Yarrows are erect perennials whose stems have fine white curly hair (some say it looks like cobwebs) and can grow from 8 to 40 inches high and are usually unbranched. All plant parts have an astringent aroma.
The leaves are fern-like, alternate and evenly distributed along the stem, stalked near the base of the stem, stalkless near the top of the stem; they are lance-shaped, widest at the middle; the leaflets are lacy and angled upward along the central stem of the leaf. The leaf blade can be considered as pinnate-pinnatifid to bi-pinnate and the leaflets range from pinnatifid to pinnate ('pinnatifid' meaning that the individual leaflets are not entirely separated, or 'pinnate' meaning they are completely separated).
The floral array consists of a number of corymbs composed each of 10 to 20 small flowers. (A corymb is a flower cluster where the individual flowers have stalks of different lengths to create the appearance of a flat top.) These clusters altogether form a flat-headed panicle.
The flower is very small, 1/4 inch, with 4 to 8 pistillate (fertile) ray florets, 1.5 to 3 mm long and just as wide, that are usually white, but which can be light pink to deep purple; these surround a central disc of 4 to 8 bi-sexual disc florets (some plants may have 10 to 20) that usually have white to grayish or shading to light yellow corollas. The throat of the corolla has five pointed lobes. The disc florets have 5 stamens that are tightly appressed around the style. Each flower head resembles a miniature daisy. The rays have two notches at the tip so the ray laminae appears to have 3 lobes. The lobes are usually white but pastel shades also exist. The underside of the flower head has 3 series of light green, hairy, oblong phyllaries, numbering 20 to 30, which have a darker midrib.
Seed: A flower cluster will produce 500 - 1500 cypselae (similar to dry achenes) which are 1 to 2 mm long with margins winged. The Garden plants are white flowered and grow one to two feet high, which is typical.
Habitat: Yarrow needs well drained soil and full sun, wet to mesic conditions, and due to its short life span, it should be divided every other year. The fibrous rhizomes divide often making this an easier task. These rhizomes help the plant grow vegetatively to form colonies, thus the plant can become weedy.
Names: The genus name Achillea, is from the Greek warrior Achilles. Legend says he always carried the plant with him to treat wounded soldiers during the Trojan war. Millefolium means a thousand leaves. There are a number of varieties and the plant is readily available in the nursery trade in several colors for home gardens where it does well. While some references list a number of varieties and subspecies, the University of Minnesota Herbarium does not recognize varieties within the state and Flora of North America does not recognize varieties in North America. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: A. millefolium resembles A. nobilis, Noble Yarrow, in appearance in that the leaves are 1 to 2 pinnately lobed. However, A. nobilis has fewer phyllaries (10 to 13 in 2 series), more ray florets (8 to 10), the ray laminae are shorter, but wider, and the cypselae are smaller.
Above: Yarrow has an unbranched stem, a composite flower head and leaves pinnately divided. Drawing courtesy Kurt Stüber's Online Library.
Above & Below - Flower detail: The white fertile ray florets have notched petals and surround a similar number of disc florets. 2nd photo below - The individual leaflets are themselves finely divided (pinnate)
Below: Leaves - 1st photo - fully extended lower leaf; 2nd photo - a stalkless upper leaf.
Below: The underside of the flower head has 3 series of light green, hairy, oblong phyllaries, numbering 20 to 30, which have a darker midrib. Flower stalks and cluster stalks all have hair..
Below: The stem is ridged with fine cobwebby hair. Upper leaves are stalkless.
Below: 1st photo - The small cypselae of Yarrow, some still enclosed in their translucent envelope. 2nd photo - a cultivar of Yarrow with red ray florets.
Below: A cluster of basal leaves of a young plant that did not produce a flower stalk.
Notes: Yarrow is indigenous to the Garden area. Eloise Butler catalogued it on May 25, 1907. Yarrow is found throughout Minnesota in introduced and native varieties. In North America there is no state or province where it has not been reported although the native species are more restricted to Canada and the western states of the U.S. Plants found in Minnesota are considered as both - some introduced and some native. Differences are not noticeable by the visual observer.
All four North American species of Achillea are known in Minnesota: A. alpina, Chinese yarrow; A. millefolium, Common Yarrow; A. nobilis, Noble Yarrow; and A. ptarmica, Sneezeweed Yarrow. The first is on the Minnesota 'Threatened' List, and the last two are introductions from Europe and Asia respectively. Only A. nobilis may be confused with A. millefolium - see notes above.
Lore and use: There is considerable literature on herbal medicine uses of this plant. Active constituents are a blue volatile oil containing cineol and a bitter principle, achillein. Densmore (Ref. #5) reports that the Minnesota Chippewa used a decoction of the leaves that was sprinkled on hot stones and inhaled as a treatment for headache. The story about Achilles has merit as Native Americans used a poultice of leaves for the treatment of skin wounds, ulcers and fistulas. 15th Century Russian Herbalists found the same use (Ref. #12). Mrs. Grieve furthers this information by noting that ancient herbalists called it Herba Militaris, the military herb. In old England the plant was called Soldier's Wound Wort and Knight's Milfoil. Yarrow tea was considered a remedy for colds. She gives the recipe. Linnaeus recommended bruised fresh leaves as a styptic (back to the Achilles principle) and in Sweden Yarrow has been used in making beer which Linnaeus considered better that beer brewed with hops. (Ref. #7)
Culpeper wrote: "An ointment of them cures wounds, and is most fit for such as have inflammations; it being an herb of Dame Venus, it stops the terms in women, being boiled in white wine, and the decoction drank; the ointment of it is not only good for green wounds, but also for ulcers and fistulas, especially such as abound with moisture. It stays the shedding of hair, the head being bathed with the decoction of it; inwardly taken it helps the retentive faculty of the stomach; ... and the leaves chewed in the mouth easeth the tooth-ach." (Ref. #4b - The English Physician)
Gerard: Some of Culpeper's thoughts were espoused earlier by Gerard when he wrote: "The leaves of Yarrow doth close up wounds, and keepeth them from inflammation, or fiery swelling; it stancheth bloud in any part of the body, and it is likewise put into bathes for women to sit in; it stoppeth the laske, and being drunke it helpeth the bloudy flixe. Most men say, that the leaves chewed, and especially greene, are a remedie for the toothach. The leaves being put into the nose, do cause it to bleede, and easeth the paine of the megrim. [sic - migraine - see Densmore above]. One dram in powder of the herbe given in wine, presently taketh away the paine of the colick." (Ref. #6a, General Historie of Plants)
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"