Himalayan knotweed identification and control
Persicaria wallichii, Buckwheat family
Himalayan knotweed (Persicaria wallichii) is an aggressive competitor in the Buckwheat Family that was introduced from Asia as an ornamental. It is a clone-forming perennial, growing up to 6 feet high, with reddish stems and leaf stalks. It tends to grow in dense, mounded clumps. The long, tapering leaves are 4 to 8 inches long, with brown, persistent sheaths at the bases of the leaf stalks. The flowers are white to pink, and occur in loose, branched clusters. Stems are solid not hollow like the other invasive knotweeds on the noxious weed list. It spreads by creeping underground rhizomes and by seed.
Himalayan knotweed will grow on most soil types, but does require some moisture. It grows in both sunlight and partial shade. It is not as common as other invasive knotweeds in King County, but where it occurs it creates dense growth that excludes other vegetation and can impact riparian habitat.
Legal status in King County, Washington
Public and private landowners are not generally required to control infestations of Himalayan knotweed that occur on their property in King County, Washington, except in selected areas on the Green River and its tributaries and on the Cedar River and its tributaries, as described on the King County Weed List. Himalayan knotweed is a Class B Noxious Weed in Washington, first listed in 2003. It has not been designated for required control in the county by the Washington State Noxious Weed Control Board, but it has been selected for required control in limited parts of the county by the King County Noxious Weed Control Board. Because control is not generally required in the county, it is on the list of Non-Regulated Noxious Weeds for King County. For more information, see Noxious Weed Lists and Laws or visit the website of the Washington State Noxious Weed Control Board.
Himalayan knotweed is on the Washington quarantine list (known as the prohibited plants list) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington. For more information, see Noxious Weed Lists and Laws.