5. Centaurea melitensis L. (Maltese star thistle)

Pl. 251 i; Map 1044

Plants annual, with a taproot, pubescent with cobwebby to woolly hairs, often appearing somewhat pale or whitened, at least when young. Stems 10–60 cm long, erect or ascending, unbranched or with loosely ascending branches, noticeably winged, the wings with irregular and sometimes broadly toothed margins. Leaves 1–15 cm long, often dotted with minute, yellow to brown resin glands; basal and lower stem leaves with the blades mostly 5–35 mm wide, oblong to oblanceolate, usually rounded at the tip, tapered gradually to a sessile or short-petiolate base, deeply pinnately lobed, usually withering by flowering time; median and upper stem leaves gradually reduced, mostly sessile, the base strongly decurrent, the blades linear to oblong-lanceolate, entire or toothed. Heads solitary or in small clusters of 2–4 at the branch tips (rarely on short, axillary branches and thus appearing lateral). Involucre 10–15 mm long (excluding spines), longer than to about as long as wide, broadly ovoid to nearly spherical. Lower and median involucral bracts with the body ovate, the margins entire, the outer surface more or less cobwebby-hairy, not concealed by the appendages; the apical appendage well differentiated, spreading, more or less narrower than the main body, straw-colored and commonly purplish-tinged, the involucre with at least some green coloration easily visible (sometimes becoming entirely straw-colored with age), the margins with 1 or 2 pairs of short, spreading, spinelike, lateral bristles and a central spine, this 5–10 mm long. Upper involucral bracts lanceolate, the appendages papery, tapered, short-spined or merely irregularly toothed at the tip. Florets all discoid and similar or the marginal florets similar in appearance but functionally sterile. Pappus of many unequal bristles, these 2–3 mm long, white, usually persistent at fruiting. Corollas 10–12 mm long, bright yellow. Fruits 2.5–3.5 mm long, somewhat flattened, the attachment scar appearing lateral, the surface yellowish brown to brown with fine, lighter, longitudinal stripes, glabrous or more commonly finely hairy, sometimes somewhat shiny. 2n=24. June–September.

Introduced, known only from historical collections from Jackson County (native of Europe, Africa; introduced widely in the western U.S. and adjacent Canada, more sporadically farther east). Railroads, roadsides, and open, disturbed areas.