This wetland annual is known from the Ozark region, southern Interior Low Plateaus and Gulf Coastal Plain. In Ky. its distribution is similar to that of capillaceum and there are no clear differences in habitat (M), but nuttallii tends flower and fruit 1-2 months earlier (F, W). P. nuttallii differs (Cr, W) in its longer styles (0.5-1.5 mm versus 0.2-0.5 mm), more elongated seeds (subglobose and ca. 1.5 mm versus ovoid and ca. 2 mm), bracts mostly entire (versus often 3-cleft), and leaves with primary divisions alternate or opposite (versus commonly 3 at a node); 2n = 14 (versus 14 and 28). There has been some confusion between these species, and some records need to be rechecked.