Eastern Mediterranean and western Asia
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.
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.
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