The close relative of pensylvanicum occurs in the Great Plains, east to c. Mo. and s. Ill. The only known coll. from Ky. was made by D. Demaree in MCRA (6 Oct 1951, at PU). Horton (1972) did report it from Ky. but details are unknown. P. bicornis differs from pensylvanicum (FNA 5) in its consistently pink and heterostylous flowers (versus white to roseate and homostylous); its achenes with "an obscure or prominent hump in the center of one face"; and its frequently narrower leaves (ca. 1-2.3 cm versus 1-4.8 cm).