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.