Native Range

Europe and western Asia

Appearance

Spotted knapweed is a winter-hardy herbaceous weed that typically grows as a perennial or (less commonly) as a biennial. Plants grow 1–3½ ft (0.3–1 m) tall, often have numerous spreading branches, and are supported by a deep taproot. Stems are ridged and covered in small hairs. Leaves are up to 8 in (20 cm) long, gray-green, woolly, and deeply divided near the plant base. Flower heads are produced at the tips of stems and branches. They are ≤½ in (1¼ cm) in diameter, have rows of fringed bracts with dark tips, and have several lavender or pink florets. Each floret produces a single brown seed topped by a tuft of bristly hairs. This plant reproduces only by seed.

Impact

Knapweeds currently infest millions of acres across North America, primarily occurring in rangelands, open forests, pastures, roadsides, vacant lots, and other disturbed areas. They are responsible for millions of dollars of damage annually in reduced agricultural yields and lowered forage value in grazing systems. Knapweeds also displace native vegetation, negatively impacting wildlife and threatening the delicate ecological balance within many habitats. Allelopathic chemicals have been isolated from North American knapweeds, though their role in plant competition is currently under debate.

Reference

Randall, C.B., J.E. Andreas, and J. Milan. 2023. Knapweeds (Centaurea 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-2023-34- KNAPWEEDS-P. https://bugwoodcloud.org/resource/files/27450.pdf

Selected Images

Maps

Invasive Listing Sources

• Alberta Weed Control Act
• Arizona Noxious Weeds
• California Invasive Plant Council
• California Noxious Weeds
• City of Ann Arbor Michigan Parks and Recreation
• Colorado Noxious Weeds
• Connecticut Invasive Plant List
• Connecticut Invasive Plant Working Group
• Delaware Invasive Species Council
• Delaware Invasive Species Council Invasive Species List
• 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.
• Invasive Plant Association of Wisconsin
• Invasive Plant Species of West Virginia
• Jil M. Swearingen, Survey of invasive plants occurring on National Park Service lands, 2000-2007
• John Randall, The Nature Conservancy, Survey of TNC Preserves, 1995.
• Kentucky Exotic Pest Plant Council - Significant Threat
• Maryland Cooperative Extension Service.  2003. Invasive Plant Control in Maryland. Home and Garden Information Center, Home and Garden Mimeo HG88. 4 pp.
• Maryland Invasive Species Council - Invasive Species of Concern in Maryland
• Massachusetts Noxious Weeds
• Massachusetts Prohibited Plant List
• Michigan Noxious Weeds
• Minnesota Noxious Weeds
• Missouri Noxious Weeds
• Missouri SRISP Species List
• Nebraska Noxious Weeds
• Nevada 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 Jersey Invasive Species Strike Team 2017 Invasive Species List
• New Mexico Noxious Weeds
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• North Dakota Noxious Weeds
• Ohio Invasive Species Council
• Ontario’s Invading Species Awareness Program Tracked Species List
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Plant Invaders of Mid-Atlantic Natural Areas
• Tennessee Exotic Pest Plant Council
• Virginia Invasive Plant Species List
• 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
• Wyoming Noxious Weeds