Oregon State University's Solve Pest Problems page has some great information about how to get rid of knotweeds Solve Pest Problems
This is a tricky weed to manage. Consider reaching out to your local Soil & Water Conservation District for technical advice.
Location
along cedar creek, at the back of my property
Expert Reviewer
Samuel Leininger Clackamas Soil and Water Conservation District 4-County Cooperative Weed Management Area Columbia Gorge Cooperative Weed Management Area