Centaurea solstitialis

Centaurea solstitialis, yellow star-thistle, is a member of the family Asteraceae, native to the Mediterranean Basin region. The plant is also known as golden starthistle, yellow cockspur and St. Barnaby's thistle (or Barnaby thistle) The plant is a thorny winter annual species in the knapweed genus. Centaurea solstitialis is an annual herb from the Asteraceae family. During the vegetative stage it forms a rosette of non-spiny leaves (5–20 cm diameter). As the summer approaches, it produces a flowering stem (1 m) which will produce numerous spinous capitula containing numerous (10-50) yellow flowers. Flowers within capitula are pollinated by insects and each capitula will produce a mix of (10-50) pappus and non-pappus seeds. It is an annual semelparous species, and will die after reproduction is completed, normally by the end of the summer. 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, 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 ecological speciation. Centaurea 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 co-evolved with it in its native habitat. However, it has been introduced in several parts of the world, including Australia, Argentina, Chile, and the USA. 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 starthistle 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. Grazing of the plant by horses can cause nigropallidal encephalomalacia or 'chewing disease', a neurological condition. The disease generally follows consumption of 60-200% of the horse's body weight over an extended period of a month or more, or 2.3-2.6 kg of starthistle per 100 kg body weight per day. Though starthistle is most dangerous when it is the only plant available or is delivered as a contaminant in dried hay, horses may develop a taste for it and seek it out. Many other species of grazers, including mules and burros, are not affected. 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 valley and foothill butterflies. Star Thistle populates ground that has been abused: dry, compacted, or scraped clean. A plant with a tap root system, it has a crucial role in restoring the soil by bringing up vital micronutrients. Similar to many plants classified as 'weeds', they 'quickly establish in, protect, and restore soil that has been left exposed by natural and human-caused disturbances'. The introduction of Centaurea 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). Introduction in other parts of the world is poorly known.