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.
FNA 2006, Kearney and Peebles 1969, Correll and Johnston 1970, Baldwin et al 2002, USDA GRIN
Duration: Perennial Nativity: Non-Native Lifeform: Forb/Herb General: Perennial herbs from slender, brown to black, creeping rhizomes; stems erect and openly branching, 30-100 cm tall, cobwebby-tomentose. Leaves: Alternate, entire, and glabrous to tomentose, the upper leaves oblong to linear-lanceolate, the lower ones pinnatifid, not much larger than the upper leaves, often deciduous by flowering. Flowers: Flower heads pinkish, discoid, arranged in flat-topped panicles; involucres urn-shaped, 1-2 cm high, the bracts (phyllaries) in 6-8 graduated series, with pointed, hyaline tips but not spine-tipped, the outermose phyllaries mostly glabrous and the inner cobwebby; florets all discs, 11-14 mm long, usually pink to lavender but can be white or blue. Fruits: Achenes ivory to grayish or brown, 2-4 mm long; topped with a pappus of white bristles, these 6-11 mm long, barbed below and short-plumose above. Ecology: Found in disturbed areas, fields, roadsides, ditches, and riverbanks, below 8,000 ft (2438) m; flowers in summer. Distribution: Native to Eurasia and naturalized in Australia and North and South America. In North America, ranges from British Columbia to Manitoba, south to Arkansas and Texas, west to California. Notes: The keys to this species are the pinnatifid lower leaves not much larger than the upper, entire leaves; the purple flowers; and the unarmed phyllaries with white hyaline (papery) tips. This introduced species is noted to be a serious pest, especially in cultivated fields as it is strongly allopathic and forms dense colonies. Ethnobotany: The bruised leaves were said to be used as a poultice for sores. FNA reports this species is poisonous, especially to horses. Etymology: Rhaponticum comes from the Greek rha for rhizomes; repens means creeping, or growing along the ground, referring to the rhizomatous growth form. Synonyms: Acroptilon repens, Centaurea repens, others, see Tropicos Editor: LCrumbacher 2011, AHazelton 2015, AHazelton 2017