Oesterr. Bot. Z. 29: 89. 1879.
Herbs with short, leafy epigeal soboles. Stems ascending to suberect, often clumped, terete, 15–50 cm, usually simple, rarely branched proximally, subglabrous proximal to inflorescence except for raised densely strigillose lines decurrent from margins of petioles, usually mixed strigillose and glandular puberulent distally. Leaves opposite proximal to inflorescence or just proximal 1/3, alternate distally, petiole 3–12 mm, ± winged; blade broadly spatulate to ovate proximally, narrowly ovate to narrowly lanceolate distally, 2–5.5 × 0.8–2.4 cm, base attenuate to cuneate, margins subentire proximally to denticulate distally with 7–16 teeth per side, more marked distally, lateral veins inconspicuous, 4–8 per side, apex obtuse proximally to subacute distally, surfaces glabrous except for strigillose margins; bracts reduced and narrower. Inflorescences nodding in bud, later erect, ± open racemes, mixed strigillose and glandular puberulent. Flowers suberect; buds 2–5 × 1.5–3.5 mm; pedicel 5–15 mm; floral tube 1–2.2 × 1–3 mm, inner surface glabrous without ring; sepals often flushed purplish red, frequently keeled, (2–)3–5.5 × 0.9–1.8 mm, abaxial surface sparsely glandular puberulent, sometimes mixed strigillose; petals white, rarely with red veins or flushed light pink, 3–8.5 × 1.6–4.5 mm, apical notch 0.7–1.4 mm; filaments white to cream, those of longer stamens 1.4–4 mm, those of shorter ones 1.1–3 mm; anthers light yellow, 0.4–0.9 × 0.3–0.6 mm; ovary 20–40 mm, glandular puberulent; style cream or white, 1.4–4.6 mm, stigma clavate or rarely subcapitate and indented apically, entire, 1.2–2.5 × 0.4–1.6 mm, surrounded by anthers. Capsules slender, sometimes flushed reddish green, ± ascending, 50–100 mm, surfaces sparsely glandular puberulent; pedicel 15–45 mm. Seeds narrowly obovoid, 1.1–1.7 × 0.4–0.6 mm, chalazal collar 0.05–0.1 mm, blond or light brown, surface reticulate or sometimes barely rugose; coma easily detached, white, 7–14 mm. 2n = 36.
Phenology: Flowering Jun–Sep.
Habitat: Montane stream banks, moist crevices and ledges, gravelly roadsides, burned-over woodlands, sandy moraines, subalpine forests, alpine meadows.
Elevation: 50–3800 m.
Greenland, Alta., B.C., N.B., Nfld. and Labr., N.W.T., N.S., Ont., Que., Yukon, Alaska, Ariz., Calif., Colo., Idaho, Maine, Mont., Nev., N.H., N.Mex., Oreg., Utah, Vt., Wash., Wyo., Eurasia.
Epilobium lactiflorum has a nearly circum-subarctic distribution in North America (including coastal Greenland) and Eurasia, extending south into alpine and cool montane habitats along mountain axes. This distribution is similar to that of E. anagallidifolium and E. hornemannii (all with CC chromosomal arrangement), and these species commonly grow in similar habitats as well.
Petal color can be variable in many Epilobium species, but E. lactiflorum (white flowers) differs quite consistently from E. hornemannii (rose-purple to light pink or rarely white) in that feature. Mature fruits and pedicels are also fairly longer in E. lactiflorum. Despite their morphological similarities and broadly overlapping ranges and habitats, E. lactiflorum and E. hornemannii subsp. hornemannii do not appear to hybridize with much frequency, although intermediates, with only moderately reduced seed fertility, might be difficult to detect.