This variable annual (2n = 34, 36) is widespread across temperate regions of North America, but largely restricted to disturbed soil and especially common in cropped areas (and locally in burned areas). Segregates have been described, but none have been recognized in recent treatments. All plants in Ky. would be referable to var. elatior (L.) Descourtis, following F. Var. artemisiifolia is native to the northeastern Coastal Plain, and remains unknown in Ky., despite some reports (M). The southern var. paniculata (Michx.) Blank. is more likely, and has also been reported (BA) but not confirmed. The pollen of this species (as in and others of the genus) is a major cause of allergic reactions by humans (White & Bernstein 2003, Wopfner et al. 2005).