Boreal yarrow is a tall forb in the aster family (Asteraceae) with finely pinnately-divided leaves and a compact cluster of white flowers arising from a basal rosette. The plant is perennial, growing from rhizomes, and occurs in dry grassy places and meadows, and is also a colonizer of roadsides and other disturbed sites. Boreal yarrow grows 20-60 cm tall with alternate leaves that are 3-15 cm long, narrow, lanceolate in outline and twice-divided into very fine segments. Stems are unbranched to few-branched above, and hairy. The many small, compound flowers grow in flat-topped branched inflorescences (a corymb) at the top of the stem. Each head is composed of several tiny flowers: ray flowers with an inconspicuous, white tube-like corolla, and a few disk flowers with a longer white petal. The composite flowers are subtended by numerous dark-margined bracts. The plants produce dry, indehiscent fruit with a persistent calyx with 1-2 mm broad wings. The smell of boreal yarrow is distinctive when the plant is crushed. Boreal yarrow can be confused with Siberian yarrow (Achillea sibirica, a wetland plant), but in that species the leaves are pinnatifid, not as finely divided and twice-pinnate as in boreal yarrow.
This plant grows a basal rosette of leaves in early spring, later sending up fruiting stalks. Plants flower in summer and fruit in fall, lying dormant through the winter. Even after senescence, the stiff stalks will often remain upright and plants in springtime sometimes can be found with the previous year's flowering stem.
Plants are monoecious with bisexual flowers. Boreal yarrow is insect pollinated, most likely by flies (Andersson 1991). Flowers are self-incompatible. The plants produce dry, indehiscent fruit with a persistent calyx with 1-2 mm broad wings. These small fruits are wind-dispersed. However, plants primarily reproduce vegetatively and dedicate fewer resources to seed production (Warwick and Black 1982). This subspecies is part of a worldwide complex, with many co-existing polyploid populations. In Alaska, most plants are tetraploids (4 sets of chromosomes) and some are hexaploids (8 sets). Hexaploids may have a fitness advantage in some habitats (Ramsey 2011).
Yarrow is widely used as a medicinal plant by Alaska Natives and modern-day Alaskans, owing to the variety of biologically active compounds it produces. All parts are used to address numerous ailments. Treatment forms are often as a tea or poultice for skin injuries and infections, coughing and chest congestion, recovery from childbirth, and to stop bleeding. (Garibaldi 1999)
Achillea millefolium subsp. borealis is a North American subspecies of the circumpolar Achillea millefolium species complex that occurs across Canada from the Maritime Provinces to British Columbia and the species occurs as far south as California on the west coast. Boreal yarrow occurs across Alaska from north to south and east to west. In Denali, boreal yarrow occurs frequently on the south side of the Alaska Range, with widely-scattered localities on the north side.
In Denali, boreal yarrow occurred across a wide range of elevations, ranging from 256-1437 m with the average occurrence at 937 m. This species occurred on widely varying slope angles from 0 to 45 degrees, with a mean slope angle of about 20 degrees. This species is more common on south-facing slopes than on northern aspects (57 vs 22 occurrences).