This annual occurs mostly in the Great Plains, and reports from Ky. have probably been erroneous (M). However, petiolaris is adventive in se. Mo., where hybrids or introgressants with annuus can be difficult to distinguish from the parents (Y; Ladd & Thomas 2015). Colls. of R. Athey (EKY, MEM) under "petiolaris" are somewhat distinct from annuus in their finely serrated leaf margins, but they are probably closer to annuus.