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2040 84th Avenue SE
Mercer Island, WA 98040
Phone: 206.275.7609
Fax: 206.275.7868
Email: miparks@mercergov.. . .
Hours: 8:30 am - 5:00 pm
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Controlling Invasive Knotweed

Knotweed - CLICK for more photos

What Is Knotweed?

Knotweed refers to several species of tall, perennial, invasive plants from Asia that have overrun numerous streamside and roadside areas in western Washington. Knotweed can be identified by its long, hollow, bamboo-like stalks, and large smooth-margined leaves. Sprays of white flowers are visible in late summer.

Knotweed spreads aggressively, and it can severely degrade wildlife habitat, push out native plants, and increase erosion. If it is mowed, dug up, or cut, stray fragments will start new plants. Because it grows rapidly to between 6 and 15 feet tall, it can dangerously reduce visibility on roads. Its spreading rhizomes can even grow up through asphalt, damaging driveways and parking areas!


What Is The City Doing About Knotweed?

Parks Management staff have been eradicating knotweed in parks for several years; now they are turning their attention to City rights-of-way and roadside areas. New official signage posted around the Island alerts residents to the plant's presence in public areas, and to upcoming treatment plans.

Click here to see a map of current knotweed populations on Mercer Island.

Question: What are the signs for?
They help road maintenance crews to avoid mowing plants until they are mature enough to treat with herbicide. Even small fragments from mowing, cutting, or digging can grow into new plants! The signs also help spread the word about knotweed to residents

Question: How will the City treat knotweed?
On City property and road rights-of-way, City staff will carefully inject the stems with herbicide in late summer. This method and timing are the safest and most effective treatment for such a vigorous invasive. Note that the City cannot treat plants on private property.
Question: How long will this process take?
Late summer herbicide injection kills most of this weed within a few weeks, but management staff will visit populations for several years to treat any regrowth.

2019 Workshop Dates and Locations

All sessions are from 6:30 p.m. to 8:30 p.m. and share the same agenda.

These workshops teach participants:

  • How invasive knotweed grows and spreads
  • How, when and where to control knotweed
  • What is being done to fight knotweed on rivers in King County
  • How to use and borrow a stem injector for use on your own land
  • What opportunities for habitat restoration may be available for your own land

Class participants can borrow King County’s knotweed stem injectors for their own knotweed projects!

The workshops are free and open to the public, but have limited capacity, so sign up early!

How to sign up: Register online

knotweek sign

What Can I Do About Knotweed?

If you know about unmarked populations on Mercer Island, or have questions about the weed and its control, please contact our City staff:

Alaine Sommargren, Natural Resources Manager
(206) 275-7879
The City cannot treat plants on private property, but homeowners are encouraged to treat populations on their property with the assistance of experienced landscape professionals. For more information or assistance in developing a plan to treat this invader, homeowners can contact one of these agencies:
Phone: (206) 477-9333
Phone: (253) 445-4657
Email: (IWCP Director)

Additional Information & Resources

Washington State Noxious Weed Control Program 
King County Noxious Weed Knotweed Page (Lots of useful photos)
Washington State Department of Agriculture Knotweed Eradication Page



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