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golden starthistle, St. Barnaby's thistle, yellow cockspur, yellow knapweed, yellow star-thistle

Habit Annuals, 10–100 cm.
Stems

simple or often branched from base, forming rounded bushy plants, gray-tomentose.

Leaves

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.

Heads

disciform, borne singly or in open leafy arrays, long-pedunculate.

Involucres

ovoid, 13–17 mm, loosely cobwebby-tomentose or becoming glabrous.

Florets

many;

corollas yellow, all ± equal, 13–20 mm;

sterile florets slender, inconspicuous.

Inner phyllaries

appendages scarious, obtuse or abruptly spine tipped.

Cypselae

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.

Principal

phyllaries: bodies pale green, ovate, appendages stramineous to brown, each with palmately radiating cluster of spines, and stout central spine 10–25 mm.

2n

= 16.

Centaurea solstitialis

Phenology Flowering mostly summer–autumn (Jun–Oct), sometimes year-round in frostfree coastal habitats.
Habitat Roadsides, fields, pastures, woodlands
Elevation 0–2000 m [0–6600 ft]
Discussion

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.)

Distribution
from FNA
AZ; CA; CO; CT; DE; FL; IA; ID; IL; IN; KS; KY; MA; MD; MI; MN; MO; MT; NC; ND; NE; NH; NJ; NM; NV; NY; OH; OK; OR; PA; RI; SC; SD; TN; TX; UT; VA; WA; WI; WV; WY; AB; MB; ON; SK; s Europe [Widely introduced]
[WildflowerSearch map]
[BONAP county map]
Parent taxa Asteraceae > tribe Cardueae > Centaurea
Sibling taxa
C. benedicta, C. calcitrapa, C. cyanus, C. depressa, C. diffusa, C. diluta, C. iberica, C. jacea, C. macrocephala, C. melitensis, C. montana, C. nigra, C. nigrescens, C. phrygia, C. scabiosa, C. stoebe, C. sulphurea, C. virgata, C. ×moncktonii
Name authority Linnaeus: Sp. Pl. 2: 917. (1753)
Source Flora of North America vol. 19, p. 193.
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