Centaurea solstitialis
Barnaby star-thistle, centauré du solstice, golden starthistle, St. Barnaby's thistle, yellow cockspur, yellow knapweed, yellow star-thistle
simple or often branched from base, forming rounded bushy plants, gray-tomentose.
gray-tomentose and scabrous to short-bristly;
basal and proximal cauline petiolate or tapered to base, usually absent at anthesis, blades 5–15 cm, margins pinnately lobed or dissected;
cauline long-decurrent, blades linear to oblong, 1–10 cm, entire.
ovoid, 13–17 mm, loosely cobwebby-tomentose or becoming glabrous.
many;
corollas yellow, all ± equal, 13–20 mm;
sterile florets slender, inconspicuous.
appendages scarious, obtuse or abruptly spine tipped.
disciform, borne singly or in open leafy arrays, long-pedunculate.
dimorphic, 2–3 mm, glabrous, outer dark brown, without pappi, inner white or light brown, mottled;
pappi of many white, unequal bristles 2–4 mm, fine.
phyllaries: bodies pale green, ovate, appendages stramineous to brown, each with palmately radiating cluster of spines, and stout central spine 10–25 mm.
= 16.
Centaurea solstitialis
Centaurea solstitialis is a serious weed pest, especially in the western United States, where it has invaded millions of acres of rangelands, and it is listed as a noxious weed in eleven western states and two Canadian provinces. It is a strong competitor in infested areas, often forming dense colonies. It is very difficult to control or eradicate once it becomes established. In addition, yellow star-thistle is poisonous to horses; when ingested over a prolonged period it causes a neurological disorder called equine nigropallidal encephalomalacia, or “chewing disease.” Although its bitter taste and spiny heads usually deter grazing animals, horses sometimes will seek it out. Yellow star-thistle tends to spread in rangelands when more palatable plants are consumed.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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