Eastern Forest Threat Center - Japanese Knotweed

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Japanese knotweed flowers

Japanese knotweed flowers

Leslie Mehrhoff, University of Connecticut, Bugwood.org

Japanese Knotweed
Polygonum cuspidatum

Japanese knotweed is native to eastern Asia. It can quickly become an invasive pest in natural areas after escaping from cultivated gardens.

Keywords: Polygonaceae, upright, herbaceous, perennial, membranous sheath, minute flowers, branched sprays, winged fruits, rhizome; Common names: Mexican bamboo

Distribution Map Distribution Source Image

Threat Description

Japanese knotweed is an upright herbaceous perennial that can grow to over 10 ft. in height. Stems are smooth, stout, surrounded by a membranous sheath at the base above each joint, and swollen at joints where the leaf meets the stem. Leaves are broadly oval to somewhat triangular and pointed at the tip. Minute greenish-white flowers occur in branched sprays in summer, followed by small winged fruits. Japanese knotweed reproduces primarily by its triangular, shiny seeds and by vegetative means with its long, stout rhizomes. It can be transported to new sites by water, wind, as a contaminant in fill-dirt, or on the soles of shoes. It tolerates a variety of adverse conditions including full shade, high temperatures, high salinity, and drought. It is found near water sources, in low-lying areas, waste places, utility rights-of-way, and around old home sites, spreading quickly to form dense thickets that exclude native vegetation and greatly alter natural ecosystems. Japanese knotweed poses a significant threat to riparian areas, where it can survive severe floods and rapidly colonize scoured shores and islands. Once established, populations are extremely persistent.