Achillea millefolium is morphologically variable and has been treated as either a single species with varieties or as multiple distinct species. At least 58 names have been used for North American specimens. Some early workers (e.g., J. Clausen et al. 1948) thought the native North American plants were taxonomically distinguishable from introduced, Old World plants. Other workers (e.g., R. J. Tyrl 1975) have treated A. millefolium as a cosmopolitan, Northern Hemisphere polyploid complex of native and introduced plants that have hybridized, forming diploid, tetraploid, pentaploid, hexaploid, septaploid, and octoploid plants and/or populations constituting a single, variable species.