Britt Slattery, USFWS

Japanese Knotweed
Polygonum cuspidatum

Origin: Eastern Asia

Background
Japanese knotweed was probably introduced into the United States in the late 1800's. It was first planted as an ornamental and has also been used for erosion control and landscape screening. Japanese knotweed is a noxious weed in the state of Washington.

Distribution and Ecological Threat
Japanese knotweed occurs across the continent from Maine to Wisconsin, south to Louisiana and in scattered locations in the Midwest and Western states. It can tolerate a variety of adverse conditions, including deep shade, high temperatures, high salinity and drought. Knotweed is commonly found near water sources, such as along streams and rivers, in low-lying areas, waste places and utility rights-of-way and around old home sites. It spreads 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.

Description and Biology

• Plant: a member of the buckwheat family (Polygonaceae), knotweed is an upright, shrubby, herbaceous perennial that can grow to over 10 feet in height. Stems of Japanese knotweed are smooth, stout and swollen where the leaf meets the stem. Like all members of the family, it has a membranous sheath surrounding the joints of the stem.
• Leaves: although variable, leaves are normally about 6 inches long by 3 to 4 inches wide, broadly oval to somewhat triangular and pointed at the tip.
• Flowers, fruits and seeds: minute greenish-white flowers occur in attractive, branched sprays in summer and are followed soon after by small winged fruits. Seeds are triangular, shiny and very small, about 1/10 inch long.
• Spreads: primarily by seed and by vegetative means with the help of 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 often escapes from neglected gardens and discarded cuttings.
• Look-alikes: Virginia knotweed (Tovara virginica), prince's feather (Polygonum orientale), and giant knotweed (Polygonum sachalinense, non-native).

Prevention and Control
Japanese knotweed is an extremely difficult plant to control due to its ability to re-grow from vegetative pieces and from seeds. Mechanical and chemical methods are most commonly used to eliminate it. Single young plants can be pulled by hand depending on soil conditions and root development. All roots and runners must be removed to prevent re-sprouting. Glyphosate and triclopyr herbicides have been used, applied either to freshly cut stems or to foliage.

Native Alternatives

sweet pepperbush (Clethra alnifolia) USFWS	Virginia sweetspire (Itea virginica) Britt Slattery, USFWS	maleberry (Lyonia ligustrina) R. Harrison Wiegand
silky dogwood (Cornus amomum) Chris Miller, NRCS	fragrant or shining sumac (Rhus aromatica or copallina) R. Harrison Wiegand

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Publication by USFWS BayScapes Conservation Landscaping Program
http://www.nps.gov/plants/alien/pubs/midatlantic/pocu.htm
Last updated: 7 October 2004