Found on the coastal plain from N.S. to Va., often with the 2 preceding spp., may be a hybrid between them. Its emersed lvs are shallowly pectinate-pinnatifid, with a flat rachis 2-3 mm wide and ascending segments 1-4 mm; the fr is as in P. palustris var. crebra.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.