This widespread southeastern and midwestern diploid (2n = 32) species has been confused with the northern tetraploid species (2n = 64), R. caricetorum (F, FNA 3). Both are wetland plants, with distinctively long repent to stoloniferous branches often produced after flowering (especially precocious in carolinianus). These two species are usually less hairy than their upland relatives, hispidus and fascicularis (both diploids). R. carolinianus differs from all of these species in its relatively short (ca. 3.5-5 mm long versus 5-11 mm), reflexed sepals (versus spreading or tardily reflexed); and its typically less numerous, larger achenes (ca. 3.7-5 mm long versus 2-4.3 mm), with a wider marginal wing (0.4-1.2 mm versus 0.1-0.2 mm). Despite revision of many Ky. colls. by Duncan (1980) and others, some colls. have remained difficult to identify. .