This tetraploid (2n = 24) perennial appears to have decreased greatly across most of its originally widespread range in eastern states. It is now concentrated in mid-western states, from Mo. to Mich. (K, NS). There are old colls. from Ky. made by A. Michaux (1803; P. kentuckiensis Michx. at Paris), C.S. Rafinesque (1836, 4:11; P. albiflora Raf. perhaps lost), R. Peter (PH), and C.W. Short (NY). However, no clues about localities can be derived from these records. The species is semi-aquatic and is usually associated with the shores of medium-sized streams on fine-textured, base-rich substrate. It is likely that persistent access for livestock and other disturbances have contributed to its decline.