Centaurea solstitialis
Centaurea macrocephala
Barnaby star-thistle, centauré du solstice, golden starthistle, St. Barnaby's thistle, yellow cockspur, yellow knapweed, yellow star-thistle
big-head knapweed, centaurée à gros capitules, giant knapweed, globe centaurea, globe knapweed, yellow Bachelor's button, yellow Bachelor's button or cornflower
simple or often branched from base, forming rounded bushy plants, gray-tomentose.
usually several, erect, unbranched or sparingly branched distally, villous with septate hairs, thinly arachnoid-tomentose, fistulose proximal to heads.
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.
short-villous and thinly arachnoid, ± glabrate, resin-gland-dotted;
basal and proximal cauline petiolate, blades oblanceolate to narrowly ovate, 10–30 cm, margins entire or shallowly dentate;
cauline sessile, shortly decurrent, not much smaller except those crowded proximal to heads, blades lanceolate to ovate, 5–10 cm, entire, often ± undulate, apices acute.
ovoid, 13–17 mm, loosely cobwebby-tomentose or becoming glabrous.
ovoid to hemispheric, 25–35 mm.
many;
corollas yellow, all ± equal, 13–20 mm;
sterile florets slender, inconspicuous.
many;
corollas yellow;
corollas of sterile florets slightly expanded, ca. 4 mm;
corollas of disc florets ca. 3.5 mm.
bodies pale green or stramineous, ovate or broadly lanceolate, glabrous, appendages erect to spreading, brown, scarious, abruptly expanded, 1–2 cm wide, ± covering phyllary bodies, lacerate fringed, sometimes tipped by weak spines 1–2 mm, glabrous.
appendages scarious, obtuse or abruptly spine tipped.
disciform, borne singly or in open leafy arrays, long-pedunculate.
disciform or weakly radiant, borne singly, sessile, closely subtended by clusters of reduced leaves.
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.
7–8 mm;
pappi of many setiform scales (“flattened bristles”), 5–8 mm.
phyllaries: bodies pale green, ovate, appendages stramineous to brown, each with palmately radiating cluster of spines, and stout central spine 10–25 mm.
= 16.
= 18 (Russia).
Centaurea solstitialis
Centaurea macrocephala
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.)
Although Centaurea macrocephala is cultivated as an ornamental and for cut flowers in many areas, it has been declared a noxious weed by the state of Washington because of its potential status as an invader.
(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, 
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MD Biodiversity, 
MI Flora, 
MN Wildflowers, 
MO Plants, 
PNW Herbaria, Turner Photog. 
WildflowerSearch
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USDA Plants Database- LBJ Wildflower Center
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