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.

Plant: Perennial, rhizomatous forb 10-60 cm; stems erect and openly branched Leaves: lower leaves deeply lobed 5-10 cm long, upper leaves entire or serrate, narrow to a sessile base INFLORESCENCE: primary inflorescence a head, each resembling a flower; heads discoid in ± flat-topped or panicle-like clusters, leafy; involucre 10-14 mm, ± ovoid; phyllaries in several unequal series, entire, ± soft-hairy, inner narrower, tips widely scarious; receptacle bristly; flowers ± 30 Flowers: flower heads 5-10 mm in diameter, discoid; phyllaries rounded with papery margins; flowers pink or lavender. Fruit: 3-4 mm achene, obovoid, slightly compressed, glabrous; pappus bristles many, 6-8 mm, ± deciduous, barbed below, short-plumose above Misc: Fields, roadsides, cultivated ground; < 1900 m.; May-Sep