Knotweed is such a fast-growing invasive species, and some infestations so extensive that all natural resources managers can do is control the spread. The plants can grow 15 feet tall and crowd out native vegetation needed for quality fish and wildlife habitat.
“During the fall, knotweed plants transfer significantly more nutrients from the leaves to the roots than do native riparian plants. When they drop their leaves, fewer nutrients are available for bacteria, insects, and fish,” said Andrew McDonnell, Sauk-Suiattle natural resources field coordinator. “Knotweed has no value for wildlife and is detrimental to salmon.”
The Nature Conservancy tracked and managed knotweed in the Sauk watershed from 2001 until the Skagit Fisheries Enhancement Program took leadership. Sauk-Suiattle joined the project in 2011, supporting knotweed eradication with McDonnell as field coordinator and a crew from the Washington Conservation Corps.
Each year, the crew spends six weeks surveying a 3-square-mile area spanning 11 river miles. When they find new patches, they spray the leaves with an herbicide that is approved for aquatic use. They also note where previously treated patches of knotweed have died, and where the plants still survive.
Since 2001, 609 knotweed patches have been documented between the mouth of the Sauk River and its confluence with the Suiattle. Last summer alone, surveyors found 130 new patches and 233 that were dead.
“Thirty-eight percent of the knotweed patches in the project area have been eliminated,” McDonnell said. “Every year we have to go back and check on the ones that were dead to make sure they haven’t come back. We try to visit every patch that’s known.”