9. Centaurea solstitialis L. (yellow star thistle, Barnaby’s thistle)

Pl. 251 d–f; Map 1048

Plants annual, with a taproot, pubescent with cobwebby to woolly hairs, appearing somewhat pale or whitened. Stems 20–80 cm long, erect or ascending, with loosely ascending branches, noticeably winged, the wings with irregular and sometimes toothed margins. Leaves 1–20 cm long; basal and lower stem leaves with the blades mostly 5–30 mm wide, oblanceolate, rounded or angled to a bluntly pointed tip, tapered gradually to a sessile or short-petiolate base, deeply pinnately lobed, often withering by flowering time; median and upper stem leaves gradually reduced, mostly sessile, the base strongly decurrent, the blades oblong-linear to narrowly lanceolate, entire or toothed. Heads solitary at the branch tips. 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, narrower than the main body, straw-colored, 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 11–30 mm long. Upper involucral bracts lanceolate, the appendages papery, tapered, merely irregularly toothed at the tip. Florets all discoid and similar (but the marginal florets usually lacking a pappus). Pappus (except in marginal florets) of many unequal bristles, these 2–5 mm long, white, usually persistent at fruiting. Corollas 13–20 mm long, bright yellow to orangish yellow. Fruits 2.5–3.5 mm long, somewhat flattened, the attachment scar appearing lateral, the surface straw-colored to yellowish brown with darker brown mottling or (in marginal florets) uniformly dark brown, glabrous, often somewhat shiny. 2n=16. June–October.

Introduced, known thus far only from Boone and Jackson Counties and the city of St. Louis (native of Europe, Asia; introduced widely in the western U.S. and sporadically elsewhere in the U.S. and Canada). Crop fields, railroads, and roadsides.

In the western states, this species is a severe pest of rangeland. Its spines can cause mechanical injury to the limbs and mouths of animals and deter grazing of mature plants by most livestock. At overgrazed sites, animals will sometimes graze the young plants, but when horses ingest yellow star thistle for lengthy periods, they can develop a usually fatal neurological disorder known as chewing disease and equine nigropallidal encephalomalacia, in which the mouth parts are affected, leading to starvation and dehydration (Burrows and Tyrl, 2001). The compounds implicated in chewing disease are not fully understood but include sesquiterpene lactones, principally repin. Centaurea repens is similarly toxic to horses, but neither species appears to affect other livestock.