Arctic willowherb, épilobe arctique

Epilobium davuricum Fischer ex Sprengel subsp. arcticum (Samuelsson) P. H. Raven; E. davuricum var. arcticum (Samuelsson) Polunin

Herbs with sessile, basal rosettes of broadly ovate to spatulate leaves 0.5–1.8 × 0.3–0.9 cm. Stems single or many, suberect or nodding in bud, often clumped, terete, (2–)5–18 cm, rarely branched, subglabrous proximal to inflores­cence with raised strigillose lines decurrent from margins of petioles, strigillose distally. Leaves opposite proximally, alternate on inflorescence, subsessile; blade obovate to narrowly elliptic to distally sublinear, 0.8–2.1 × 0.2–0.5 cm, base cuneate to attenuate, margins subentire to minutely denticulate, 2–5 low teeth per side, veins inconspicuous, apex obtuse to truncate, surfaces glabrous or sparsely strigillose on abaxial midrib; bracts reduced and narrower. Inflorescences often nodding in bud, erect later, few-flowered racemes, subglabrous or sparsely strigillose. Flowers nodding to suberect; buds 2–5 × 1–2.5 mm; pedicel 6–12(–18) mm, exceeding subtending bracts; floral tube 0.5–1.1 × 0.6–1.3 mm, with or without sparse ring of hairs at mouth inside; sepals green or flushed purple, 1.1–1.8 × 0.6–1.2 mm; petals white, sometimes flushed pink, 2.2–4.5 × 1.4–2.5 mm, apical notch 0.5–0.7 mm; filaments white or light pink, those of longer stamens 4–5 mm, those of shorter ones 2–3 mm; anthers cream, 0.3–0.4 × 0.2–0.3 mm; ovary 8–18 mm, subglabrous to strigillose; style white, 2.5–3.5mm, stigma clavate, 1–1.8 × 0.5–1 mm, surrounded by anthers. Capsules erect, often reddish purple, 20–42 mm, surfaces sparsely strigillose; pedicel 25–40 mm. Seeds narrowly fusiform to narrowly obovoid, 1.1–1.7 × 0.3–0.5 mm, with distinct chalazal collar 0.1–0.2 mm, light brown, surface rugose or reticulate; coma persistent, dull white, 5–7 mm. 2n = 36.

Flowering Jul–Aug. Boggy, wet meadows, along streams, seepage slopes, depressions of low-center polygons; 50–500 m; Greenland; N.W.T., Nunavut, Que., Yukon; Alaska; Europe (n Russia).

Epilobium arcticum has the most northern distribu­tion in the genus, occurring almost exclusively above the Arctic Circle (about 66ºN) in Alaska, Canada, coastal Greenland north of 69ºN, and the Russian Federation, although it apparently is absent from northern Europe (P. H. Raven 1968).

Epilobium arcticum has often been combined or confused with E. davuricum, but they differ in size, leaf shape and size, and seed size, as well as in distribution (S. G. Aiken et al., http://nature.ca/aaflora/data). Most populations of E. arcticum occur at (62–)67–80ºN, in the Tundra zone, often on the islands of the Arctic Archipelago (especially Axel Heiberg, Baffin, and Ellesmere islands). Most populations of E. davuricum, on the other hand, occur south of the Arctic Circle in the Boreal and Taiga zones, and rarely, if ever, on those islands.

Because Epilobium arcticum grows at latitudes with extremely short growing seasons, plants often com­mence flowering at the second or third most-proximal node.