Perennials, 30-150 cm. Stems 1-few, erect or ascending, openly branched distally, villous to scabrous with septate hairs and loosely tomentose, ± glabrate. Leaves: basal and proximal cauline petiolate, blades oblanceolate or elliptic, 5-25 cm, margins entire or shallowly dentate to irregularly pinnately lobed; distal cauline sessile, not decurrent, gradually smaller, blades linear to lanceolate, entire or dentate. Heads discoid, in few-headed corymbiform arrays, borne on leafy-bracted peduncles. Involucres ovoid to campanulate or hemispheric, 15-l8 mm, usually ± as wide as high. Principal phyllaries: bodies lanceolate to ovate, loosely tomentose or glabrous, bases usually ± concealed by expanded appendages, appendages erect, overlapping, dark brown to black, flat, margins pectinately dissected into numerous wiry lobes. Inner phyllaries: tips truncate, irregularly dentate or lobed. Florets 40-100+, all fertile; corollas purple (rarely white), 15-18 mm. Cypselae tan, 2.5-3 mm, finely hairy; pappi of many blackish , unequal, sometimes deciduous bristles 0.5-1 mm. 2n = 22, 44. Flowering summer-fall (Jun-Oct). Roadsides, fields, clearings, waste areas; 0-300 m; introduced; St. Pierre and Miquelon; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Calif., Conn., Del., Idaho, Ill., Iowa, Ky., Maine, Md., Mass., Mich., Mo., Mont., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Vt., Va., Wash., W.Va., Wis.; Europe. Black knapweed is listed as a noxious weed in Colorado and Washington.
Perennial, 2-8 dm, rough-puberulent and sometimes arachnoid when young; lvs entire or toothed, the basal ones broadly oblanceolate or elliptic, entire or toothed to sometimes few-lobed, mostly (1-)1.5-4(-6) cm wide, petiolate, the cauline ones reduced upward and becoming sessile; heads terminating the often numerous branches; invol 12-19 mm, broader than high; appendages of the invol bracts well developed, conspicuously blackish at least in part, the middle and outer deeply and fairly regularly pectinate, the larger ones mostly (3-)4-6 mm long, seldom any of them markedly bifid; fls pink-purple, the marginal ones typically not enlarged; pappus ca 1 mm or less; 2n=22, 44. Fields, roadsides, and waste places; native of Europe, now widely established in s. Can. and n. U.S., s. to Va. and W.Va. July-Oct. Hybridizes with no. 7 [Centaurea jacea L.], producing segregating or stabilized intermediates called C. ذratensis Thuill., these often approaching C. nigra as to invol, but subradiate as in C. jacea. (C. nigra var. radiata)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.