This is widespread in mid-temperate to arctic regions across the Northern Hemisphere. Within Ky. it becomes less frequent to the southwest, and further south it is largely absent on the southeastern Coastal Plain (K, W). Several segregates have been named but these have not been recognized in recent treatments (FNA 2; W).Christenhus et al. (2019) have provided details of variation and nomenclature, stating: "...arvense is incredibly plastic in morphology and adapts readily to various environmental conditions, giving rise to the numerous varieties, forms and monstrosities recognized in the past". Also hybrids are known with several other related species elsewhere in North America (but not with the praealtum-hyemale group). E. sylvaticum L. and E. pratense Ehrh. are related northern (circumboreal) species that have also been reported from Ky.,, probably in error (M) but relictual hybrids are possible. Variation in stem anatomy of arvense and its aliies deserves closer examination, following F, Hauke (1965; FNA 2), Cobb et al. (2005) and others. D. Dourson & C. Christensen have recently found unusual arvense-like plants in WOLF (along Devils Cr.) with the central cavity only 1/8 to 1/6 as wide as the stem, published treatments indicate that the cavity in arvense is 1/5 to 2/3 as wide. Thus, these plants might suggest hybridization with another northern species: E. palustre L., which has distinctively narrow central cavities. Hauke (1965) separated pratense and palustre from arvense (and the more robust sylvaticum) as follows: first internode of branch shorter than or about equal to subtending stem sheath, at least in lower part of plant (versus longer). E. arvense is potentially somewhat toxic to mammals because of its thiaminase content and possibly other slowly-acting chemicals (Pohl 1955, Tago et al. 2010). However, this species can be reduced by deer (Kellogg & Bridgham 2004).