1. Acroptilon repens (Linnaeus) de Candolle in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 6: 663. 1838.
Centaurea repens Linnaeus, Sp. Pl. ed. 2, 2: 1293. 1763; Acroptilon picris (Pallas ex Willdenow) C. A. Meyer; C. picris Pallas ex Willdenow
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.