This "water-pepper" is a Eurasian annual is now scattered across much of North America except for warmer or drier regions (F, FNA 5, W). In Ky. most records are from the northern Bluegrass, but it may be more widespread. P. hydropiper is a variable diploid (2n = 18-24. cited by Kim et al. 2008) that can be confused with punctata, especially the annual var. confertifolium (F, FNA 5). Many published records from southeastern states (especially Tenn., Ala., and Miss.) appear to be based on misidentified punctata. P. hydropiper has seeds that are minutely roughened and dull (versus smooth, lustrous). Its calices are mostly greenish or pink-purplish (versus whitish). Its inflorescences have lax arching-drooping branches (versus erect to spreading), and are generally more condensed, with axillary branches sometimes enclosed in ocreae (versus never) and subtended by regular leaves (versus often reduced). The acrid or pungent chemistry of this hydropiper (and presumably punctata) involves diverse flavonoids (some sulphated), sesquiterpenoids, glucosides and formic acid (e.g. Haraguchi et al. 1996, Starkenmann et al. 2006, Ayaz et al. 2015), but there has been little definitive summary. There has been much traditional medicinal or culinary use of hydropiper in east Asia, with selection of cultivars. It has a sharp to acrid peppery taste. See also notes on "persicarin" under Oenanthe javanica.