This is native to Eurasia and apparently alien in North America but spreading south from northeastern states; see notes under gracilis.. Compared to gracilis (2n = 26), dioica sensu stricto (2n = 56) is a generally less tall and less branched plant; rhizomes are bright yellow (versus pale yellowish-white); leaves tend to have broader shape (often with cordate base) and deeper teeth (mostly 5-6 mm versus 2-3.5 mm), it is generally (versus rarely) dioecious (W). U. dioica has more stinging hairs, including upper leaf surfaces, where absent in gracilis; hairs increase in plants exposed to herbivory or similar damage (Dalin et al. 2008). The stinging mechanisms use histamine, serotonin and acetylcholine (Cumming & Olsen 2011). In Ky., dioica is known only from a few recent colls. made after 1980, mostly near the Ohio Rv., in LEWI (BEREA), JEFF (EKY), SHEL (EKY) and MEAD (APSC); M. Brock at APSC first determined some of these colls. There are also scattered reports from most adjacent states, especially in the central Appalachians (FNA 3; W). Although the two species are intermixed at some sites, and hybrids have been suspected, artificial attempts to make crosses have failed (citations of W).