Centaurea solstitialis, Yellow Star-thistle
Scientific Name: Centaurea solstitialis
Common Name: Yellow Star-thistle
Also Called: Barnaby Thistle, Golden Starthistle, St. Barnaby's Thistle, Yellow Cockspur, Yellow Star Thistle, Yellow Starthistle
Family: Asteraceae, Sunflower Family
Synonyms: (Leucantha solstitialis)
Status: Introduced, naturalized in California; from Europe.
Duration: Annual or biennial.
Size: Up to 3 feet more or less.
Growth Form: Forb/herb; large taproot; stems simple or often branched from the base, stems are winged; rounded bushy plants; plants gray-tomentose.
Leaves: Leaves gray-tomentose, scabrid to short-bristly; cauline leaves petiolate or tapered to base; leaves usually have fallen off during flowering; leaves; leaf margins pinnately lobed or dissected; linear to oblong.
Flower Color: Yellow; flowers discoid only; solitary or multiples flower heads, glabrous to loosely cobwebby-tomentose; flower with long stems (peduncles); phyllaries; terminal spine of the phyllary yellowish, comparatively stout, 1.2 to 2 cm long; plants thinly tomentose
Flowering Season: May or June to October or sometimes year-round.
Elevation: Up to 6,500 feet or higher.
Habitat Preferences: Waste places, roadsides, abandoned fields, pastures, recreational areas, disturbed grasslands, woodlands, not common in desert communities. In the northwestern part of the country Yellow Star-thistle co-dominates with Cheatgreas (Bromus tectorum) and other annual-grasses. In California Yellow Star-thistle is found in the Central Valley and nearby foothills; now moving into mountainous regions below 7,000 feet.
Recorded Range: Yellow Star-thistle is found throughout North America and most of Canada.
North America & US County Distribution Map for Centaurea solstitialis.
U.S. Weed Information: In North America Centaurea solstitialis can be weedy or invasive according to the following authoritative sources: California Invasive Plant Council (Cal-IPC); Weeds of the United States and Canada; Western Society of Weed Society.
Plants included here may become weedy or invasive.
Invasive/Noxious Weed Information: In North America Centaurea solstitialis is listed as a Prohibited, Restricted, Quarantined, Regulated or Noxious Weed by AZ, CA, CO, MT, NV, NM, ND, OR, SD, UT, WA.
Plants included here are invasive or noxious.
Wetland Indicator: No information available. Threatened/Endangered Information: No information available.
In the Southwestern United States: Arizona and New Mexico each have 8 species of Centaurea, California has 17 species, Nevada has 5 species, Texas has 4 species, Utah has 10 species. All data is approximate and subject to taxonomic changes.
Comments: Both Yellow and Maltese Star-thistle plants are aggressive exotic winter annual weeds. Yellow Star-thistle is thought to have had several introductions including from contaminated alfalfa seed (Medicago sativa). This species is thought to be toxic to horses if cumulatively ingested.
For a comprehensive thoroughly documented review of Yellow Star-thistle see the USDA USFS Fire Effects Information System, or FEIS.
Special Value to Native Bees; European Honey Bees (Apis mellifera) are important pollinators of Yellow Star-thistle. Bumblebees (Bombus) are also important floral visitors.
Also see in Southwest Desert Flora; Maltese Star-thistle, Centaurea melitensis.
The species epithet solstitialis refers to the summer or mid-summer or the solstice.