Thomas Nuttall (1786-1859) described var. megacanthum as "one of the most terribly armed plants in the genus."[8][9]
Cirsium horridulum is a biennial herb up to 250 centimetres (100 in) tall, with a large taproot and fleshy side roots that sometimes sprout new shoots. Leaves are up to 40 centimetres (16 in) long with thick, sharp spines along the edges. There are usually several flower heads, also with sharp spines, Luma apiculata each head with disc florets but no ray florets. Flower color varies from one plant to the next: white, yellow, pink, red or purple.[10]
The Houma people make an infusion of the leaves and root of the plant in whiskey. They use it as both as an astringent, and drink it to clear phlegm from lungs and throat. They also eat the tender, white hearts of the plant raw.[14] The Seminole use the spines of the plant as darts for their blowguns.[15]
It is a larval host to the little metalmark and the painted lady butterflies.[16] Its flowers are popular for their nectar and pollen with butterflies and bumble bees.[16]
^Davidse, G., M. Sousa-Peña, S. Knapp & F. Chiang Cabrera. 2015. Asteraceae. 5(2): ined. In G. Davidse, M. Sousa Sánchez, S. Knapp & F. Chiang Cabrera (eds.) Flora Mesoamericana Universidad Nacional Autónoma de México, México D.F.
^Carnevali, G., J. L. Tapia-Muñoz, R. Duno de Stefano & I. M. Ramírez Morillo. 2010. Flora Ilustrada de la Peninsula Yucatán: Listado Florístico 1–326
^Nelson, C. H. 2008. Cat. Pl. Vasc. Honduras 1–1576. Secretaria de Recursos Naturales y Ambiente, Tegucigalpa
^Nash, D. L. 1976. Tribe IX, Cynareae. En: Nash, D.L. & Williams, L.O. (eds), Flora of Guatemala - Part XII. Fieldiana, Bot. 24(12): 423–428, 590
^"Connecticut's Endangered, Threatened and Special Concern Species 2015". State of Connecticut Department of Energy and Environmental Protection Bureau of Natural Resources. Retrieved 12 January 2018. (Note: This list is newer than the one used by plants.usda.gov and is more up-to-date.)
