Know Your Natives – Yarrow

Yarrow (Achillea millefolium) of the Sunflower, Aster or Composite (Asteraceae) family has distinctive, highly dissected, frilly leaves. The species, the only Arkansas member of the genus Achillea, is native to North America, Asia, and Europe––one of the widest worldwide ranges among the flowering plants. It occurs across almost the entire U.S., including throughout all of Arkansas. The genus name originates from “Achilles,” the mythological Greek hero of the Trojan Wars, who is said to have discovered the plant’s healing powers. The specific epithet, from Latin, means “thousand-leaved.” Other vernacular names include milfoil (based on the French word for “thousand-leaved”), soldier’s woundwort, woolly yarrow, and thousand-leaf. Preferred habitat is well drained soils in full sun in prairies, grasslands, and roadsides, and occasionally, in partial sun of open woodlands.

This herbaceous perennial may occur as individual clumps, larger communities of intertwined clones, or as thick “weedy” stands. It has shallow, skinny, horizontal rhizomes that give rise to long thread-like roots. New rosettes of basal leaves, growing from rhizome tips, are present in mid-winter. Erect stems, beginning development in late winter, eventually mature at 2-3 feet tall. Stems are unbranched to the inflorescence and typically covered with short woolly pubescence. Flowering occurs from May to June. After seeds mature in August, dead stems recline and persist into winter. Crushed plants have a strong scent that is comparable to chrysanthemums.

Photo 1: Rosettes of basal leaves grow during winter months. Photo – late March.

Compound basal and cauline leaves are bipinnately divided and dissected, with closely spaced, finely cut leaflets throughout. Basal leaves grow to 8 inches long and ½ inch wide, their largest leaflets occurring near mid-leaf. Leaflets terminate in sharp, yet flexible mucronate points. Leaflets and sub-leaflets are lightly twisted in various planes so that a leaf appears frilly and soft. Alternate, well-spaced cauline leaves are sessile to sub-sessile and cyclically arranged. Leaf pubescence varies from hairy to nearly glabrous.

Photo 2: Basal leaves, showing upper side (above) and lower side (below). Complexity of sub-leaflets decreases toward leaf base and apex. Photo – mid-January.

 

Photo 3: Cauline leaves are alternate and cyclic about the stems. Stems are erect, but easily damaged by wind and rain. Photo – late April.
Photo 4: Cauline leaves have the same appearance as basal leaves (see Photo 2). Note pubescence of stems. Photo – late April.

Yarrow produces a terminal inflorescence in April and May. Branches are cyclically arranged below the stem apex, with the longest branches lower on the stem. The ascending sturdy branches divide into peduncles (stalks of multiple flower heads) and, ultimately, into pedicels (stalks of individual flower heads). Each branch produces a corymbose inflorescence which, merging with other branches, forms a compound corymb with a flat to slightly domed top 2 to 4 inches broad. Compound corymbs may consist of 150 to 200+ small flower heads (¼ inch long and across) that are tightly spaced to partially overlapping. Branches, peduncles, and pedicels are a pale green with short pilose pubescence.

Photo 5: This compound corymb is composed of several smaller corymbs, each on a separate branch. Nectar and pollen attract a wide variety of insects. Photo – early June.

Each composite flower head comprises 4-6 pistillate ray florets which surround 8-20 perfect disk florets. Corollas of the ray florets, white to occasionally pink, bear a broadly rounded, shallowly 3-lobed, white ligule atop a slender hidden tube that attaches to the apex of an inferior ovary. The cream-colored tubular disk florets have five recurved, short, triangular corolla lobes. A ring of 5 yellow disk floret anthers emerges from the disk corolla, after which the style, acting as a kind of plunger, is exserted through the anther ring and presents the pollen, making it available to pollinating insects. After pollen is disbursed, the styles (of both ray and disk florets) split and recurve as their stigmas become receptive.

Florets are tightly contained by an elongate involucre composed of imbricated, appressed, oblong-lanceolate, hairy phyllaries (bracts). Phyllaries have yellow apices, a narrow pale-green lanceolate midrib, and wide light-colored scarious (thin and dry) borders. 

Photo 6: Display shows lower and upper side of a portion of a compound corymb. Phyllaries have a green midrib and scarious borders. Yellow elongate anthers of several disk florets that are approaching anthesis can be seen. Photo – mid-May.
Photo 7: The composite flower heads have 4 to 6 pistillate ray florets surrounding 8 to 20 perfect disk florets. As shown, anthers of most disk florets have already dispersed pollen and exserted styles are bifurcated. Photo- mid-May.

With fertilization, ray and disk florets produce light tan, oblong, flattened, 1-seeded achenes. No pappus is present. The flat apical edge of the achenes bears a central raised scar which results from attachment of the corolla tube. The light-weight seeds are dispersed short distances by wind.

In well-drained soil, yarrow is both a widely cultivated ornamental as well as a lovely addition to a native plant garden or natural area, appreciated for its winter foliage, its distinctive finely textured leaves year round, and its (usually) bright white inflorescence. The slender stems are subject to wind and rain damage and decline soon after seeds mature. Plants can spread aggressively by rhizomes and self-seeding. Removal of spent stems after bloom restricts plant-spread by self-seeding and keeps plants tidy in late summer. Nectar and pollen attract a wide variety of insects. Yarrow is drought-resistant and is not preferred by deer or rabbits. It has a rich history of medicinal and herbal use.

Characteristics of yarrow’s leaves are sufficiently distinct so that it is readily distinguished from any other native (or non-native) species occurring in the state.

Article and photographs by ANPS member Sid Vogelpohl

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