6. Centaurea nigra L. (black knapweed, Spanish buttons)

C. jacea L. ssp. nigra (L.) Bonnier & Layens

Pl. 252 f, g; Map 1045

Plants perennial, often with short rhizomes, pubescent with short, stiff hairs or cobwebby hairs when young, not appearing pale or whitened, sometimes nearly glabrous at maturity. Stems 30–150 cm long, erect or ascending, sometimes from a prostrate base, with loosely ascending or spreading branches at or above the midpoint, somewhat angled but not winged. Leaves 3–25 cm long; basal and lower stem leaves with the blades mostly 12–50 mm wide, oblanceolate to elliptic, angled to a sharply pointed tip, tapered gradually to a sessile or short-petiolate base, the margins entire or few-toothed to shallowly lobed; median and upper stem leaves gradually reduced, mostly sessile, the base not decurrent, the blades linear to lanceolate, entire or toothed. Heads solitary at the branch tips. Involucre 14–18 mm long, about as long as wide, bell-shaped to hemispheric. Lower and median involucral bracts with the body lanceolate to ovate, the margins entire, the outer surface glabrous or cobwebby-hairy, often concealed by the appendages; the apical appendage well differentiated, ascending, broader than the main body, overlapping, brown to dark brown or black, the involucre thus often appearing solid brown or black, the margins comblike with a fringe of stiff, spreading or loosely upward-curved, parallel bristles. Upper involucral bracts similar but the appendages merely irregularly toothed along the margins, the tips rounded to truncate. Florets all discoid and similar. Pappus of many unequal bristles, these 0.5–1.0 mm long, straw-colored, sometimes shed by fruiting. Corollas 15–18 mm long, purple or rarely white. Fruits 2.5–3.0 mm long, somewhat 4-angled, the attachment scar appearing lateral, the surface tan to grayish brown with lighter stripes, finely hairy. 2n=22. June–September.

Introduced, known only from Boone County and the city of St. Louis (native of Europe; introduced in the northeastern and western U.S. and adjacent Canada). Railroads and roadsides.

Plants that appear to represent fertile hybrids between C. nigra and C. jacea L. (brown knapweed, not yet reported from Missouri) have been called C. Hpratensis Thuill. or more correctly C. Hmoncktonii C.E. Britton (meadow knapweed), and were collected by Viktor Mühlenbach during his botanical inventories of the St. Louis railyards.