A. millefolium is a perennial herb that usually spreads via rhizome. It is characterized by feathered leaves, proximally petiolate and distally sessile, that gradually reduce in size as they approach the apex of the plant. The ray flowers can range from white to pink to dark purple, but the disc flowers are always white or grayish white. It has a variety of ploidy levels, from 2n = 18 to 2n = 72. Because of its huge degree of variability, it has often been treated as several distinct species or as one species with several varieties. It has one of the most expansive ranges, growing in virtually every corner of North America. This may be due to its ability to tolerate meadows and woodlands, dry or damp soils, and to accomodate altitudes from sea level to 3600 m. In fact, much observable variation in A. millefolium can be contributed to environmental differences.
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