Achillea millefolium

common yarrow

Asteraceae

The Basics

USFS Plant Database

Flora of North America

Western yarrow is a pioneer species everywhere it is found. It is an invader species on disturbed rangeland sites. Western yarrow also appears to be tolerant of competition but not tolerant of excessive shade. It is usually present in the earliest stages of vegetation development and persists throughout succession. It dominates on overgrazed high summer ranges, where the undisturbed climax vegetation would be made up of wheatgrasses (Triticeae).

Native Americans used tea made from western yarrow to relieve ear-, tooth-, and headaches; as an eyewash; to reduce swelling; and as a tonic or stimulant. Western yarrow varies in taste and in potency depending on where it grows and at what stage of growth it is in. The best time to collect yarrow for tea is right before the flowers are produced, using only the new succulent leaves. During the Civil War, western yarrow was widely used to treat wounds and became known as "soldiers' woundwort." An ethanol extract of western yarrow has mosquito- repelling properties.

The scientific name of western yarrow is Achillea millefolium L. (Asteraceae). There are both native and introduced phases of western yarrow in North America. Introduced and native phases differ primarily in chromosome number and are difficult to distinguish morphologically. Native and introduced phases hybridize. The intricate pattern of morphologic, geographic, and ecologic variation within the species has frustrated all efforts to organize an intraspecific taxonomy on a circumboreal or even a strictly North American basis.

Western yarrow varies greatly in forage value, depending on locality and seasonal development. It is generally unpalatable, although domestic livestock and wildlife occasionally consume the flowers. Cattle and horses usually do not graze western yarrow, but bighorn sheep, pronghorn, and deer may use it.

Western yarrow contains volatile oils, alkaloids, and glycosides but is not generally considered a toxic plant because it is so seldom consumed by livestock. Milk from cows consuming western yarrow has a "disagreeable" flavor. (Plant Database)

Identification

Western yarrow is a perennial forb 30-100 cm in height with extensive rhizomes. It has few to numerous erect stems. The basal rosette of leaves may remain green throughout the winter. Plants grow in a somewhat scattered fashion and seldom form pure stands in areas larger than 5 square meters. Typical European Achillea millefolium is hexaploid with flat leaves. Native forms are mostly tetraploid, with narrow leaf-segments disposed in various planes so that the leaf is 3-dimensional. (Plant Database)

Perennials, 6–65+ cm (usually rhizomatous, sometimes stoloniferous). Stems 1(–4), erect, simple or branched, densely lanate-tomentose to glabrate. Leaves petiolate (proximally) or sessile (distally, weakly clasping and gradually reduced); blades oblong or lanceolate, 3.5–35+ cm × 5–35 mm, 1–2-pinnately lobed (ultimate lobes ± lanceolate, often arrayed in multiple planes), faces glabrate to sparsely tomentose or densely lanate. Heads 10–100+, in simple or compound, corymbiform arrays. Phyllaries 20–30 in ± 3 series, (light green, midribs dark green to yellowish, margins green to light or dark brown) ovate to lanceolate, abaxial faces tomentose. Receptacles convex; paleae lanceolate, 1.5–4 mm. Ray florets (3–)5–8, pistillate, fertile; corollas white or light pink to deep purple, laminae 1.5–3 × 1.5–3 mm. Disc florets 10–20; corollas white to grayish white, 2–4.5 mm. Cypselae 1–2 mm (margins broadly winged). 2n = 18, 27, 36, 45, 54, 63, 72 (including counts from Europe). (Flora of North America)

Threats

Western yarrow is not highly flammable...Western yarrow's rhizomes and mycorrhizae are usually only slightly damaged by fire, although western yarrow is susceptible to fire-kill and reduction by severe fire...Fire results in fragmentation of western yarrow's rhizomes stimulating regeneration. Cover and frequency of western yarrow generally increase 1 to 2 years after fire but not with any consistent pattern...Following fire, regeneration is from rapid rhizome spread and wind dispersal of seeds onto burned sites from adjacent unburned areas. (Plant Database)

Reproduction

Flowering and Fruiting - The fruit is a small achenes weighing about 0.17 mg. They are produced in large numbers. Several thousand achenes may be produced per flowering stem. The viability of freshly shed seeds exceeds 90%. Western yarrow seed showed 41% germination after 9 years in dry storage.

Vegetative Reproduction - Western yarrow regenerates from fragments of rhizomes and from colonization through short-distance (1-2 m) wind dispersal of seeds. In disturbed soils, fragmented rhizomes regenerate shoots which can emerge from soil depths as great as 30 cm. In undisturbed soil the rhizomes remain attached to the parent plant, forming new plants at the rhizome apices. (Plant Database)

Species Distribution

Western yarrow is circumboreal. In North America, it occurs in every state, province, and in Mexico. It is adventitious in Hawaii. (Plant Database)

Citation

USDA Plant Database http://plants.usda.gov/characteristics.html USDA, NRCS. 2016. The PLANTS Database (http://plants.usda.gov, 4 February 2016). National Plant Data Team, Greensboro, NC 27401-4901 USA.

Flora of North America http://www.efloras.org/flora_page.aspx?flora_id=1 Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

Silvics of North America Burns, R.M., and B.H. Honkala. 1990. Silvics of North America (Volume 1: Conifers, Volume 2: Hardwoods). USDA Forest Service Agricultural Handbook 654.

Intermountain Herbarium http://intermountainbiota.org/portal/collections/harvestparams.php Consortium of Intermountain Herbaria. 2016. http//:intermountainbiota.org/portal/index.php. Accessed on February 04.

Burke Museum Plant Image Collection The plant image collection at the Burke Museum, University of Washington.

Jepson Manual http://ucjeps.berkeley.edu/eflora/ The Jepson Manual: Vascular Plants of California. B.G. Baldwin, D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken [editors]. 2012. 2nd edition, thoroughly revised and expanded. University of California Press, Berkeley, CA., hardcover; 1600 pages. ISBN-13: 978-0520253124.

USGS Plant Species Range Maps http://esp.cr.usgs.gov/data/little/ Critchfield, W.B., and Little, E.L., Jr., 1966, Geographic distribution of the pines of the world: U.S. Department of Agriculture Miscellaneous Publication 991, p. 1-97. Little, E.L., Jr., 1971-1978, Atlas of United States trees, volume 1,3,13,17, conifers and important hardwoods: U.S. Department of Agriculture Miscellaneous Publications.