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).
Thistles are known for their effusive flower heads, usually purple or rose to pink, also yellow or white. The radially symmetrical disk flowers are at the end of the branches. They have erect stems and prickly leaves, with a characteristic enlarged base of the flower which is commonly spiny. The leaves are alternate, and some species can be slightly 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 seed has tufts of tiny 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.
Most species are considered weeds. Cirsium vulgare (Bull Thistle, Common Thistle, or Spear Thistle) is listed as a noxious weed in nine US states. Some species are cultivated in gardens for their aesthetic value and to attract butterflies. Some other common species are: Cirsium lanceolatum, Cirsium palustre, Cirsium oleraceum.
Certain species of Cirsium, like Cirsium monspessulanum, Cirsium pyrenaicum and Cirsium vulgare, have been traditionally used as food in rural areas of Southern Europe. Cirsium oleraceum is cultivated as a food source in Japan and India.
The word 'Cirsium' derives from the Greek word kirsos meaning 'swollen vein'. Thistles were used as a remedy against swollen veins. The flower blooms April to August.