This Appalachian species is a tetraploid (2n = 48), apparently derived from hybridization of the southern Appalachian P. mengesii (W. Wolf) Kiger and the widespread western P. parviflorus (Nutt.) Kiger. Both of these parents are mostly diploid but with some local tetraploid races (FNA 4). These two species may also be expected in Ky., since they occur within 50 miles of the state line (K): mengesii in Morgan Co. of e. Tenn.; parviflorus in Pope Co., of s. Ill. P. teretifolius has intermediate flower morphology and, reportedly, intermediate diurnal time for openings (Murdy & Carter 2001): mid-afternoon (versus early in mengesii and late in parviflorus).