Rep. (Annual) Missouri Bot. Gard. 2: 111, plate 48. 1891.
Herbs with wiry, scaly soboles just below ground level, often with extended semi-woody rootstock. Stems numerous, ascending, clumped, terete, 5–15(–22) cm, usually simple, rarely branched, subglabrous, with raised strigillose lines decurrent from petioles, ± densely strigillose and often mixed glandular puberulent distally. Leaves crowded and opposite proximal to inflorescence, alternate distally, petiole 0–3 mm; blade obovate proximally to ovate or elliptic distally, 0.5–2.8 ×0.6–1.6 cm, base attenuate proximally to obtuse distally, margins subentire to sparsely denticulate, 3–6 teeth per side, veins obscure, 2–4 per side, apex obtuse proximally to subacute distally, surfaces subglabrous or sparsely strigillose margins and abaxial midrib, sometimes subglaucous; bracts not much reduced, sessile. Inflorescences usually erect, rarely slightly nodding, racemes, strigillose and glandular puberulent, sometimes sparsely so. Flowers erect; buds often purplish green, 3–4.5 × 1.4–2.2 mm; pedicel 3–9 mm; floral tube 0.6–2 × 1–2 mm, glabrous or with a raised ring of sparse hairs at mouth inside; sepals often purplish green, 2.5–4.2 × 1–2 mm, abaxial surface sparsely glandular puberulent to subglabrous; petals rose-purple to pale pink, obcordate, 3.5–6(–7) ×2–4 mm, apical notch 0.5–1 mm; filaments cream, those of longer stamens 1.8–4 mm, those of shorter ones 1–3 mm; anthers light yellow, 0.4–0.9 × 0.25–0.5 mm; ovary often reddish purple, 8–20 mm, densely glandular puberulent, often mixed strigillose, rarely subglabrous; style white or pale pink, 1.4–3.2 mm, glabrous, stigma cream, narrowly clavate to subcapitate, 0.8–1.4 × 0.3–0.8 mm, surrounded by at least longer anthers. Capsules often purplish red, 20–42 mm, surfaces sparsely pubescent or subglabrous; pedicel 2–21 mm. Seeds narrowly obovoid or fusiform, (1.3–)1.5–2.1 × 0.4–0.7 mm, chalazal collar conspicuous, 0.04–0.16 × 0.2–0.4 mm, blond, surface finely reticulate; coma easily detached, white, 5–8 mm. 2n = 36.
Phenology: Flowering May–Sep.
Habitat: Rocky crevices, scree slopes, ledges, stream banks, often near snow banks or moraines in upper montane to alpine zones.
Elevation: 800–4200 m.
Alta., B.C., N.W.T., Yukon, Alaska, Calif., Colo., Idaho, Mont., Nev., Oreg., Utah, Wash., Wyo.
Epilobium clavatum shares a clumped habit and the CC chromosomal arrangement with related species in the Alpinae group, but differs from them by its dense, wiry mass of basal soboles arising from an extended and somewhat woody caudex and relatively thick capsules and large seeds (1.3–2 mm). This unusual habit morphology may be the result of growing on unstable, shifting scree slopes. Like E. anagallidifolium, with which it often grows in near-sympatry in alpine areas, it is of notably low stature, often less than 15 cm, and has subentire leaves and capsules rarely exceeding 4 cm. However, E. clavatum does not nod in bud, and generally is more robust than E. anagallidifolium, and it has a much smaller range, being endemic only to the western North American cordilleran region, whereas E. anagallidifolium has a discontinuous circumboreal range.