This narrow-leaved aquatic is widespread in most humid temperate regions of North and Central America, but it is rare to absent in more southeastern states, especially on the Coastal Plain (K, W). In Ky. it occurs in various kinds of streams, but it is concentrated close to the Ohio Rv. and its major tributaries. C.W.Short collected it from JEFF during 1840: "In shallows of the Ohio River, called "grassy flats"" (PH); A.T. Hotchkiss also collected it from the Falls area in 1971 (DHL). H. dubia (and perhaps other species in the genus) may be susceptible to grazing by vertebrate herbivores, with relatively low alkaloid content (Ostrofsky & Zettler 1986, Dick et al. 2016).