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

centaurée scabieuse, greater centaurea, greater knapweed, hardheads

Habit Annuals, 10–100 cm. Perennials, 30–150 cm.
Stems

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

1–several, branches ascending, glabrous to ± hirsute.

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.

minutely hispid, resin-gland-dotted;

basal and proximal cauline petiolate, blades 10–25 cm, margins usually 1–2-pinnately divided into linear or oblong segments;

mid and distal cauline smaller, entire or once dissected.

Involucres

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

ovoid to hemispheric, becoming campanulate, 15–25 mm.

Florets

many;

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

sterile florets slender, inconspicuous.

many;

corollas reddish purple (white), those of sterile florets 35–40 mm, often conspicuously enlarged, those of fertile florets 20–25 mm.

Phyllaries

bodies dark green, ovate (outer) to oblong-lanceolate (inner), glabrous or finely arachnoid, margins and erect appendages black, ± fringed distally with slender teeth, inner phyllaries with brownish scarious, expanded, erose dissected appendages.

Inner phyllaries

appendages scarious, obtuse or abruptly spine tipped.

Heads

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

borne singly or few in open cymiform arrays, pedunculate.

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.

brown, 4.5–5 mm, puberulent;

pappi of many unequal stiff bristles, white, 4–5 mm.

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.

= 20 (Russia), 40.

Centaurea solstitialis

Centaurea scabiosa

Phenology Flowering mostly summer–autumn (Jun–Oct), sometimes year-round in frostfree coastal habitats. Flowering summer (Jun–Sep).
Habitat Roadsides, fields, pastures, woodlands Disturbed sites, pastures, sparingly escaped from cultivation, probably not persisting in all areas where reported
Elevation 0–2000 m (0–6600 ft) 50–1800 m (200–5900 ft)
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]
from FNA
CT; IA; ID; KY; ME; MT; ND; NH; NJ; NY; OH; PA; UT; WY; BC; NB; ON; QC; Europe [Introduced in North America]
[BONAP county map]
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.)

Source FNA vol. 19, p. 193. FNA vol. 19, p. 185.
Parent taxa Asteraceae > tribe Cardueae > Centaurea 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
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. solstitialis, C. stoebe, C. sulphurea, C. virgata, C. ×moncktonii
Name authority Linnaeus: Sp. Pl. 2: 917. (1753) Linnaeus: Sp. Pl. 2: 913. (1753)
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