Black knapweed, lesser knapweed, centaurée noire

Centaurea jacea Linnaeus subsp. nigra (Linnaeus) Bonnier & Layens; C. nemoralis Jordan

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.

SELECTED REFERENCE

Ockendon, D. L., S. M. Walters, and T. P. Whiffen. 1969. Variation within Centaurea nigra. Proc. Bot. Soc. Brit. Isles 7: 549–552.