King County working with landowners, agencies to protect river banks from invasive knotweed

Free workshops offered for residents living in affected watersheds

The battle is on to protect some of King County’s highest quality riverside habitat from knotweed, a tough, invasive plant.

With help from state and federal funding, the cooperation and support of many landowners and public land managers the King County Noxious Weed Program is leading a cooperative effort to get control of this fast-spreading invasive weed.

“Knotweed spreads when floods move root fragments downstream, so control involves starting at the top of the waterway, then moving downstream and systematically removing all knotweed in the floodplain,” said Steven Burke, manager of the noxious weed program.

“Removing knotweed from the riverbanks and adjacent floodplains reverses the harm this plant has done to the fish and wildlife habitat along many rivers,” Burke said.

Free public workshops in Renton, Covington and North Bend will provide landowners with detailed information on how to remove knotweed from their property. The meetings are scheduled for:

• June 24, 7-8:30 p.m. - at Maplewood Greens, 4050 Maple Valley Hwy, Renton, hosted by the Cedar River Council;
• July 9, 7-8:30 p.m. – at Meadowbrook Farm Interpretive Center, 1711 Boalch Ave, North Bend. This session will also include control methods for other invasive weeds of concern to Snoqualmie River basin landowners;
• July 15, 7-8:30 p.m. – at the Covington Library, 27100 164th Ave. S.E., Covington.

King County’s efforts to remove this invasive plant began in 2004 along the upper and middle reaches of the Green River. The battle then moved to the South Fork Skykomish, the South and Middle forks of the Snoqualmie, and, beginning this year, the Cedar River. The county is also working closely with the Mountains to Sound Greenway Trust to begin knotweed control projects on the Raging River and Issaquah Creek.

Unfortunately, knotweed has had a huge head start and has been able to spread unabated on many county rivers.

“Knotweed doesn’t stop at property lines and invades some of the most sensitive and inaccessible areas.  Stopping this plant is no simple matter and will require the cooperation of many public and private landowners working together for many years,” Burke said.

Results can already be seen on the upper and middle Green River where knotweed has been reduced to small, scattered patches, rather than the continuous large patches that had existed previously.

Burke said much work remains on other rivers where the eradication program has begun, but progress is evident.

“There is strong evidence that this kind of systematic, strategic control plan can reverse the damage done by knotweed,” he said.

Monica Walker, the noxious weed program’s knotweed project manager, said the big invasive knotweed species, including Japanese, giant, Bohemian and Himalayan, are collectively one of the biggest invasive weed threats to the health of the county's rivers.

“Traditional weed control methods often fail to eliminate invasive knotweed and often only increase the problem by spreading it more,” she said. “Mowing along roads or riversides just increases the population, while pulling the roots out leaves fragments that re-sprout or, worse, moves the knotweed to new locations.”

Instead, knotweed is controlled by injecting an herbicide directly into the plant’s hollow stems or by reducing the roots over many years by covering or other methods.

The noxious weed program regularly receives calls from homeowners who have been unable to get rid of knotweed on their property in spite of years of hard work, said Sasha Shaw, noxious weed program education specialist.

“Homeowners are often frustrated by the massive roots of the knotweed and its ability to regenerate year after year despite their best efforts,” she said.

The noxious weed program's knotweed projects address the highest priority infestations, including those that are directly in upper watershed shorelines and floodplains.  Knotweed growing in people's backyards, along roadsides, in parks and elsewhere in the county, is beyond the scope of these projects.

The upcoming knotweed workshops will teach participants why knotweed is so invasive and hard to control, and offer practical information on how to effectively get rid of knotweed.

Class participants will get to see a knotweed stem injector in action, and learn how this tool can be used in an overall strategy to eliminate knotweed. Workshop participants will also learn what is being done to fight knotweed in each watershed, and what work remains.

The noxious weed program receives grant funding from the King Conservation District, Washington State Department of Agriculture, US Fish and Wildlife Service, and US Forest Service that can be used to control knotweed along the entire course of a river, regardless of property ownership.

For more information, visit www.kingcounty.gov/weeds or call the noxious weed program at 206-296-0290.