Most species, about 70 %, are diploid with a chromosome number 2n of 34; about 10 % of the species are tetraploid (2n = 68), other counts being less frequent. Cirsium is a typical example of a genus with a high affinity to form natural interspecific hybrids. Several species are serious weeds. The genus Cirsium comprises both gynodioecious and dioecious species.
The species of the Cirsium genus are at times quite similar to the species of the Carduus 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 and to the Cardueae tribe of this family.
The involucral bracts are in many series, imbricate, usually with a simple apical spine. The capitula. do not have ray florets. The disk florets are usually hermaphrodite, purple or yellowish, and rarely white. The seeds are oblong achenes with a pappus of several rows of plumose setae united in a ring at the base, the inner setae somewhat longer than the outer and simple.
Morphological intermediates, quite often hybryds, are frequent in the genus. Devising an identification key to the Cirsium species is quite difficult. Extensive and overlapping ranges of variation in morphologic characteristics often require that a species be keyed two or more times.
The reputation of the genus has suffered greatly as a result of the introduction to North America of a few invasive weedy species from Eurasia. Cirsium vulgare and Cirsium arvense (picture above) have long been despised as noxious weeds. In recent years Cirsium palustre has joined their ranks. Additionally, weedy Eurasian species of Carduus, Centaurea, etc., add to the public perception that all thistles are bad. Most North American native Cirsium are not at all weedy, and many are strikingly attractive plants. All are spiny plants that command respect, but they deserve a better reputation.
In an old Russian fairy tale, fairies in a wildflower meadow protect themselves from a greedy merchant by turning all the lovely blossoms into prickly Cirsium.