This is largely restricted to the southern Interior Low Plateaus and the Black Belt. It occurs mostly on base-rich soils in rocky glades, old fields and roadsides. Distinction from hirsutus can be difficult (Pennell 1935, Clements et al. 1998); see also otes under pallidus, which may hybridize.. P. tenuiflorus differs from hirsutus in its generally pubescent leaves (versus thin hairy to glabrate between veins), with stellate hairs frequent (versus usually infrequent), blades remotely serrate to entire (versus usually with numerous teeth), relatively firm and gray-green (versus thin and bright green). Its flower buds are yellow (versus yellow-purple), the corolla creamy white at anthesis (versus flushed violet-purple in tube); sepals are 2-5 mm long (versus 3-8 mm). Hybrids have not been reported from the wild but they can be artificially produced, as reviewed by Clements et al. (1998).