Help, my knotweed is a mile high and I don’t know what to do!

It can feel a little overwhelming to confront a full grown patch of knotweed.  June is the month knotweed, also known as false bamboo, reaches its full height and starts to think about flowering.  With lots of rain and sunshine this spring, this plant is having a great year and is looking even more invasive than usual. You might be tempted to just attack it with a machete and chop it all down to get some relief. Before you grab the cutting tools, you might want to do some research to make sure this is the best approach for your site.

Knotweed on the Sandy River in Oregon. Photo courtesy of Jason Dumont, The Nature Conservancy.

Knotweed control is all about the roots. Although it can make seeds, most of knotweed’s spread is from root and stem fragmentation.  And because of its massive underground root reserves, eradicating knotweed means trying to destroy the roots or use up that root energy without allowing the plant to replenish its roots.

Digging up a huge knotweed stand with a small shovel is probably not the most sensible approach.

There are several ways to approach this goal depending on what you are comfortable doing, where the knotweed is growing, and how close you are to water, honeybees and desirable plants and trees.

Also, knotweed has a very regular annual cycle of upward growth, flowering, energy storage and going dormant. Effective control means disrupting this cycle at the right time of year to get the best results. No matter what you choose to do, getting the timing right for the method is critical.

King County Noxious Weeds has a team of people and a wealth of online resources to help you solve your knotweed problems. Our riparian team controls knotweed on many rivers in King County and also educates people about how to control knotweed. Our website has resources on control methods including do-it-yourself videos, detailed Best Management Practices and a simpler fact sheet.

If you would like to learn how to control knotweed in a classroom setting and find out how to use a knotweed stem injector, we have one more homeowner knotweed workshop coming up on August 18 in North Bend.

If you attend one of our knotweed workshops or take our online quiz after watching the stem-injector training videos, you can qualify to borrow a knotweed stem injector for your own use. Contact Sayward Glise to find out more about this opportunity.

The stem-injection method is a highly selective and effective method for controlling knotweed.

If you live on a river in King County, there is a good chance that our riparian team or another organization will be coming your way to offer to control the knotweed on the riverbanks. You can find out more about our knotweed projects by contacting Justin Brooks at King County.

King County Noxious Weed Control Program maps and controls knotweed along many county rivers in order to protect habitat and improve water quality.


    • Our program doesn’t have any lake projects for knotweed right now. Sorry about that! Send us an email though and we can help you sort out the best options for you.

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