7. Centaurea nigrescens Willd. (Tyrol knapweed, short-fringed knapweed)

C. vochinensis Bernh. ex Rchb.

C. dubia Suter ssp. vochinensis (Bernh. ex Rchb.) Hayek

Pl. 252 j, k; Map 1046

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 12–16 mm long, longer than wide, narrowly cup-shaped or narrowly bell-shaped to ovoid. Lower and median involucral bracts with the body lanceolate to ovate, the margins entire, the outer surface glabrous or cobwebby-hairy, not concealed by the appendages; the apical appendage well differentiated, ascending, as wide as or narrower than the main body, not or only slightly overlapping, brown to dark brown or black, the involucre with at least some green coloration visible, 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 or the marginal florets raylike. Pappus absent or of a low crown of unequal bristles, these 0.1–0.5 mm long, straw-colored, sometimes shed by fruiting. Corollas of discoid florets 15–18 mm long, those of raylike florets (when present) 20–25 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 thus far only from Boone and Jackson Counties (native of Europe; introduced sporadically in the northeastern and northwestern U.S. and adjacent Canada). Pastures, railroads, and open, disturbed areas.

Moore and Frankton (1974) discussed the taxonomy and nomenclature of this species. It seems likely that the name C. dubia Suter also should be considered a synonym of C. nigrescens (Voss, 1996); however, the taxonomic status of native populations in Europe requires further research. Steyermark’s (1963) report of Tyrol knapweed (as C. vochinensis) from the city of St. Louis could not be confirmed during the present study.