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 feathered hairs to their achenes. The other genera have a pappus of simple unbranched hairs.[2]
They are mostly native to Eurasia and northern Africa, with about 60[3] species from North America (although several species have been introduced outside their native ranges).
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.[4] 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, typically by agricultural interests. Cirsium vulgare (bull thistle, common thistle, or spear thistle) is listed as a noxious weed in nine US states.[5] Some species in particular are cultivated in gardens and wildflower plantings for their aesthetic value and/or to support pollinators such as butterflies. Some species dubbed weeds by various interest groups can also provide these benefits. Cirsium vulgare, for instance, ranked in the top 10 for nectar production in a UK plants survey conducted by the AgriLand project which is supported by the UK Insect Pollinators Initiative. Bull thistle was also a top producer of nectar sugar in another study in Britain, ranked third with a production per floral unit of (2323 ± 418μg).[6] Not only does it provide abundant nectar, it provides seeds and floss for birds, such as the American goldfinch, Spinus tristis, and supports the larvae of a Painted Lady butterfly, Vanessa cardui.[7] A great many native North American plants have weed in their common names, despite their beneficial qualities, such as Asclepias tuberosa, commonly known as butterflyweed. Some other common species are: Cirsium lanceolatum, Cirsium palustre, Cirsium oleraceum.
Some ecological organizations, such as the Xerces Society, have attempted to raise awareness of the benefits of thistles, to counteract the general agricultural and home garden labeling of thistles as unwanted weeds. The monarch butterfly (Danaus plexippus), for instance, was highlighted as relying upon thistles such as Tall thistle (Cirsium altissimum) as nectar sources during its migration.[8] Although such organizations focus on the benefits of native thistles, non-native thistles, such as Cirsium vulgarein North America, may provide similar benefits to wildlife. Some prairie and wildflower seed production companies supply bulk seed for native North American thistle species, for wildlife habitat restoration, although availability tends to be low. Thistles are particularly valued by bumblebees for their high nectar production.