Europe and western Asia
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.
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 (Centaureaspp.): 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