Centaurea solstitialis, the yellow star-thistle, is a species of thorny plant in the genus Centaurea, which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other places. It is also known as golden starthistle, yellow cockspur and St. Barnaby's thistle (or Barnaby thistle).
Description
Centaurea solstitialis is an annual herb from the family Asteraceae. During the vegetative stage it forms a rosette of non-spiny leaves, between in diameter. As the summer approaches, it produces a flowering stem up to in height. The leaves at the base are lobed and range between 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 across, containing between 10–50 yellow flowers, with spines between . 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.
Ecology
Centaurea solstitialis is 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, is a major nectar source for many central valleys and foothill butterflies. 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. solstitialis grows as a balanced part of the ecosystems in Eurasia, where it is kept in check by an assortment of natural herbivore enemies and other plants that have with it in its native habitat. However, it has been introduced in several parts of the world, including Australia, Argentina, Chile, and the U.S. In many of these non-native regions, where the particular array of natural biological controls found in the native regions do not exist, it has become an invasive species and noxious weed.
The yellow star-thistle plant has the ability to create monotypic stands and habitats in the cultivated soil of fields, graded dirt sites, and disturbed natural ecosystem lands. Its colonization eliminates and prevents other plant species from growing, terminating the habitat's biodiversity. Extensive spreading monotypic fields of yellow star-thistle are not uncommon. Its growth plasticity, competitiveness, preference for the Mediterranean climate, and a lack of natural herbivore enemies and co-evolved species, make it a very successful invader. The plant is an invasive pest in field crops, degrades native plant habitats and natural ecosystems, prevents the grazing of domestic animals in rangelands, and is a physical barrier to indigenous animal movement in wildlands.
As an invasive species
The introduction of C. solstitialis in North America probably occurred in California sometime after the start of the California Gold Rush, as a fodder seed contaminant in imported Chilean-harvested alfalfa seed, also known as Chilean clover (Trifolium macraei). Star-thistle has been introduced throughout North and South America, Africa, and Europe.