Cirsium horridulum was first described as Carduus spinossimus by Thomas Walter in 1788 in the publication Flora caroliniana :secundum systema vegetabilium perillustris Linnaei digesta; characteres essentiales naturalesve et differentias veras exhibens; cum emendationibus numerosis: descriptionum antea evulgatarum: adumbrationes stirpium plus mille continens: necnon, generibus novis non paucis, speciebus plurimis novisq. ornata /auctore Thomas Walter, Agricola. This specimen was later identified as the variety horridulum under the species Cirsium horridulum as coined by Andre Michaux in Flora boreali-american :sistens caracteres plantarum quas in America septentrionali collegit et detexit Andreas Michaux in 1803. C. horridulum var. horridulum is also known as C. horridulum var. elliottii. C. horridulum var. megacanthum was originally described in 1841 as Cirsium megacanthum by Nuttall, but was included as a variety under C. horridulum in 2004 by D. J. Keil. C. horridulum var. vittatum was first described as Carduus vittatus by Small in 1905, but was incorporated as a variety under C. horridulum in 1970 by R. W. Long. Other known synonyms for this variety are Carduus smallii, Cirsium smallii, and Cirsium vittatum.
Cirsium horridulum is a biennial or perennial herb belonging to the New World lineage of true thistles. It can often be distinguished by its well-distributed, increasingly spiny cauline leaves toward the distal end of the plant, and the densely spiny, leaf-like bracts that surround each head. It can be found growing in dumpsters or on roadsides, in meadows, pastures, or woodlands, preferring damp soil and full sun. Flowering times differ across the varieties, as early as February and as late as August. The varieties found in the United States are horridulum, with yellow to red-purple flowers, megacanthum, with pink to purple flowers, and vittatum, solely with purple corollas. Distribution also correlates with variety, but the species can be found from Maine, down the coast to Florida and west to Texas. Like other members of Cirsium, it is considered to be endangered in some states, most likely due to habitat loss, and classified as a weed in others whose economy greatly depends on grazing. Its flowers are attractive to many types of butterflies, bees and beetles, but it is scarcely used as a food source for any other animal.