Cirsium

Cirsium is a genus of perennial and biennial flowering plants in the Asteraceae, one of several genera known commonly as thistles. They are more precisely known as plume thistles. These differ from other thistle genera (Carduus, Silybum and Onopordum) in having a seed with a pappus of feathered hairs on their achenes. The other genera have a pappus of simple unbranched hairs. They are mostly native to Eurasia and northern Africa, with about 60 species from North America (although several species have been introduced outside their native ranges). The lectotype species of the genus is Cirsium heterophyllum (L.) Hill. Cirsium thistles are known for their effusive flower heads, usually purple, rose or pink, also yellow or white. The radially symmetrical disc flowers are at the end of the branches and are visited by many kinds of insects, featuring a generalised pollination syndrome. They have erect stems, with a characteristic enlarged base of the flower which is often spiny. The leaves are alternate, spiny in many (but not all) species, and in some species can be slightly to densely hairy. Extensions from the leaf base down the stem, called wings, can be lacking (Cirsium arvense), conspicuous (Cirsium vulgare), or inconspicuous. They can spread by seed, and also by rhizomes below the surface (Cirsium arvense). The seeds have a tuft of hair, or pappus, which can carry them far by wind. Cirsium thistles are used as food plants by the larvae of some Lepidoptera species—see list of Lepidoptera that feed on Cirsium. The seeds are attractive to small finches such as American goldfinch. Many species are considered weeds, typically by agricultural interests. Cirsium vulgare (spear thistle) is listed in the United States (where as a non-native invasive species it has been renamed "bull thistle") as a noxious weed in nine states. Some...