Yellow Star Thistle. Centaurea solstitialis, Sunflower Family ( Asteraceae ), Yellow Star Thistle. Also called: yellow star thistle, geeldissel, golden star thistle, St. Barnaby's thistle, yellow centaury, yellow cockspur. We wish to thank Wikipedia, the free encyclopedia for some of the information on this page. We share images and information with Wikipedia. Yellow starthistle is a gray-green to blue-green annual or biennial invasive weed with a deep taproot. The stems are erect, 24 inches tall, freely branching, covered with a thin tomentum and prominently winged by decurrent leaf bases. The basal leaves are lyrate or pinnatifid, early deciduous, 2" wide to 9" long; middle and upper leaves are smaller becoming linear and entire. The yellow flowers are in heads that terminate the branches. The middle and outer involucral bracts are spine tipped, the larger central spines commonly 4" - 10" long; inner bracts unarmed. Pappus lacking on marginal flowers, that of others 2" - 4" long. The fruit is a yellowish achene with brown marbling. Quick Notes: Height: 24" tall and usually less wide. Flowers: Yellow flowers in heads that terminate the branches. The middle and outer involucral bracts are spine tipped, the larger central spines commonly 4" - 10" long; with inner bracts unarmed. Pappus is lacking on marginal flowers, that of others is 2" - 4" long. Flowering Time: January to October. Seeds: Pappus-bearing seed usually dispersed after the flowers senesce. Non-pappus-bearing seeds are retained in the seed head. Leaves: The basal leaves are lyrate or pinnatifid, early deciduous, 2" wide to 9" long; middle and upper leaves are smaller becoming linear and entire. Found: Found throughout the Sonoran and southern Mojave Deserts, southern Arizona, southeastern California, Arizona, northwestern Mexico Hardiness: USDA Zone 7a: to -17.7 �C (0 �F) USDA Zone 7b: to -14.9 �C (5 �F) USDA Zone 8a: to -12.2 �C (10 �F) USDA Zone 8b: to -9.4 �C (15 �F) USDA Zone 9a: to -6.6 �C (20 �F) USDA Zone 9b: to -3.8 �C (25 �F) USDA Zone 10a: to -1.1 �C (30 �F) USDA Zone 10b: to 1.7 �C (35 �F) USDA Zone 11: above 4.5 �C (40 �F) Soil pH requirements: 6.1 to 6.5 (mildly acidic) 6.6 to 7.5 (neutral) 7.6 to 7.8 (mildly alkaline) 7.9 to 8.5 (alkaline) 8.6 to 9.0 (strongly alkaline) Sun Exposure: Full Sun Sun to Partial Shade Light Shade Elevation: 0 - 6,000 Feet. Habitat: Riparian habitats. Miscellaneous: Flowering Photos Taken March 31, 2003. Glendale, Arizona.

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