Japanese Knotweed

Japanese Knotweed

Japanese knotweed (Polygonum cuspidatum: Polygonaceae) is an invasive species that has established numerous populations in New River Gorge National River. Originally from eastern Asia, Japanese knotweed was introduced to North America in the late 1800s as an ornamental. It then escaped from cultivation and is now well established in the northeast and mid-Atlantic regions (Gleason and Cronquist 1991). Japanese knotweed is found in 36 of the 50 states (USDA Plants Database).

Japanese knotweed is a shrub like herbaceous perennial and can grow as tall as 10 feet (3m). Leaves are simple, alternate and average 3-6 inches long by 2-5 inches wide (8-15 cm x 5-12 cm). Like other members of the Polygonaceae family, Japanese knotweeds have the characteristic sheathed nodes (ocrea) (Gleason and Cronquist 1991). The main stem of the plant is hollow and is sometimes compared to a bamboo stem.

Japanese knotweed primarily reproduces vegatatively from the rhizomes (underground stems) or shoot fragments. Seeds are also produced and distributed via wind, water, and birds. Individual plants contain either male or female parts, or are dioecious. Small greenish-white flowers bloom in mid-late summer (Huebner et al. 2007).

Japanese knotweed can quickly spread into dense thickets along waterways, wetlands, roadsides, hillsides, and disturbed areas. A variety of soil types and pH levels can be tolerated by Japanese knotweed, but it has a preference for wet habitats and is shade intolerant (Huebner et al. 2007). A similar native species is Virginia knotweed (Polygonum virginianum) which differs because it is not a shrub and the ocreae have bristles (Huebner et al. 2007).

Japanese knotweed poses a great threat to the ecological systems of New River Gorge National River, Gauley National Recreation Area, and Bluestone National Scenic River. Populations of Japanese knotweed are currently established along the New River and Gauley River, but not on the Bluestone River. The Early Detection List for the Bluestone National Scenic River includes Japanese knotweed (https://science.nature.nps.gov/im/units/ermn/).

To counteract the threat posed by Japanese knotweed, the Invasive Pests Management Team tries to control large patches of Japanese knotweed with chemical herbicides and manual cutting. Foliar sprays and cut stump treatments with glyphosate are used to kill Japanese knotweed.

Literature Cited
Gleason, H.A. and A. Cronquist.. 1991. Manual of Vascular Plants of Northeastern United States and Canada. 2 ed. Bronx (NY): The New York Botanical Garden Press, p. 139.

Huebner, C.D., C. Olson, and H.C.Smith. 2007. Invasive plants field and reference guide: an ecological perspective of plant invaders of forests and woodlands.

USDA Natural Resources Conservation Service PLANTS database. PLANTS profile: Polygonum cuspidatum Siebold & Zucc. Japanese knotweed. Available from https://plants.usda.gov/java/nameSearch. (Accessed 25 March 2013).

Last updated: February 26, 2015

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