8. Centaurea repens L. (Russian knapweed)

Acroptilon repens (L.) DC.

Pl. 252 h, i; Map 1047

Plants perennial, suckering from deep-set, creeping, dark brown to black roots, pubescent with fine, cobwebby hairs when young (appearing somewhat grayish when young), sometimes appearing nearly glabrous at maturity. Stems 20–100 cm long, erect or ascending, with loosely ascending branches, angled and/or ridged but not winged. Leaves 1–15 cm long; basal and lower stem leaves with the blades mostly 20–40 mm wide, oblanceolate, rounded or angled to a bluntly or sharply pointed tip, tapered to a sessile or short-petiolate base, the margins with coarse teeth or more commonly deeply pinnately lobed with ascending, oblong-triangular lobes; median and upper stem leaves progressively reduced, mostly sessile, the base not or only slightly decurrent, the blades lanceolate to linear, mostly with a few widely spaced teeth, the larger ones sometimes shallowly lobed. Heads solitary at the branch tips. Involucre 9–15 mm long, longer than wide (sometimes about as long as wide when pressed), ovoid. Lower and median involucral bracts with the body broadly ovate to ovate, the margins entire, the outer surface glabrous, becoming straw-colored to light brown at maturity, the apical appendage well differentiated, ascending, as wide as or slightly narrower than the main body, broadly rounded at the tip not or only slightly overlapping, thin, papery, white, the margins entire. Upper involucral bracts narrowly ovate, narrowed to a sharply pointed tip, this sometimes irregularly and finely toothed along the margins, plumose-hairy. Florets all discoid and similar. Pappus of many unequal bristles (the longest often somewhat plumose), these 6–11 mm long, white, shed by fruiting. Corollas 12–14 mm long, pinkish purple to reddish purple or purple. Fruits 3.0–3.5 mm long, somewhat flattened, the attachment scar appearing nearly basal (slightly oblique), the surface pale grayish white, with faint, fine, darker ridges, glabrous. 2n=24. May–September.

Introduced, known only from historical collections from Jackson County (native of Asia; introduced widely in the western U.S. and Canada east to Ohio, Kentucky, Arkansas, and Texas). Railroads and open, disturbed areas.

This species was introduced into the northern states and Canada in the late 1800s as a contaminant in alfalfa seeds. Because of its deep-set rootstock, it is a very difficult species to eradicate. It has allelopathic properties similar to those of C. stoebe and is poisonous to horses (see discussion under the treatment of C. solstitialis).