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Russian knapweed (Acroptilon repens, Rhaponticum repens) is a bushy rhizomatous perennial, up to 8 dm tall. Stems and leaves are finely arachnoid-tomentose becoming glabrous and green with age. The rosette leaves are oblanceolate, pinnately lobed to entire, 2–3 cm wide by 3–8 cm long. The lower cauline leaves are smaller, pinnately lobed; the upper leaves become much reduced, sessile, serrate to entire. The heads are numerous terminating the branches. Flowers are pink to purplish, the marginal ones not enlarged. The outer and middle involucral bracts are broad, striate, smooth with broadly rounded tips; the inner bracts are narrower with hairy tips. Pappus present with bristles 6–11 mm long. Fruit is a whitish, slightly ridged achene.
Russian knapweed is a deep-rooted long lived perennial. Some stands have been in existence for 75 years. It forms dense colonies in cultivated fields, orchards, pastures, and roadsides.
A native to Eurasia, Russian knapweed was introduced into North America in the late 19th century. Absent only from southeastern U.S., it has become widespread in other regions, especially in the western United States.
Russian knapweed is the sole member of the monotypic genus Acroptilon. The genus name derives from acro- (high) and ptilo- (feather). Although some sources include this species in the genus Centaurea, molecular phylogeny results, the structure of the flower, and the chromosome number support recognizing it as a separate genus.
- "The Plant List: A Working List of All Plant Species". Retrieved 3 October 2014.
- "Acroptilon repens". Flora of North America.
- "Acroptilon". Flora of North America.