The genus Carduus Linnaeus

Remark The words or terms in red (actually dark orange) in the text are defined in a glossary.

jul_07_11n.gthmb can't be loaded. The genus Carduus is not native to North America; it was introduced in the late 1800s, crowding out the native species and forage for the livestock; the genus is native to Eurasia and Africa. The genus has about 90 species, 5 of which can be found in North America and 3 of them in Québec. The Carduus species are annuals or biennials but rarely perennials. They have a chromosome number (2n) of 8, 9, 10, 11 or 13.

The species of the Carduus genus are at times quite similar to the species of the Cirsium genus. In fact, at one time botanists considered the species of Cirsium to be Carduus taxa. The only significant difference between these two genera is the nature of the pappus. The Cirsium species can be distinguished from the Carduus species by the plumose pappus of their seeds. The members of the Carduus genus are known as plumeless thistles because of their simple, not plumose, pappus. In the genus Carduus the pappus is composed of capillary bristles that are naked, single, and silky; whereas in the genus Cirsium the pappus is composed of featherlike bristles. The pappus must always be examined to determine the genus of an unidentified thistle.

The genus belongs to the Asteraceae family.


In Latin, cardus or carduus is the generic name for the thistles as cited by the Roman poet Publius Vergilius Maro (Vergil), 70 BC-19 BC, in De Georgica (The Georgics), a poem on the subject of agriculture. Thistles include many genera and many species of the subfamily Cynareae of the Asteraceae family, Cirsium, Silybum, Carduus etc. It is also the name of the artichoke, (Cynara cardunculus var. scolymus), cited by Plinius Secundus (Pliny the Elder), 23 AD-70 AD, in Naturalis Historia (Natural History). Although the artichoke does not belong to the Carduus genus there is some similarity between the artichoke and thistles.


Carduus can reach 2 m in height and more, but some are not more than 30 cm high. They have erect stems that are simple too much branched. They have basal and cauline leaves. Their blade margins are dentate and spiny. The flower heads have disk florets only, that are pink or purple. Their phyllaries are in series, of half a dozen to a dozen and tipped by a spine. The seeds are ovoid cypselae with a barbed pappus.

The Carduus of Québec

In Québec one can find three species of the Carduus genus: It is not to difficult to differentiate these species; Carduus nutans plants have larger involucres, form 20 mm up to 70 mm, than the other two species, up to or less than 20 mm. Then, the abaxial leaf faces of Carduus acanthoides are glabrate except for long, curled, septate hairs along the veins. The abaxial leaf faces of Carduus crispus are sparsely to densely tomentose with fine non septate hairs and often with curled, septate hairs along the veins as well.