Centaurea calcitrapa |
Centaurea nigra |
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caltrops, centaurée chausse-trappe, chausse-trappe, purple knapweed, purple star-thistle, red star-thistle |
black knapweed, centaurée noire, common knapweed, hardheads, lesser knapweed |
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Habit | Annuals, biennials, or short-lived perennials, 20–100 cm. | Perennials, 30–150 cm. |
Stems | 1–several, often forming rounded mounds, puberulent to loosely tomentose. |
1–few, erect or ascending, openly branched distally, villous to scabrous with septate hairs and loosely tomentose, ± glabrate. |
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. |
basal and proximal cauline petiolate, blades oblanceolate or elliptic, 5–25 cm, margins entire or shallowly dentate to irregularly pinnately lobed; distal cauline sessile, not decurrent, gradually smaller, blades linear to lanceolate, entire or dentate. |
Involucres | ovoid, 15–20 × 6–8 mm. |
ovoid to campanulate or hemispheric, 15–18 mm, usually ± as wide as high. |
Florets | 25–40; corollas purple, all ± equal, 15–24 mm; sterile corollas slender. |
40–100+, all fertile; corollas purple (rarely white), 15–18 mm. |
Inner phyllaries | appendages truncate, spineless. |
tips truncate, irregularly dentate or lobed. |
Heads | disciform, borne singly or in leafy cymiform arrays, sessile or short-pedunculate. |
discoid, in few-headed corymbiform arrays, borne on leafy-bracted peduncles. |
Cypselae | white or brown-streaked, 2.5–3.4 mm, glabrous; pappi 0. |
tan, 2.5–3 mm, finely hairy; pappi of many blackish, unequal, sometimes deciduous bristles 0.5–1 mm. |
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 lanceolate to ovate, loosely tomentose or glabrous, bases usually ± concealed by expanded appendages, appendages erect, overlapping, dark brown to black, flat, margins pectinately dissected into numerous wiry lobes. |
2n | = 20. |
= 22, 44. |
Centaurea calcitrapa |
Centaurea nigra |
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Phenology | Flowering summer–autumn (Jun–Nov). | Flowering summer–fall (Jun–Oct). |
Habitat | Pastures, fields, roadsides | Roadsides, fields, clearings, waste areas |
Elevation | 0–1700 m [0–5600 ft] | 0–300 m [0–1000 ft] |
Distribution |
AL; AZ; CA; DC; FL; GA; IA; IL; MA; MD; NJ; NM; NY; OR; PA; UT; VA; WA; ON; Europe; Africa
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CA; CT; DE; IA; ID; IL; KY; MA; MD; ME; MI; MO; MT; NH; NJ; NY; OH; OR; PA; RI; VA; VT; WA; WI; WV; BC; NB; NF; NS; ON; PE; QC; SPM; Europe [Introduced in North America]
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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.) |
Black knapweed is listed as a noxious weed in Colorado and Washington. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Source | FNA vol. 19, p. 191. | FNA vol. 19, p. 187. |
Parent taxa | ||
Sibling taxa | ||
Synonyms | C. jacea subsp. nigra, C. nemoralis | |
Name authority | Linnaeus: Sp. Pl. 2: 917. (1753) | Linnaeus: Sp. Pl. 2: 911. (1753) |
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