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Mat-forming herb with a horizontal, branched rhizome and short, vertical branches of caudex ending in leaf rosettes. Stems ascending to erect, up to 20—30 cm, with numerous leaves (sometimes more than 10). Leaves, stems and inflorescence branches with a more (stems, inflorescence) or less (leaves) dense pubescence of 1—2 mm long, very soft, white hairs.
Leaves alternate, up to 12(14) × 2—3 cm, narrowly oblong in outline, with short petioles (less than 1/8 of leaf length), 2—3 times pinnatisect with narrowly lanceolate, acuminate lobes with white, coriaceous tips.
The primary inflorescence of Asteraceae is a head (capitulum) surrounded by an involucrum of one or more rows of phyllaries (involucral bracts). The flowers sit on a flat, concave or convex receptacle, sometimes with scales (the bracts of the single flowers). The flowers are epigynous with perianth at top of the gynoecium. The sepals are transformed into a pappus, mostly by hairs or sometimes by scales (but absent in Achillea). The 5 stamens are inserted in the corolla tube and the anthers form a ring through which the style grows and pushes the pollen outwards. Gynoecium of two fused carpels, 2 stigmas. The fruit is an achene with one seed.
Inflorescence a corymb of numerous, often 30—50, small heads, 4—6 × 3—4 mm. Phyllaries ca. 4 × 1 mm, in 2—3 indistinct rows, appressed, narrowly oblong to ovate, obtuse, sparsely hairy, with green central part with raised yellow mid vein and a broad brown hyaline margin ca. 0.3 mm broad, dentate and in the apex fimbriate. Receptacle with scales. Outer flowers in the head female, monosymmetric, ligulate (with all petals fused into a tongue facing outwards); ligula ca. 3—4 × 2 mm, apically with 3 irregular, obtuse teeth, patent, pink or white. Inner flowers bisexual, radial symmetric, tubular, with 5 petal lobes, pale yellow or white, corolla tube compressed.
Fruit (not seen in Svalbard) compressed, obovate or oblong, without pappus.g.
Sexual reproduction by seeds (not observed yet in Svalbard); efficient local vegetative reproduction by clonal growth with rhizomes.
There is nothing similar in the Svalbard flora; the leaf architecture and the collection of heads are unique.
The majority of stands in Svalbard, at least in Spitsbergen, are located in south-facing scree and bird cliff meadows. The species may be rather indifferent as to soil reaction (pH) but the finds have been made on circumneutral to basic substrates. The situation may be different on Bjørnøya where the species more often is found on dolomite or schist gravel on frost-patterned ground.
Introduced. Achillea millefolium has been found in several settlements but may be permanent only in three: Longyearbyen 1939—2013 (vegetative and in bud), Pyramiden 1961—2014 (vegetative one year, in bud three years, flowering 1993), and Barentsburg 1988—2011 (in bud four years, flowering two years). The other finds were made at Hotellneset near Longyearbyen 1897 (vegetative), in Ny-Ålesund 1928 (vegetative), in Colesbukta 1936 (in bud), and at Hiorthhamn near Longyearbyen 1939 (phenological stage not reported). There is no evidence for the species reproducing by seeds in Svalbard, but this may change as it seems to persist by vegetative means for long periods.
The global range is circumpolar in the temperate, boreal and low arctic zones.
Achillea millefolium is a polymorphic species, suggested with different races in Eurasia and North America and possibly also with an arctic race (ssp. borealis (Bong.) Breitung) mainly in N North America (see Elven et al. 2011). The Svalbard plants belong to the ordinary, temperate to boreal Eurasian race (ssp. millefolium), as expected from their most probable places of origin (mainland Norway and W Russia).
Elven, R., Murray, D.F., Razzhivin, V. & Yurtsev, B.A. (eds.) 2011. Annotated Checklist of the Panarctic Flora (PAF).