Centaurea solstitialis
Centaurea phrygia
Barnaby star-thistle, centauré du solstice, golden starthistle, St. Barnaby's thistle, yellow cockspur, yellow knapweed, yellow star-thistle
wig knapweed
simple or often branched from base, forming rounded bushy plants, gray-tomentose.
few–many, erect, simple or branched.
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.
± arachnoid-tomentose;
basal and proximal cauline winged-petiolate, blades lanceolate to ovate, 3–15 cm, margins entire or dentate;
distal cauline sessile, sometimes clasping, not decurrent, well developed.
ovoid, 13–17 mm, loosely cobwebby-tomentose or becoming glabrous.
ovoid to ± spheric, 15–20 mm.
many;
corollas yellow, all ± equal, 13–20 mm;
sterile florets slender, inconspicuous.
many, the peripheral sterile;
corollas pink or purple, those of sterile much expanded and exceeding corollas of fertile florets, those of fertile 20–25 mm.
appendages scarious, obtuse or abruptly spine tipped.
tips erect, ovate or orbiculate, irregularly dentate or lobed.
disciform, borne singly or in open leafy arrays, long-pedunculate.
usually radiant, usually borne singly.
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.
tan, 3–4 mm, finely hairy;
pappi 0 or of many unequal bristles 0.5–2 mm.
phyllaries: bodies pale green, ovate, appendages stramineous to brown, each with palmately radiating cluster of spines, and stout central spine 10–25 mm.
phyllaries: bodies lanceolate to ovate, loosely tomentose or glabrous, appendages brown or blackish, lanceolate to ovate, ± covering bodies of adjacent phyllaries, tips often recurved, elongate, featherlike, pectinately dissected into long, filiform lobes.
= 16.
= 22 (Russia), 44 (Slovenia).
Centaurea solstitialis
Centaurea phrygia
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.)
According to R. J. Moore (1972), reports of Centaurea nervosa Willdenow [C. uniflora Turra subsp. nervosa (Willdenow) Bonnier & Layens] from New York were based on a specimen referable to C. phrygia subsp. phrygia. Moore called these plants C. austriaca Willdenow, which J. Dostál (1976) treated as a synonym of C. phrygia subsp. phrygia.
Specimens of Centaurea phrygia are sometimes misidentified as C. nigrescens (or one or another of its synonyms) or as C. nigra. The elongate, often recurved, setose-ciliate tips of the phyllary appendages are a readily recognizable characteristic of this species. Considerable morphologic variation occurs in vegetative features and head dimensions in American material of the species, and it is possible that one or more of the specimens we have identified as C. phrygia represent an extreme variant of one of the members of the C. jacea complex. J. Dostál (1976) recognized 10 subspecies of C. phrygia in Europe. We have chosen not to assign the sparse North American material to subspecies.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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