This annual weed is widespread across temperate North America. In Ky. it may have been widespread before settlement, but increased greatly during the 1800s to become a major problem in hay-meadows and over-grazed pastures. It is not clear if there are records from Ky. earlier than 1914, when Gm considered it "introduced." Typical annuus is mostly triploid (2n = 27) and apomictic (FNA 19, Y). There is considerable variation and possible intergradation with E. strigosus, but varieties have not been widely recognized within annuus. Both species flower mostly during Jun-Sep in Ky. According to FNA 19, annuus can be distinguished by its stems "sparsely piloso-hispid" with spreading hairs or "sometimes strigose distally" (versus "± strigose to strigillose or hirsuto-strigillose", with hairs usually ascending); also basal leaves usually wither by flowering (versus usually persistent); cauline leaves are little reduced up to mid-stem (versus gradually reduced); blades in general are lanceolate to oblong or ovate (versus spathulate to oblanceolate to linear), with margins often coarsely serrate (versus no more than slightly serrate or crenate).