Creeping roots usually dark brown or black, with scaly adventitious buds. Stems ± cobwebby-tomentose. Leaves: basal and proximal cauline often deciduous by flowering, blades oblong, 4-15 cm; mid and distal linear to linear-lanceolate or oblong, 1-7 cm. Involucres 9-17 mm, loosely cobwebby. Phyllaries: apices of inner acute or acuminate, densely short-pilose. Corollas 11-14 mm, tubes 6.5-7.5 mm, throats 2-3.5 mm, lobes 3-3.5 mm. Cypselae ivory to grayish or brown, 2-4 mm; pappus bristles white, 6-11 mm. 2n = 26. Flowering late spring-summer (May-Sep). Fields, roadsides, riverbanks, ditch banks, clearcuts, cultivated ground; 0-2300 m; introduced; Alta., B.C., Man., Ont., Sask.; Ariz., Calif., Colo., Idaho, Iowa, Kans., Minn., Mont., Nebr., Nev., N.Mex., N.Dak., Okla., Oreg., S.Dak., Tex., Utah, Wash., Wyo.; Mexico (Baja California); c Asia. Acroptilon repens has been reported also from Arkansas, Illinois, Indiana, Kentucky, Louisiana, Michigan, Missouri, New York, Ohio, Virginia, and Wisconsin; I have not seen specimens from those states.
Acroptilon repens is a serious weed pest, especially in the western United States. It is a strong competitor in infested areas, often forming dense colonies, and has allelopathic effects on other plants growing nearby. It is very difficult to control or eradicate once it becomes established. It reproduces vigorously from seed and spreads from adventitious buds borne on deep-seated runner roots. Root fragments readily regenerate as new individuals after cultivation. In addition, Russian knapweed is very poisonous to horses, causing neurological symptoms. Because of its bitter taste, it is usually avoided by grazing animals, and consequently it tends to spread when more palatable plants are consumed.
Coarse, bushy-branched, colonial perennial from deep- seated creeping roots, 4-8 dm, finely arachnoid-tomentose, becoming glabrate; lvs rather small, the lower cauline ones to 15 נ4 cm and often few-toothed, the others numerous, smaller, entire or few-toothed; heads numerous, terminating the branches; invol pale, 9-15 mm, the middle and outer bracts broad, striate, glabrous, with large, broadly rounded, subentire hyaline tip, the inner bracts narrower, tapering to a plumose- hairy tip; fls purple, the marginal ones not enlarged; larger pappus-bristles subplumose above, 6-11 mm; achenes basilaterally attached; 2n=26. Fields, roadsides, and waste places; native of Asia, now widely established in w. U.S. and occasional in our range. June-Sept. (C. picris; Acroptilon r.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.