This annual is widely scattered in northern, Appalachian and Ozarkian regions. It may be uncomforably close to the more southern P. helleri (Britt.) A. Anderb., as established by Nesom (2001b); see also FNA 19 and Schilling & Florden (2012). P. micradenium is also close to obtusifolium, and there has probably been some confusion. All Ky. records date from before 1950 (M), and most are colls. of B that should be rechecked.