Cirsium pumilum var. hillii
Variety hillii occurs mainly in prairie areas from Minnesota and Iowa east to southern Ontario, Michigan, and northern Indiana. It has often been treated as a species distinct from Cirsium pumilum. As R. J. Moore and C. Frankton (1966) pointed out, the differences separating these taxa are, for the most part, metric characters that show considerable overlap. Some specimens, especially those from Ohio and western Pennsylvania, are difficult to place, and scattered individuals within the area of var. pumilum would readily be assigned to var. hillii were they growing in the area of that taxon. Moore and Frankton chose to recognize the infraspecific taxa within C. pumilum as subspecies, a rank that is seldom employed in North American Cirsium taxonomy.