Centaurea calcitrapa
Centaurea diffusa
caltrops, centaurée chausse-trappe, chausse-trappe, purple knapweed, purple star-thistle, red star-thistle
centaurée diffuse, diffuse knapweed, diffuse or tumble or white knapweed, tumble knapweed, white knapweed
1–several, often forming rounded mounds, puberulent to loosely tomentose.
1–several, much-branched throughout, puberulent and ± gray tomentose.
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.
hispidulous and ± short-tomentose;
basal and proximal cauline petiolate, often absent at anthesis, blades 10–20 cm, margins bipinnately dissected into narrow lobes;
mid cauline sessile, bipinnately dissected;
distal much smaller, entire or pinnately lobed.
ovoid, 15–20 × 6–8 mm.
narrowly ovoid or cylindric, 10–13 × 3–5 mm.
25–40;
corollas purple, all ± equal, 15–24 mm;
sterile corollas slender.
25–35;
corollas cream white (rarely pink or pale purple), those of sterile florets 12–13 mm, slender, inconspicuous, those of fertile florets 12–13 mm.
appendages truncate, spineless.
lanceolate, ± acute, appendage lacerate or spine-tipped.
disciform, borne singly or in leafy cymiform arrays, sessile or short-pedunculate.
disciform, in open paniculiform arrays.
white or brown-streaked, 2.5–3.4 mm, glabrous;
pappi 0.
dark brown, ca. 2–3 mm;
pappi 0 or less than 0.5 mm, only rudimentary.
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 to lanceolate, glabrous or finely tomentose, with a few prominent parallel veins, margins and erect appendages fringed with slender stramineous spines, each phyllary tipped by spine 1–3 mm.
= 20.
= 18, 36.
Centaurea calcitrapa
Centaurea diffusa
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 diffusa is native to southeastern Europe and casually adventive in central and western Europe.
Centaurea diffusa readily hybridizes with C. stoebe subsp. micranthos and is often confused with their fertile hybrid (C. ×psammogena G. Gáyer); the latter can be recognized by its cypselae bearing pappi and having conspicuously radiant heads. Morphologically the hybrids are extremely variable; they may be intermediate or may closely resemble one or the other of the parents. Conspicuously radiant heads and pappi are always present; appendages of the phyllaries are brown to black, or rarely stramineous; spines are absent or short and 2n = 18. Centaurea ×psammogena is known from waste places, roadsides, railway tracks; 50–2500 m; B.C., Ont., Que.; Colo., Mass., Mich., Mo., N.C., Oreg., Tenn., Wash. It may occur spontaneously where the ranges of the parent species overlap; they may also be distributed separately. In mixed stands it replaces C. diffusa by introgression. Hybrids are often misidentified as C. diffusa.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
- Local floras:
BC, 
CA, 
OR, 
WA - Local Web sites: CalFlora, CalPhotos,
Flora NW, 
Go Botany, 
IL Wildflowers, 
MD Biodiversity, 
PNW Herbaria 
WildflowerSearch
iNaturalist (observations)
USDA Plants Database- LBJ Wildflower Center
- SEINet
- Plants of the World Online
- Encyclopedia of Life
- Wikipedia
- Google Image Search
- Local floras: BC, CA, OR, WA
- Local Web sites: CalFlora, CalPhotos, Flora NW, Go Botany, IL Wildflowers,
MI Flora, 
MO Plants, PNW Herbaria, Turner Photog. - WildflowerSearch
- iNaturalist (observations)
- USDA Plants Database
- LBJ Wildflower Center
- SEINet
- Plants of the World Online
- Encyclopedia of Life
- Wikipedia
- Google Image Search
