This occurs in the Southern Appalachians, Appalachian Plateaus and Shawnee Hills, plus disjunctions in the Ozarks and Chihuahua (Mexico). It is restricted to damp recessed walls of non-calcareous cliffs, especially sandstone. There are diploids (2n = 72), tetraploids and occasional sterile triploids. Diploids are prevalent in western regions, but they have not been distinguished with morphological characters (FNA 2). See W for taxonomic literature.