Large leaf wetlands plant
It looks like knotweed--an really nasty invasive, noxious weed. Yhere are several varieties in our area. It is recommended that you do NOT try to pull it out, because that just encourages growth. This is actually the best time of year to attempt eradication of Knotweed. This article gives more information. If you do choose to use an herbicide, be sure to FOLLOW ALL DIRECTIONS ON THE LABEL EXACTLY!.
In King county, control of Bohemian, Japanese, giant and Himalayan knotweed is required on the Green River and its tributaries upstream of the Auburn City Limits and on the Cedar River and its tributaries upstream of the Renton City Limits. If you have any questions about this, contact your local Noxious Weed Control Program, send an email, or call 206-477-WEED (206-477-9333)