Originally introduced to the United States from Asia in the late 1800s, knotweed has since spread across the country, and is now present throughout the Pacific Northwest. The main species of invasive knotweed are Bohemian knotweed (Polygonum x bohemicum), Japanese knotweed (P. cuspidatum), and giant knotweed (P. sachalinense). All invasive knotweed species are Washington State Class B Noxious Weeds. Control is required in Snohomish County.
Knotweed is a shrub-like plant that grows in dense patches 6-12 feet tall. Its stems are smooth and hollow, resembling bamboo. Leaves are large, bright green and spade-shaped, with smooth edges. Small, whitish-green flowers appear in upright sprays during July and August. Although knotweed is dormant in winter, the dry, reddish-brown stems often remain standing. Knotweed is most often found in moist, open areas like stream banks and roadway ditches, but can also be present in forests, parks, backyards, and farms.
A massive root system and rapid, early spring growth allows knotweed to effectively outcompete most other plants. Root systems can spread 20 feet from the parent plant and penetrate up to 7 feet into the soil. Root and stem fragments as small as ½” can form new plant colonies. Cut or broken stems or roots will resprout if left in moist soil or water. Most often, knotweed spreads when roots or stems are moved by seasonal floods or contaminated soil.