Achillea millefolium
Achillea millefolium was first described by Linnaeus in Species Plantarum in 1853.
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.
The morphological variant previously recognized as Achillea Borealis or Achillea millefolium var. Borealis is a species of special concern in Canada. (USDA, 2010.) Similarly, the eco-morphotype previously known as A. megacephala or A. millefolifum var. megacephala is of special concern in Canada. (Trock, 2006.)
Although A. millefolium contains alkaloids and other volatile compounds, it is not considered toxic because it is so rarely consumed by livestock. (US Forest Service, 2010.)
The chromosome number of A. millefolium varies, and has been observed as 2n = 18, 27, 36, 45, 54, 63, 72. (Trock, 2006.)
In the Western US, the tetraploid (2n=36) and the hexaploid (2n=54) are the dominant ploidy levels. (Tyrl, 1969.) Capable of hybridization between ploidies, A. millefolium has been observed at every ploidy level up to octoploid (2n=72) worldwide.
Achillea millefolium has a moderate growth rate.
The feathered leaves of Achillea millefolium resemble those of Anthemis cotula (mayweed chamomile), Matricaria discoidea (pineapple weed) and Daucus carota (wild carrot), though all can easily be distinguished by their blooms. (Calhoun, 2010.) Conversely, the flowers of A. millefolium are easily confused with those of Lepidium latifolium (perennial pepperweed) and Cardaria draba (hoary cress). The highly divided leaves of A. millefolium allow for distinction from these look-alikes. (Donaldson, 2004.)
Achillea millefolium has pollen classified as trizonocolporate, with a lacunate aperture and echinate sculpturing. (Meo & Khan, 2003.)
According to a 2008 study, A. millefolium has travelled from Europe and southwest Asia to east Asia, then to North America. The A. millefolium aggregate has been shaped by cycles of differentiation, hybridization and polyplodization, resulting in the now worldwide expanse of the hexaploid (and other polyploids). (Guo, Saukel & Ehrendorfer, 2008.)
Achillea millefolium is a source of nectar for many species of flies, wasps and a few bees. Some grasshoppers, aphids, beetles and caterpillars may feed on it.
Achillea millefolium spans the entire continental U.S. (including Alaska) and every Canadian province.
For more information, see the map above provided by Flora of North America. (Trock, 2006.)
A. millefolium occupies many diverse habitats, including sunny pastures, meadows and roadsides with dry soils, stream sides and waste grounds with sandy or salty soils, and damp woodlands with clay soils. (Trock, 2006.) There is, however, some morphological variation (eco-morphotypes or ecotypes) among the species that is dependent on its habitat.
Perennial
Axillary rhizomes of A. millefolium produce new plants annually at their apices. Vegetative reproduction occurs when the rhizomes are fragmented. (Bourdot, Field & White, 1985.)
Achillea millefolium has different flowering periods depending on where it's located. In the south, it flowers late April to early July, but in the north it doesn't begin flowering until mid-July and will continue through mid-September. (Trock, 2006.)
Achillea millefolium is subject to powdery mildew, root and stem rot.
Achillea millefolium has a long history of cultural and medicinal uses. In some eastern cultures it has been used in the mystical practice of divination. A. millefolium can be distilled to form an oil that possesses known anti-inflammatory and blood-staunching abilities. It has been used to treat internal bleeding, excessive menstrual bleeding, high blood pressure, fever reduction and more. The leaves have been used, both fresh and dry, as a garnish and in salads. (Morgenstern, 2010.)