Thistle, the common name of various herbaceous plants. There are annual, biennial, and perennial thistles. Most thistles belong to the composite family; they have prickly leaves and showy purple, pink, white, or yellow heads densely packed with tubelike flowers. The flower heads are usually surrounded by spiny, leaflike structures. Most thistles are weeds, although a few species, such as the blessed thistle, are occasionally cultivated for their flowers. The Scotch thistle is depicted on the national emblem of Scotland. A number of thistles, including the bull thistle, the Canada thistle, and sow thistles, are troublesome field and pasture weeds. The Russian thistle is a tumbleweed. Thistles are found throughout most regions of the Northern Hemisphere. There are more than 200 species.
Most thistles belong to one of three genera (Carduus, Cirsium, or Onopordon) of the composite family, Compositae. The blessed thistle is Carduus benedictus; the Canada thistle, Cirsium arvense; the bull thistle, Cirsium vulgare; the Scotch thistle, Onopordon acanthium. Sow thistles belong to the genus Sonchus of the family Compositae. The Russian thistle is Salsola kali tenuifolia of the goosefoot family, Chenopodiaceae.