Non-native Invasive Freshwater Plants

Japanese Knotweed (Polygonum cuspidatum)

Technical Information


Japanese knotweed (Polygonum cuspidatum) is a perennial species with spreading rhizomes and numerous reddish-brown, freely branched stems. The plant can reach four to eight feet in height and is often shrubby. The petioled leaves are four to six inches long and generally ovate with an abrupt point. The whitish flowers are borne in open, drooping panicles. The plant is dioecious, so male and female versions of the inconspicuous flowers are produced on separate plants. The approximately 1/8 inch long fruits are brown, shiny, triangular achenes, (Hitchcock and Cronquist 1964; Hickman 1993).

Economic Importance

Japanese knotweed is a very aggressive species (Hitchcock and Cronquist 1964) that is capable of crowding out all other vegetation (Ahrens 1975); Hickman (1993) lists the species as a noxious weed. In addition, the plant can create a fire hazard in the dormant season (Ahrens 1975). Japanese knotweed is an escaped rnamental that is becoming increasingly common along stream corridors and rights-of-way in Washington. The species forms dense stands that crowd out all other vegetation, degrading native plant and animal habitat. This perennial plant is difficult to control because it has extremely vigorous rhizomes that form a deep, dense mat. In addition, the plant can resprout from fragments; along streams, plant parts may fall into the water to create new infestations downstream.

Geographic Distribution

As its name indicates, Japanese knotweed is a native of Japan (Hickman 1993). However, it has become naturalized in North America, where it is found from Newfoundland and many parts of the northeastern U.S. (Muenscher 1955), west to California (Hickman 1993) and the Pacific Northwest (Hitchcock and Cronquist 1964).


An escaped ornamental, Japanese knotweed is often found in waste places, neglected gardens, roadsides, and along streambanks (Muenscher 1955; Figueroa 1989).


A native of Asia, this species was introduced to England in 1825 for use as an ornamental (Patterson 1976). Japanese knotweed was subsequently introduced to the U.S. for use in ornamental hedges and for erosion control (Pridham and Bing 1975).

Growth, Development, and Reproduction

Japanese knotweed is a perennial plant. This species spreads by seed and by long, stout rhizomes (Muenscher 1955). However, colonies rarely establish from seed. Primary spread of the species is reported to be through mechanical movement of plant parts (Figueroa 1989).

Management Methods: See the information at these sites for current control methods.

Biocontrol Potential

No information is available