This robust annual or short-lived perennial occurs mostly in swamps and seeps of the southeastern and Atlantic Coastal Plains, Ouachita Mts. and southern Cumberland Plateau. However, there are disjunct interior records as far north as s. Mich. (Cr, FNA 23, K). The two Ky. records come from depressional wetlands on karst plains of the Mississippian Plateaus; both sites have diverse native flora with several conservative and rare species. Although the perianth bristles of macrostachya are distinct from those of corniculata in length (11-14 mm versus 2-4 mm) and other features (Y), these two species are sometimes confused. R. macrostachya also tends to have more congested inflorescences, with mostly 10-20 florets per cluster (versus 3-5); its leaves tend to be narrower, up to 10 (15) mm (versus 20 mm); and it lacks rhizomes (versus often densely colonial).