Native Range

East Asia

Appearance

Giant knotweed is an upright herbaceous perennial that grows up to 16 ft (1m) tall. It has deep taproots as well as rhizomes that may extend up to 65 ft (20 m) laterally. The above-ground stems are pale green, hollow, smooth, and often woody at their base. The nodes are jointed, swollen, and have papery sheaths (ocrea). The leaves are alternately arranged along the stem, 6–12+ in (15–30+ cm) long and 4–10 in (10–25 cm) wide. They have a heart-shaped base, tapering tip, somewhat wavy margins, and long, fine, wavy hairs on their undersides. Knotweed flowers are small, greenish to creamy-white, and have five petal-like sepals that are fused at the base, forming a narrow tube. Flowers grow in branched clusters from leaf axils near stem tips. Giant knotweed flower clusters are typically up to 4 in (10 cm) long, and the leaf immediately beneath each flower cluster is always much longer than the flower cluster. Knotweed fruits are papery and have three wings and one seed.

Impact

Knotweeds are a major concern for displacing native species in riparian areas and in native and commercial forests. Knotweeds have demonstrated allelopathic tendencies where they release compounds into the soil that inhibit the growth of other plant species. They alter nutrient cycling in soil and streams, disrupt food chains, and can exclude trees, which is potentially detrimental to fish and other stream inhabitants that benefit from tree shade. Their lack of fine surface roots can lead to increased erosion, which is problematic along waterways. Their forceful rhizomes are capable of cracking concrete and asphalt, thus causing costly damage to roadways, parking lots, and building foundations.

Reference

Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Reynoutria spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. https://bugwoodcloud.org/resource/files/23198.pdf

Selected Images

Maps

Invasive Listing Sources

• Alberta Weed Control Act
• California Invasive Plant Council
• City of Ann Arbor Michigan Parks and Recreation
• Colorado Noxious Weeds
• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Georgia Exotic Pest Plant Council - Category 4
• Hoffman, R. & K. Kearns, Eds. 1997. Wisconsin manual of control recommendations for ecologically invasive plants. Wisconsin Dept. Natural Resources, Bureau of Endangered Resources. Madison, Wisconsin. 102pp.
• Idaho Noxious Weeds
• Illinois Exotic Weed Act
• Indiana Invasive Species Council - Invasive Plant List
• Indiana Terrestrial Plant Rule
• Invasive Plant Species of West Virginia
• Jil M. Swearingen, Survey of invasive plants occurring on National Park Service lands, 2000-2007
• Kentucky Exotic Pest Plant Council - Watch List
• Maryland Department of Natural Resources Policy: Restriction on Planting Exotic Invasive Plants
• Mid-Atlantic Field Guide to Aquatic Invasive Species
• Minnesota Noxious Weeds
• Montana Noxious Weeds
• National Park Service, Mid-Atlantic Exotic Plant Management Team Invasive Plant List
• National Park Service, National Capital Region Exotic Plant Management Team Invasive Plant List
• Native Plant Society of Oregon, 2008
• Nebraska Noxious Weeds
• New Hampshire Invasive Species Committee. 2005. Guide to Invasive Upland Plant Species in New Hampshire. New Hampshire Department of Agriculture,  Markets and Food Plant Industry Division and New Hampshire Invasive Species Committee.
• New Hampshire Prohibited Invasive Species
• New Jersey Invasive Species Strike Team 2017 Invasive Species List
• New York Regulated and Prohibited Invasive Species - Prohibited
• Non-Native Invasive Plants of the City of Alexandria, Virginia
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• Ohio Invasive Species Council
• Ontario’s Invading Species Awareness Program Tracked Species List
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Pennsylvania Department of Conservation and Natural Resources Invasive Plants
• Pennsylvania Noxious Weeds
• Pennsylvania's Field Guide to Aquatic Invasive Species
• Rhode Island Natural History Survey
• To be proposed for MISC
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States
• West Virginia Invasive Species Strategic Plan and Volunteer Guidelines 2014
• West Virginia Native Plant Society, Flora West Virginia Project, and West Virginia Curatorial Database System, September 3, 1999
• Wisconsin Noxious Weeds
• Wisconsin's Invasive species rule – NR 40

References