Weed class: B
Year Listed: 1995
Native to: Asia
Is this Weed Toxic?:
not known to be
Why Is It a Noxious Weed?
It is a very aggressive escaped ornamental that is capable of forming dense stands, crowding out all other vegetation and degrading wildlife habitat. It can also create a fire hazard in the dormant season. It is difficult to control once established.
How would I identify it?
It is a perennial species with spreading rhizomes and numerous reddish-brown, freely branched stems. Plants can reach 4 to 8 feet in height and is often shrubby.
The whitish to whitish-green flowers are in drooping panicles (clusters) from leaf axils. Male and female flowers are on separate plants.
Leaves are alternately arranged with petioles (stalks) and are 4 to 6 inches long, ovate and have a truncated base and an abrupt tip.
Stems are upright, branching and deciduous.
Fruit Seed Description
The fruits are approximately 1/8 inch long, shiny brown and triangular.
May Be Confused With
Knotweed species resemble each other, Japanese knotweed is shorter than Bohemian knotweed or giant knotweed. If you need help with plant identification, please contact your county noxious weed coordinator.
Where does it grow?
It is often found in waste places, gardens, roadsides and stream and riverbanks. Please click here to see a county level distribution map of Japanese knotweed in Washington.
How Does it Reproduce?
It spreads by seed and by long stout rhizomes. Colonies rarely establish from seed.
How Do I Control It?
General Control Strategy
Mowing or cutting plant shoots is ineffective alone; however, mowing followed by herbicide treatments will provide some control. Methods must be repeated if infestation is very large. Care must be taken not to produce new plants. All plant material should be removed, dried and burned if possible. New plants can sprout from very small fragments.
Grubbing out small clumps when discovered can prevent new colonies from establishing. Rhizomes and fragments left in the ground, or nearby, can regenerate and spread infestations. The entire root system must be removed since re-sprouting can occur from rhizomes.
There are currently no registered biological control agents for use on any of these Polygonum species. Grazing may be an effective strategy to prevent establishment. Any grazing strategy should be carefully controlled to prevent damage in riparian areas.
For More Information
See our Written Findings for more information about Japanese knotweed (Polygonum cuspidatum).
Cowlitz County NWCB Fact Sheet on Japanese knotweed
Asotin County NWCB Fact Sheet on Japanese knotweed
Douglas County NWCB Fact Sheet on Japanese knotweed
Clark County NWCB Fact Sheet on Japanese knotweed
Whatcom County NWCB Fact Sheet on Knotweed spp.
Mason County NWCB Fact Sheet on Knotweed spp.
Jefferson County NWCB Fact Sheet on Knotweed spp.
Thurston County NWCB Fact Sheet on Knotweed spp.
King County NWCB Fact Sheet on Knotweed spp.
Control Options for Japanese knotweed from Lincoln County NWCB
Control options for Knotweed spp. from Whatcom County NWCB
Best Practices for Control of Knotweed spp. from King NWCB