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Thistle is the common name given to some plants in several genera of the Cynareae tribe, family Asteraceae. These genera include Cirsium, Carduus, Echinops, Onopordum, Silybum, Centaurea, and Cnicus. The name thistle most often refers to the weedy, prickly plants belonging to the genera Cirsium and Carduus. Thistles are composite flowers, which means their flower is actually a group of small flowers that give the appearance of one, larger flower. The flowers, which are of the disc type, are surrounded by bracts. Many people group Carduus and Cirsium together, but a closer look at these two genera would reveal a distinct difference. While both genera have hairs or bristles mixed with the flowers and a hairy Prairie thistle (Cirsium undulatum). Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission. pappus, the hairs on Cirsium species are plumose (they are feathery—the hairs have branch hairs) and the Carduus hairs are not.

The most notable characteristic of thistles are the prickly stems, leaves, and the bracts around the flower head. Among the thistle genera, there are various leaf shapes and colors. The flowers are most often purple, but some species have pink, white, or yellow flowers. Thistles can be found in temperate regions, can grow in many climates and soils, and their height can range from 1 foot (0.3 m) to 12 ft (3.5 m). Some well known Cirsium thistles are C. vulgare or bull thistle, C . arvense or Canada thistle, and C. eriophorum or woolly thistle. C. nutans or nodding thistle is the best known of the Carduus species. Globe thistles of the Echinops genus are often grown for ornamental purposes. Onopordum acanthium or Scottish thistle is well known for its large purple flower. Silybum marianum or milk thistle, Centaurea calcitrapa or star thistle, and Cnicus benedictus or holy thistle, have been used and cultivated for medicinal purposes.

Cynara scolymus or globe artichoke is closely related to thistle. What is eaten of the artichoke that shows up on our dinner plate is the unopened flower head and bracts of the plant. Some plants such as sow thistle (genus Sonchus of the Chicory tribe-Lactuceaea) and Russian thistle (Salsola kali of the Goosefoot family), are called thistle because of their prickliness but are not true thistles.

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