This variable subaquatic species, reportedly tetraploid (2n = 40), is widespread across North and Central America, especially in stagnant water that can drop in the summer. Typical hydropiperoides in Ky. (including var. buschianum Stanford) has largely whitish calices (not roseate or purplish as stated in F), with glands concentrated towards the base or absent. However, there is much variation. The species is closely related to punctatum and setaceum, so that further study is needed for reliable separation in the state; see notes under those names. Also, a southern segregate to be expected here has been named P. opelousanum (Riddell) Small, but this reportedly intergrades extensively with typical hydropiperoides (FNA 5). As a species (F) or variety (Cr), opelousanum is reported to differ from hydropiperoides in its slightly exserted achenes (versus included), calices dull purple, greenish or creamy (versus bright pink to white), and narrower leaves (5-15 mm versus 5-25 mm), with more distinct glands on lower surfaces (and on calices). Both hydropiperoides and opelousanum have "multicellular plate-glands" (especially on the lower surface of their leaves), unlilke any other North American species in the genus, but also found in some maculosa (FNA 5). Hobbs (1992) noted: "The sepals and other inflorescence parts sometimes bear flat, sparse, whitish dots or excrescences which are easily dislodged. These should not be confused with the resin glands of P. hydropiper and P. punctatum which are imbedded in the tissue and cannot be dislodged." P. hydropiperoides sometimes has a sharp taste like that of punctatum and hydropiper; its strength may be correlated with the degree of glandularity in flowers or leaves.