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Acroptilon repens

Russian knapweed

Photographer: Norman E. Rees
Source: USDA Agricultural Research Service,


Noxious perennial to 1 meter tall, with dark, creeping rhizomes. Plants exhibit allelopathic effects and are aggressively competitive, facilitating rapid colonization and development of dense stands. Infestations can be extremely long-lived due to extensive root and rhizome systems. Stems dieback after flowering in summer, and new shoots are generated in spring. Introduced from Central Asia. Like yellow starthistle (Centaurea solstitialis L.], Russian knapweed is toxic to horses, causing nigropallidal encephalomalacia or "chewing disease" when sufficient quantities are consumed. Under most circumstances livestock will avoid grazing Russian knapweed because of its bitter taste.

Other white, pink, and purple-flowered knapweeds in the genus Centaurea and bearded creeper (Crupina vulgaris Cass.) are most easily distinguished by their lack of dark, spreading rhizomes and by phyllary and achene characteristics. In addition, only bearded creeper has leaf margins with stiff hairs barbed at the tips (glochidiate hairs) and flower receptacles with flattened, scale-like, chaffy bracts.

Photographer: John M. Randall, The Nature Conservancy,

Close-up of flower. Note that the leaves are a different shape and color than the leaves lower on the plant (see next photo), and that the lower and middle involucral bracts are broader and more rounded then the upper ones, which have a hairy tip.

Photographer: Bonnie Million, Bureau of Land Management,

Lower leaves. Note leaves are more lobed the lower on the stem they are.