Knotweed is a highly invasive plant that takes over riverside habitats. That’s why King County is working with landowners and agencies to weed it out.

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 an effort to control this fast-spreading 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 county’s 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 added.

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

King County’s efforts to remove knotweed 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.

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

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’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.

The noxious weed program receives grant funding from the King Conservation District, Washington State Department of Agriculture, U.S. Fish and Wildlife Service, and U.S. Forest Service. This funding allows the county to control knotweed along the entire course of a river, regardless of property ownership.

Knotweed workshop

A free workshops on knotweed removal hosted by the Cedar River Council is June 24, 7-8:30 p.m. at Maplewood Greens, 4050 Maple Valley Highway, Renton.

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