Further study of colls. is needed. This reportedly diploid (2n = 20) segregate of hydropiperoides occurs mostly on the Coastal Plain of southeastern states, but it has probably been overlooked inland, especially in the Ohio Valley; it is known as far north as Mich. (FNA 5; Weakley et al. 2023). P. setacea tends to occur in or around ponds that dry out more than typical habitat of hydropiperoides. Although variety status has often been preferred (B, Cr), little or no intergradation is reported (F, FNA 5, J, W, Kim et al. 2008). P. setacea is also close to P. hirsuta (Walt.) Small, a more southern species that restricted to the Coastal Plain. Mapped records of setacea here include var. interjecta Fern (to which most material is referable) and var. tonsa Fern., which are not generally recognized in recent treatments. P. setacea differs from hydropiperoides in its whiter calyx (versus usually pink or roseate at least towards the base); ocreae long-strigose, the fringing bristles ca. 5-15 mm long (versus short-strigose, 1-7 mm); larger leaves 12-35 mm wide and usually strigose (versus 5-25 mm, glabrous or scabrous); stems erect, 2.5-6 mm thick at base, strigose, often branching near middle, from rhizomes (versus ascending, 1-4 mm thick, usually glabrous, mostly simple, decumbent at base). Hairs on the ocreae may be most diagnostic in plants with intermediate characters (FNA 5, based on work of McDonald, 1980): "Specimens of P. setacea without the characteristic ascending or spreading hairs on the ocreae usually can be distinguished from P. hydropiperoides by the extent of adnation of the hairs to the ocreae—up to one-third their lengths in P. setacea, but one-third to two-thirds their lengths in P. hydropiperoides." Taste is just mildly sharp, based on subjective experience.