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caltrops, centaurée chausse-trappe, chausse-trappe, purple knapweed, purple star-thistle, red star-thistle

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

Habit Annuals, biennials, or short-lived perennials, 20–100 cm. Annuals, 10–100 cm.
Stems

1–several, often forming rounded mounds, puberulent to loosely tomentose.

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

Leaves

puberulent to loosely gray-tomentose, becoming ± glabrous, minutely resin-gland-dotted;

proximal leaves petiolate, blades 10–20 cm, 1–3 times pinnately dissected, rosette with central cluster of spines;

mid sessile, not decurrent, blades ovate, usually less than 10 cm, narrowly lobed;

distal blades linear to oblong, entire to shallowly lobed.

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.

Involucres

ovoid, 15–20 × 6–8 mm.

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

Florets

25–40;

corollas purple, all ± equal, 15–24 mm;

sterile corollas slender.

many;

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

sterile florets slender, inconspicuous.

Inner phyllaries

appendages truncate, spineless.

appendages scarious, obtuse or abruptly spine tipped.

Heads

disciform, borne singly or in leafy cymiform arrays, sessile or short-pedunculate.

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

Cypselae

white or brown-streaked, 2.5–3.4 mm, glabrous;

pappi 0.

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 greenish or stramineous, ovate, scarious-margined, appendages stramineous, spiny fringed at base, each tipped by a stout spreading spine 10–25 mm.

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

2n

= 20.

= 16.

Centaurea calcitrapa

Centaurea solstitialis

Phenology Flowering summer–autumn (Jun–Nov). Flowering mostly summer–autumn (Jun–Oct), sometimes year-round in frostfree coastal habitats.
Habitat Pastures, fields, roadsides Roadsides, fields, pastures, woodlands
Elevation 0–1700 m (0–5600 ft) 0–2000 m (0–6600 ft)
Distribution
from FNA
AL; AZ; CA; DC; FL; GA; IA; IL; MA; MD; NJ; NM; NY; OR; PA; UT; VA; WA; ON; Europe; Africa
[WildflowerSearch map]
[BONAP county map]
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]
Discussion

Centaurea calcitrapa is native to southern Europe and northern Africa. It is listed as a noxious weed in Arizona, California, Nevada, New Mexico, Oregon, and Washington. These plants are unpalatable and increase on rangelands as more desirable forage plants are consumed. Dense stands are impenetrable because of the vicious spines on the mature involucres.

Centaurea ×pouzinii de Candolle, an apparently stabilized hybrid between Centaurea aspera (2n = 22) and C. calcitrapa (2n = 20), has been reported from California. A chromosome count of 2n = 42 has been reported from California material of this nothospecies (A. M. Powell et al. 1974). Centaurea ×pouzinii can be distinguished from C. calcitrapa by its shorter spines and by cypselae with a short pappus. Reports of C. calcitrapoides Linnaeus from North America are apparently based on this hybrid.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

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