Cirsium scariosum var. americanum
Variety americanum is a variable taxon widely distributed in mountains and valleys of the western United States from the eastern slope of the Cascades and Sierra Nevada across the northern Great Basin to the central Rocky Mountains. Within some populations dominated by acaulescent forms, occasional short, caulescent individuals or even taller plants may occur. These approach var. coloradense in the Colorado Rockies and resemble var. scariosum in the valleys of Oregon. Short caulescent individuals within populations of var. coloradense and var. scariosum approach var. americanum. Although the extremes are strikingly different, the genetic differences are apparently slight. Acaulescent individuals from the San Francisco Peaks and White Mountains of Arizona are treated here as var. coloradense. In the mountains of Colorado putative hybrids have been documented between Cirsium scariosum var. americanum and C. canescens and between C. scariosum var. americanum and C. undulatum.