Native Range

Eastern Mediterranean and western Asia

Appearance

Diffuse knapweed is a winter-hardy forb that usually grows as a biennial but may at times grow as an annual or short-lived perennial. Plants grow 1–3½ ft (0.3–1 m) tall, often have numerous spreading branches, and are supported by a long taproot. Stems are ridged and covered in small hairs. Leaves are up to 8 in (20 cm) long, gray-green, woolly, and frequently deeply divided. Flower heads are produced at the tips of stems and branches. They are ≤S in (0.8 cm) in diameter, and have bracts fringed with spines, of which the terminal spine is distinctly longer. The florets are typically white but may occasionally be pink or lavender. Each floret produces a single brown seed topped by a tuft of very short, 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

Resources

• Program Aid - USDA APHIS PPQ
• Know Your Knapweeds - North Dakota State University
• Element Stewardship Abstract - The Nature Conservancy
• Weed Field Guide - USDA Forest Service

Selected Images

Maps

Invasive Listing Sources

• Alberta Weed Control Act
• Arizona Noxious Weeds
• California Invasive Plant Council
• California Noxious Weeds
• Colorado Noxious Weeds
• Idaho Noxious Weeds
• 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.
• Minnesota Noxious Weeds
• Montana Noxious Weeds
• 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 Mexico Noxious Weeds
• Nonnative Invasive Species in Southern Forest and Grassland Ecosystems
• North Dakota Noxious Weeds
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Utah Noxious Weeds - Control
• Washington Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States
• Wisconsin Noxious Weeds
• Wisconsin's Invasive species rule – NR 40
• Wyoming Noxious Weeds

References