Knotweeds
Description
Knotweeds are a group of four closely related plants that belong to the buckwheat family (Polygonaceae). They are: giant knotweed (Polygonum sachilinense) Japanese knotweed (P. cuspidatum) bohemian knotweed (P. Xbohemicum) and Himalayan knotweed (P. polystachyum). All have similar behavioral and morphological characteristics. All are perennials with upright jointed bamboo-like stems and alternate, thick leaves. Stems are hollow and bamboo-like. Giant knotweed grows up to 12-feet tall, while Japanese knotweed grows four to nine-feet tall. Leaves of giant knotweed are heart-shaped and can exceed one foot in length; leaves of Japanese knotweed are much shorter and are oval to round with a flat base, versus the heart shaped base of giant knotweed. The flowers of Japanese knotweed are prominently winged and greenish-white, growing larger with maturity, while the flowers of giant knotweed are sparse, smaller and do not grow larger with maturity. Bohemian knotweed has leaf, flower, and height characteristics intermediate to Japanese and giant knotweed. Himalayan knotweed is not typically confused with the others; its leaves are lanceolate (long and pointed),10-20 cm long with bases that are slightly heart-shaped to tapered.Habitat
All are found along stream banks, in moist waste places, neglected gardens, roadsides, and railroad right-of-waysDistribution
Primarily west of the CascadesImpacts
Dominate and out-compete native plant species, prevent streamside revegatation in riparian areasDispersal Methods
Natural causes (flood, erosion) or man-made dispersal (road side clearing, fill dirt)Prevention
Look for knotweed species along roadsides, stream banks, and in other disturbed areas. Use native alternatives in gardens; never dig up and attempt to transplant knotweed species from home gardens. Also, do not dispose of knotweed garden waste in natural areas, as plants can revegetate from stem and root fragments and rhizomes.Listings
ODAFactsheets
Washington State Noxious Weed Control Board Fact Sheet on Giant Knotweed
Washington State Noxious Weed Control Board Fact Sheet on Japanese Knotweed
Washington State Noxious Weed Control Board Fact Sheet on Himalayan Knotweed
Photos
Photos of Japanese Knotweed
Photos of Himalayan Knotweed
Identification Help
Other Links
Japanese Knotweed Profile from Oregon Department of Agriculture Noxious Weed Control Program
Document on Controlling Knotweed from The Nature Conservancy
Washington State Department of Agriculture Document on Knotweed Eradication