OregonFlora: Centaurea solstitialis vs. Centaurea virgata

	Centaurea solstitialis	Centaurea virgata
	St. Barnaby's thistle, yellow starthistle	squarrose knapweed
Habit	Plants annual or biennial, 1–10 dm.
Stems	1–many, branched from bases, tomentose.
Leaves	tomentose and scabrous to bristly, bases decurrent; basal and lower cauline blades often absent at maturity, 5–15 cm, pinnately dissected or lobed; petioles winged; upper cauline blades oblong to linear, 1–10 cm; margins entire or slightly dentate, sessile.
Inflorescences	cyme-like clusters or heads solitary.
Involucres	ovoid, 10–15 mm.
Florets	yellow, sterile florets ? fertile florets, fertile florets 13–20 mm.
Phyllaries	outer ovate; bodies green, palmately spiny; central spines stout, 10–25 mm; straw-colored; inner with smaller hyaline appendages.
Fruits	dimorphic; outer 2–3 mm, dark brown, glabrous; pappi 0; inner light brown or mottled, pappi of white bristles, 2–4 mm.
Heads	disciform, pedunculate.
2n	=16.

	Centaurea solstitialis	Centaurea virgata
Distribution	[WildflowerSearch map] [BONAP county map]	[WildflowerSearch map] [BONAP county map]
Discussion	Disturbed areas, roadsides, woods, fields, pastures, streambanks. Flowering Jul–Oct. 0–2000 m. BR, BW, Col, CR, Est, Lava, Sisk, WV. CA, ID, NV, WA; throughout North America; Europe. Exotic. Centaurea solstitialis is an aggressive weed of pastures and rangeland in western North America, often forming dense stands. This species is cumulatively toxic to horses, potentially causing a disorder known as “chewing disease.” See also C. × gerstlaueri.
Source	Flora of Oregon, volume 2, page 215 Bridget Chipman
Sibling taxa	C. benedicta, C. calcitrapa, C. cyanus, C. diffusa, C. iberica, C. jacea, C. jacea x Centaurea nigra, C. melitensis, C. montana, C. nigra, C. nigrescens, C. stoebe, C. virgata	C. benedicta, C. calcitrapa, C. cyanus, C. diffusa, C. iberica, C. jacea, C. jacea x Centaurea nigra, C. melitensis, C. montana, C. nigra, C. nigrescens, C. solstitialis, C. stoebe
Subordinate taxa		C. virgata ssp. squarrosa
Web links	Local floras: BC, CA, OR, WA Local Web sites: CalFlora, CalPhotos, Flora NW, 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	Local floras: OR Local Web sites: Flora NW, PNW Herbaria WildflowerSearch iNaturalist (observations) USDA Plants Database LBJ Wildflower Center SEINet Plants of the World Online Encyclopedia of Life Wikipedia Google Image Search

Centaurea virgata

St. Barnaby's thistle, yellow starthistle

squarrose knapweed

Habit

Plants annual or biennial, 1–10 dm.

Stems

1–many, branched from bases, tomentose.

Leaves

tomentose and scabrous to bristly, bases decurrent;

basal and lower cauline blades often absent at maturity, 5–15 cm, pinnately dissected or lobed;

petioles winged; upper cauline blades oblong to linear, 1–10 cm;

margins entire or slightly dentate, sessile.

Inflorescences

cyme-like clusters or heads solitary.

Involucres

ovoid, 10–15 mm.

Florets

yellow, sterile florets ? fertile florets, fertile florets 13–20 mm.

Phyllaries

outer ovate;

bodies green, palmately spiny;

central spines stout, 10–25 mm; straw-colored; inner with smaller hyaline appendages.

Fruits

dimorphic; outer 2–3 mm, dark brown, glabrous;

pappi 0; inner light brown or mottled, pappi of white bristles, 2–4 mm.

Heads

disciform, pedunculate.

=16.

Centaurea virgata

Distribution

[WildflowerSearch map]

[BONAP county map]

[WildflowerSearch map]

[BONAP county map]

Discussion

Disturbed areas, roadsides, woods, fields, pastures, streambanks. Flowering Jul–Oct. 0–2000 m. BR, BW, Col, CR, Est, Lava, Sisk, WV. CA, ID, NV, WA; throughout North America; Europe. Exotic.

Centaurea solstitialis is an aggressive weed of pastures and rangeland in western North America, often forming dense stands. This species is cumulatively toxic to horses, potentially causing a disorder known as “chewing disease.” See also C. × gerstlaueri.

Source

Flora of Oregon, volume 2, page 215
Bridget Chipman

Sibling taxa