Plant 243

Achillea millefolium L. (Asteraceae)



Yarrow is a common grassland and woodland herb, familiar because of its heads of white or pink flowers, and its highly dissected leaves which produce a highly aromatic, dark blue essential oil. The name Achillea was associated with yarrow long before Carolus Linnaeus adopted it as one of his genera when binomial scientific naming was established in 1753.

In 1960, in the Shanidar Cave in Iraqi Kurdistan, an archaeological site more than 60,000 years old, a skeleton of a middle-aged, Neanderthal male was discovered in soil with a high density of pollen grains, including yarrow. Given the context, archaeologists inferred these plants were related to either Neanderthal burial rituals or their medicinal practices. However, such conclusions have been cast into doubt as some rodents in the region cache flowers.

Whether yarrow is the herb Achilles used to treat wounded soldiers is a matter of academic dispute. However, many traditional names for the plant have been spawned that reflect associations with battle, blood and wounds, e.g., herba militaris, soldier's woundwort and stanchweed. The flower heads of yarrow have also been used in beer as substitute hops. Yarrow is also used to create drought-tolerant pasture and breed horticulturally-important plants.

Yarrow is a classic example of a widespread species that shows ecotypic variation, that is specific plant forms are associated with particular habitats. For example, short yarrow plants might be associated with coastlines and high-altitude habitats, whilst tall plants occur in damp and woodland sites at low altitudes. Such patterns of variation, combined with differences in the number of whole chromosome sets (polyploidy), mean discrete species are often difficult to define and name. The result is a species complex.

Achillea millefolium, found throughout temperate regions of the Eurasia and North America and extending from sea level to over 3,000 metres above sea level, is used as a model to study the evolution of such complexes. In Eurasia, diploid races (two chromosome sets) are most commonly found together with some races with four (tetraploid) or six (hexaploid) sets of chromosomes. In North America, the polyploids are most frequent.

Detailed phylogenetic analysis of nuclear and chloroplast DNA sequence data show that the North American populations are distinct from their European and Asian relatives. Furthermore, these data support the idea that yarrow rapidly differentiated ecologically following its colonisation of North America via the Bering Land Bridge during the Pleistocene (1.8 million years to 11,500 years ago).

Further reading

Applequist WJ et al. 2011. Yarrow (Achillea millefolium L.): a neglected panacea? A review of ethnobotany, bioactivity and biomedical research. Economic Botany 65: 209-225.

Ramsey J et al. 2008. Rapid adaptive divergence in New World Achillea, an autopolyploid complex of ecological races. Evolution 62: 639-653.

Warwick SI and Black L 1982. The biology of Canadian weeds. 52. Achillea millefolium L. s.l. Canadian Journal of Plant Sciences 62: 163-182.

Stephen Harris