Centaurea diffusa
Centaurea solstitialis
centaurée diffuse, diffuse knapweed, diffuse or tumble or white knapweed, tumble knapweed, white knapweed
Barnaby star-thistle, centauré du solstice, golden starthistle, St. Barnaby's thistle, yellow cockspur, yellow knapweed, yellow star-thistle
1–several, much-branched throughout, puberulent and ± gray tomentose.
simple or often branched from base, forming rounded bushy plants, gray-tomentose.
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.
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.
narrowly ovoid or cylindric, 10–13 × 3–5 mm.
ovoid, 13–17 mm, loosely cobwebby-tomentose or becoming glabrous.
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.
many;
corollas yellow, all ± equal, 13–20 mm;
sterile florets slender, inconspicuous.
lanceolate, ± acute, appendage lacerate or spine-tipped.
appendages scarious, obtuse or abruptly spine tipped.
disciform, in open paniculiform arrays.
disciform, borne singly or in open leafy arrays, long-pedunculate.
dark brown, ca. 2–3 mm;
pappi 0 or less than 0.5 mm, only rudimentary.
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.
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.
phyllaries: bodies pale green, ovate, appendages stramineous to brown, each with palmately radiating cluster of spines, and stout central spine 10–25 mm.
= 18, 36.
= 16.
Centaurea diffusa
Centaurea solstitialis
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.)
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.)
- 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
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- Local floras: BC, CA, OR, WA
- Local Web sites: CalFlora, CalPhotos, Flora NW, Go Botany, IL Wildflowers,
KS Wildflowers, 
MD Biodiversity, MI Flora, 
MN Wildflowers, 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
