Native Range

East Asia

Appearance

Bohemian knotweed is a hybrid of Japanese and giant knotweed and has features intermediate between the two parents. However, not all Bohemian knotweed plants will look the same; sometimes they look closer to the Japanese knotweed parent, and other times they look more similar to the giant knotweed parent.

Bohemian, Japanese, and giant knotweeds are upright herbaceous perennials that typically grow 3.5–16 ft (1–5 m) tall. They have deep taproots as well as rhizomes that may extend up to 65 ft (20 m) laterally. The above-ground stems are 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. 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. Knotweed fruits each contain just one seed. Fruits/ seeds are papery and have three wings.

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
• Colorado Noxious Weeds
• Idaho Noxious Weeds
• Illinois Exotic Weed Act
• Indiana Invasive Species Council - Invasive Plant List
• Indiana Terrestrial Plant Rule
• Invasive Plant Species of West Virginia
• Minnesota Noxious Weeds
• Montana Noxious Weeds
• 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 York Regulated and Prohibited Invasive Species - Prohibited
• 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
• Washington Noxious Weeds
• Wisconsin Noxious Weeds
• Wisconsin's Invasive species rule – NR 40

References