A. millefolium L. sensu lato is a variable, circumboreal weed that has often been considered alien in eastern states (Gray 1864; Gm), However, most or all Ky. plants of Achillea appear referrable to the east North American tetraploids (2n = 36) now known as A. gracilis (= A. millefolium ssp. lanulosa (Nutt.) Piper), "with narrower leaf-segments disposed in various planes" (Cr); see also review of W. Closely related plants now known as A. borealis Bongard, sensu stricto, are more western in North America (W and his citations). The European hexaploids (2n = 54), A. millefolium sensu stricto, tend to have flatter, smoother, greener leaves. A practical treatment is still needed, based on the complex variation that has been revealed by molecular analysis (e.g. Guo et al. 2008). Apart from W, there has been little attempt to subdivide the species in recent North American floras (e.g. FNA 19). Reported typical flowering dates for the whole complex range from Apr to Nov, and in western Europe plants sometimes flower through the winter (pers. obs.)..However, flowering in Ky. occurs mostly during mid-May to early Jul. and never in the winter.