources - Knotweed Injection [link to PDF]
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Pick a nice day and get started
You have a 10-day window to inject your knotweed, but it won’t take you all 10 days. Set aside the best-weather days in this window to tackle your infestation.
• Don’t forget to take before, during, and after pictures (it’s gratifying to see the progress)!
• Be sure to have your personal protection gear & spill kit handy.
• Clean the injector after use and check to make sure all the parts are together. If you need a refresher on cleaning or anything else day-of, consult our injector training videos. -
Fill out the survey in your confirmation email
After using the injector, be sure to return it at the time you chose in step 3, 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. This is a good place to provide feedback about the program.
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All done for now!
You’re welcome to come back next season to treat regrowth as needed. Thank you for reducing the impact of knotweed in King County.
Frequently Asked Questions
Also known as "hollow stem injection" this method uses a specialized tool (injector gun) to deliver a small dose of concentrated herbicide (41% glyphosate) directly inside the hollow knotweed stem. The plant absorbs the chemical and it moves throughout its system, reaching the roots to the leaves. It is very effective and significantly decreases the potential for off target impacts but is time and resource intensive. It is most effective on larger stems and is a great option for chemical control at sensitive sites.
There are many considerations when it comes to knotweed injection, including the following:
- Physically demanding/access considerations: this method requires the applicator to be near or at ground level in the brush for an extended period (long enough to inject each individual stem of the infestation). If this does not sound doable, consider asking for support through friends/family, or hiring a contractor (email us for a list). Other methods like foliar spray may be easier to try depending on your site.
- Site location: do not use herbicides near water unless you are licensed applicator with an aquatic endorsement and have permission to apply from the state department of ecology. Read your product label for more information.
- Infest size: this method requires you to inject each individual stem and thus is best for sites where the knotweed is relatively sparse (<2500 stems per acre) and smaller than 1 acre in size. If your infest is larger/denser, consider foliar applications for its first treatment (can plan to inject in future seasons when appropriate).
- Timing: if your knotweed is too small (stems < ½ inch wide) or too woody (later season, needle won’t push through), consider other methods or waiting to borrow in the appropriate season July through October.
- Municipality/local regulations: King County regulations allow residents to control listed noxious weeds on properties they manage in accordance to the herbicide label. For those who reside in any city’s boundaries, the city’s municipal regulations take precedence. Check with your local government to check for any permitting needs or other regulations to be aware of around herbicide application on listed noxious weeds.
In some circumstances, another control method may be more appropriate – consult our Best Management Practices Guide or contact us for recommendations.
- Be sure to secure your herbicide ahead of time! Products that work for injection are getting harder to find, play it safe and get yours before picking up your injector.
- Read the label of the herbicide you will be using! Prepare yourself with the appropriate Personal Protection Equipment (i.e. gloves, eye protection, long sleeves) and be aware of any environmental/physical constraints.
- Know your boundaries! King County residents can find their parcel boundaries here: King County Parcel Viewer. Non-certified applicators may apply general use herbicides on their own properties. Treatments performed on land not owned by the applicator are considered commercial and require certification – even if no compensation is provided for the treatment.
- Have the knotweed injector video link nearby for easy reference on injection day! Familiarize yourself with the process before using the tool.
If possible, it’s best to let the canes dry and die in place (several weeks, weather dependent), then you can knock them down to compost on site or move into yard waste.
For more urgent timelines, please still wait at least 1 week after treatment to cut down canes. Be careful moving recently treated canes, canes contain herbicide and could cause unintentional damage to nearby vegetation. Let cut canes dry out on a tarp/non-soil area for a few days to dry out before putting into yard waste. Alternatively, non-dried canes can be bagged and put into the garbage.
You should not dig up the remaining knotweed. When given the time to work into the system, the herbicide should travel through the root system of the plants – leaving dead roots that can naturally compost back into the soil. If digging is necessary, wait at least 1 month if possible following treatment. If it is unavoidable, be prepared for concentrated regrowth at the digging site next season (new plants will grow from broken/living root fragments).
Wait to plant into the area at least 1 month following treatment, with the understanding that knotweed takes usually 2-5 treatments/seasons to be eradicated. While there is less every year, anticipate herbicide treatment in that area for the first couple years following initial treatment.