Native Range

Eastern Europe and Western Asia

Appearance

Squarrose knapweed is a long-lived perennial forb. 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 ≤¼ in (≤2/3 cm) in diameter, and have bracts fringed with spines, of which the terminal spine curves distinctly backward. The florets are pink or lavender. 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

Resources

• Weed Field Guide - USDA Forest Service

Selected Images

Maps

Invasive Listing Sources

• California Noxious Weeds
• Colorado Noxious Weeds
• Idaho Noxious Weeds
• Nevada EDRR
• Nevada Noxious Weeds
• New Invaders of the Northwest
• Oregon Noxious Weeds
• Pacific Northwest Exotic Pest Plant Council, 1998
• Utah Noxious Weeds - Control
• WeedUS - Database of Plants Invading Natural Areas in the United States

References