Achillea millefolium L., common yarrow. Perennial herb, becoming dormant during late summer, rhizomatous, fibrous–rooted, each shoot rosetted with ascending leaves, typically unbranched below inflorescence, erect, in range 30—110 cm tall; shoots with finely dissected basal leaves and cauline leaves, long–hairy and glandular–hairy with sessile, spheric hairs, strongly but pleasantly aromatic; rhizomes shallow, horizontal, 2—5+ mm diameter.
Stems many–ridged, to 5 mm diameter, with 3 ridges descending from each leaf, straight, green becoming pinkish to light brown, woolly and somewhat cobwebby.
Leaves helically alternate, 2—3–pinnately lobed with ultimate lobes arranged in 3–dimensions, 35—370 × 5—35 mm, decreasing upward along stem, petiolate (basal and lower cauline leaves) and sessile (upper cauline leaves), without stipules; petiole to 35 mm long, indistinct from leaf axis, thin and strongly flattened, partially sheathing stem, with several parallel veins; blade ± narrowly oblong to narrowly oblanceolate in outline, with alternate to opposite primary and secondary leaflets; rachis ridged along edges with 3 veins most of length (5–veined at base to 1–veined near tip), midvein larger than bordering veins and veins conspicuously raised on lower side, ridges green or pinkish rose; primary leaflets arising along marginal ridge of rachis, the lowest leaflets pinnately dissected with 2—5 divisions, 1—2 mm long, other leaflets 1—2–pinnate, with 2—11 divisions and to 25 × 8 mm, venation inconspicuous on lobes; ultimate segments linear–lanceolate, mostly < 0.5 mm wide, ending as a colorless point at tip, upper surface sparsely long–hairy and with sessile glandular hairs.
Inflorescence heads in flat–topped or convex arrays, terminal, having 3—4 orders of branching, array mostly 50—170 × 35—70 mm, of many heads, head radiate, ca. 9 mm across, with (3—)5(—8) ray flowers and 8—16 disc flowers, bracteate, having scattered hairs to tomentose on axes; bract subtending peduncle of array leaflike; axis of array 5—55 mm long; bracts present along some branches or subtending and along branchlets, decreasing upward, lower bracts pinnately lobed, ca. 10 mm long, bracts of upper branchlet lanceolate to awl–shaped, ca. 2 mm long; peduncle (= 3rd or 4th order branchlet) 1—5 mm long; involucre urn–shaped aging bell–shaped in fruit, 4—5 × 2.3—3 mm, phyllaries typically 10—14 in 3 or 4 unequal series, ovate, 2—3.5 mm long, tomentose and mostly glandular, widely membranous on margins, obtuse to acute at tip and fringed or jagged on upper margin, green along midvein, becoming scarious in fruit; receptacle with ovate, transparent bractlets (paleae), palea as long as unopened flower bud, the outermost paleae subtending the outermost disc flowers and ± like the innermost phyllaries subtending ray flowers.
Ray flower pistillate, bilateral, ca. 2 mm across; calyx (pappus) absent; corolla mostly 3–toothed, glandular–hairy on outer surface of tube and limb; tube 1.2—1.5 mm long, greenish, expanded at base and covering tip of ovary, tapered to mouth; limb ± circular (orbiculate), 1.8—2.3 × 1.8—2.1 mm, white, rounded or with shallow teeth at tip; stamens absent; pistil 1; ovary inferior, flattened front–to–back and narrowly tapered, ca. 1.8 × 0.6 mm, whitish, 1–chambered with 1 brownish ovule visible within; style 1.5—1.7 mm long, swollen at base, 2–branched, pale green or whitish below branches, the branches exserted, arching, ca. 0.4 mm long, purplish, truncate to slightly 2–toothed at stigmatic tip.
Disc flower bisexual, radial, ca. 1 mm wide; calyx (pappus) absent; corolla 5–lobed, 2.3—2.5 mm long, white, glandular on outer surface of tube, throat, and lobes; tube cylindric, ca. 0.5 mm long, flared at base and covering top of ovary; throat cylindric but expanding in upper 1/3, 1.5—1.7 mm long, green below midpoint and white above; lobes deltate, 0.3—0.6 mm, white; stamens 5, fused to corolla at base of throat; filaments ca. 0.7 mm long, white; anthers fused into cylinder surrounding style, somewhat exserted, basifixed, dithecal, 1.2—1.5 mm long, with an appendage at tip 0.2—0.4 mm long, light yellow, the sacs rounded at base, longitudinally dehiscent; pollen yellow; pistil 1; ovary inferior, narrowly inversely conic and compressed side–to–side, ± 2 mm long, whitish or pale green, 1–chambered with 1 brownish ovule visible within, glabrous; style ± 1.7 mm long, swollen and greenish at base, purplish red approaching tip, 2–branched, the branches exserted, ca. 0.5 mm long, dark purplish red, slightly expanded and truncate at stigmatic tips.
Fruits cypselae, dimorphic, of ray flowers cypselae compressed–obovate, 1.8—2 × 0.7—0.9 mm, whitish and glossy with dark brown seed visible, ± ribbed on margins, of disc flower cypselae (often absent) compressed wedge–shaped, 2.5—2.6 × 0.6—0.9 mm, pale to light tan with dark brown seed visible.
A. C. Gibson & B. A. Prigge