Yellow Star-Thistle

Centaurea solstitialis is an annual herb from the family Asteraceae. During the vegetative stage it forms a rosette of non-spiny leaves, between 5–20 centimetres (2–8 inches) in diameter. As the summer approaches, it produces a flowering stem up to 1 metre (3+1⁄2 feet) in height. The leaves at the base are lobed and range between 5–8 cm (2–3 in) in length, while the ones on the stem are unlobed and smaller. Between May and October, the stem produces numerous spinous flower heads (capitula) about 12–16 millimetres (1⁄2–5⁄8 in) across, containing between 10 and 50 yellow flowers, with spines between 1–2.5 cm (3⁄8–1 in). Flowers within capitula are pollinated by insects and each capitula produces 10–50 seeds, some with and some without a pappus. It is an annual semelparous species, and will die after reproduction is completed, normally by the end of the summer.

Similar species. Similar species include purple star-thistle (Centaurea calcitrapa), sulphur star-thistle (C. sulphurea), Maltese star-thistle (C. melitensis), and rough star-thistle (C. aspera). Bachelor's button (C. cyaneus) is a relative.

Centaurea solstitialis is considered a weed also on its native European range (e.g., Turkey, Greece, Italy, France, Spain). Consequently, it inhabits highly disturbed ruderal habitats, being typically found on roadsides and cereal crop margins. After introduction in several parts of the world as an exotic species (e.g., Australia, Argentina, Chile, the United States), it has developed local adaptations to the different habitats colonized, and an incipient level of reproductive isolation between native and non-native ranges has been detected—a case of ecological speciation. Star-thistle is a valuable source of pollen, thus nectar for pollinators. Yellow star-thistle, a noted pest plant in California, is a major nectar source for many butterflies of the state's central valleys and foothills. Star-thistle populates ground that has been abused: dry, compacted, or scraped clean. A plant with a taproot system, it has a crucial role in restoring the soil by bringing up vital micronutrients. Similar to many plants classified as 'weeds', they (in the words of Mark Schonbeck) "quickly establish in, protect, and restore soil that has been left exposed by natural and human-caused disturbances". C. …

Although the spines make the plant a pain to deal with, it produces a light honey.