4. Centaurea diluta Aiton (North African knapweed, lesser star thistle)

Pl. 251 h; Map 1043

Plants annual or less commonly perennial, usually with a taproot, pubescent with cobwebby hairs when young, not appearing pale or whitened, sometimes nearly glabrous at maturity. Stems 30–150 cm long, erect or ascending with loosely ascending or spreading branches at or above the midpoint, somewhat angled but not winged. Leaves 2–15 cm long, often dotted with minute, yellow to brown resin glands; basal and lower stem leaves with the blades mostly 12–80 mm wide, oblanceolate to obovate, rounded or angled to a bluntly pointed tip, tapered gradually to a sessile or short-petiolate, usually somewhat expanded base, mostly deeply pinnately lobed, the margins otherwise entire or finely toothed; median and upper stem leaves gradually reduced, mostly sessile, the base sometimes slightly clasping the stem, not or only slightly decurrent, the blades linear to oblong-oblanceolate, entire or rarely toothed or shallowly lobed. Heads solitary at the branch tips. Involucre 15–18 mm long, longer than to about as long as wide, ovoid (sometimes appearing narrowly bell-shaped when pressed). Lower and median involucral bracts with the body ovate to narrowly ovate, the margins narrowly thin and papery, entire or somewhat irregular, the outer surface glabrous or nearly so, not concealed by the appendages; the apical appendage well differentiated, ascending, narrower than the main body, not or only slightly overlapping, white to straw-colored or brownish-tinged, the involucre with at least some green coloration visible, the margins with a fringe of slender, spreading teeth and a short central spine, this 1–5 mm long. Upper involucral bracts lanceolate, the appendages not well differentiated from the relatively broad (at least toward the tip), thin, white, papery margins, the tips bluntly or sharply pointed, irregularly toothed or narrowly lobed. Florets discoid, but the marginal florets raylike. Pappus of many unequal bristles, these 3–5 mm long, white, usually persistent at fruiting. Corollas of discoid florets 18–22 mm long, those of raylike florets 25–30 mm long, pinkish purple to reddish purple, rarely white. Fruits 3.0–3.5 mm long, somewhat flattened, the attachment scar appearing lateral, the surface tan to grayish brown with lighter stripes, finely hairy. 2n=20. June–September.

Introduced, known thus far only from the city of St. Louis (native of Europe, Africa; introduced sporadically in California, Missouri, and New York). Railroads.

This species was first reported for Missouri by Mühlenbach (1979). It is much less common in the United States than C. diffusa, with which it sometimes has been confused.