This species of eutrophic woodlands was recently described, but R. Brooks and others have been aware of its distinctiveness for several decades (Campbell 2000). It is the earliest flowering wild rye (Elymus) in eastern North America, with anthesis usually during mid- May to mid-Jun (versus mid-Jun to late Jul+ in other taxa of the virginicus group). Typical woods in Ky. have frequent Acer saccharum (sensu lato), Aesculus glabra, Carya cordiformis, Celtis occidentalis, Fraxinus americana, Juglans nigra or Ulmus americana; oaks (Quercus spp.) generally form a minor component. C.W. Short (PH) identified this grass in ca. 1830 as Elymus canadensis: "Hab. deep shaded woods near Lex[ington]." The correct name is in honor of the singular Clan MacGregor, not the plural "macgregoriorum" (J). ("Elymus interior" was an early provisional unpublished name). Variation deserves further study. Some colls. from CAMP, FAYE and elsewhere have distinctly pubescent spikes, sheaths or other plant parts, but these may not be common enough to deserve recognition as a variety; they have been named var. hirsutisplicus A. Haines (W). E. macgregorii appears to form occasional hybrids with hystrix, virginicus and, more rarely, the later flowering glabriflorus, which it most resembles; 2n = 28 in these and most other eastern Elymus species (FNA 24). Some populations have unusually narrow glumes, suggesting introgression from hystrix.