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Beating Back Knotweed on the Sauk River

Searching for knotweed in the shadow of Whitehorse Mountain      © TNC

Knotweed is a nasty invasive weed that invades river banks and gravel bars and chokes out other vegetation. But we’re beating it back on the Sauk River, east of Mount Vernon, where Conservancy volunteers and staff and a Washington Conservation Corps crew spent three days hiking and rafting the river and eradicating patches of the noxious plant. They know they’re making progress: Where they found 200 patches last year, this year they found only about 20.

The Sauk River is a major tributary to the Skagit River, and an important salmon stream. The Nature Conservancy is working with partners and local landowners to eradicate knotweed on the Sauk, working from just above Bedal, which is the highest point where human development is present, to the confluence with the White Chuck River. Eradication efforts start high in the river system to control knotweed, then move downstream, so that cleared areas are not recolonized from upstream patches.

Searching a gravel bar for knowtweed.  © TNC

In 2006, Melisa Holman, the Conservancy’s stewardship manager for the upper Skagit, led a survey trip by raft down that stretch of river that identified about 200 patches of knotweed. “We stopped at every exposed gravel bar and backwater on the river,” Holman said. Each location was identified by GPS. She led a second trip down to treat every patch of knotweed.

This year, at the end of June, Holman was able to combine the survey and treatment trip into one swoop down the river. With rafts and guides from Chinook Expeditions, Holman, the crew from Washington Conservation Corps, and volunteers Ann Schaechtel and Darren Kavanagh, they revisited every location marked last year, and searched for new patches of knotweed. “The difference was dramatic,” Holman said.

Knotweed thicket on the riverbank.  © TNC

Why such a big effort? Knotweed invades riparian areas and gravel bars, and spreads quickly to out-compete other vegetation. Where the knotweed grows, you will not find the forested riverbanks that are so crucial to salmon health. Trees keep the river cool by offering shade, and fallen trees provide plenty of habitat for young salmon.

The Nature Conservancy is also engaged in battling knotweed in the lower Chehalis and Black river systems near Olympia, as well as fighting invasive plants across the globe. For more on our efforts against invasives, check out the Conservancy's Invasive Species Initiative.