Phytologia 73: 457. 1993.
Herbs usually with taproot, sometimes with loose network of roots. Stems erect or ascending, terete, 4–150 cm, simple or branched with strong central axis, proximal branches ascending or suberect, villous or strigillose, often mixed glandular puberulent distally. Leaves opposite and often early-deciduous proximally, alternate and crowded distally, usually subsessile, rarely petiole 1–2 mm, blade usually narrowly lanceolate to sublinear, rarely to lanceolate, 1.4–7.5(–9.2) × 0.5–1.4 cm, base cuneate to attenuate, margins remotely to sharply serrulate, 5–12 teeth per side, lateral veins obscure, 2–5 per side, apex acute, surfaces densely villous and/or strigillose; bracts broader than cauline leaves, broadly lanceolate to ovate or subrotund, 0.5–2.5 × 0.3–1.8 cm, long-acuminate, sometimes folded on midrib. Inflorescences erect spikes, congested, simple, densely villous and strigillose, sometimes mixed glandular puberulent. Flowers erect, often hidden within subtending bracts, usually chasmogamous; buds sessile, narrowly elongate, 2–4 mm; floral tube 1.3–3.8 × 1–2.2 mm, ring of hairs 0.6–2 mm distal to base inside; sepals 2–7.5 × 0.5–2.2 mm, apex acute; petals rose-purple, magenta, pink, or white, 3–9.5(–11.5) × 1.2–5(–6.2) mm, apical notch 0.8–3.8 mm; filaments dark pink, those of longer stamens 1.5–4.5 mm, those of shorter ones 0.5–1.9 mm; anthers yellow, 0.5–1.2 × 0.3–0.7 mm; ovary 2–5 mm, densely villous, often mixed glandular puberulent; style white, 2.2–5.5(–7.5) mm, glabrous, stigma subcapitate to irregularly 4-lobed, 0.3–0.8 × 0.3–1 mm, surrounded by longer anthers. Capsules cylindrical to subfusiform, 4–11 mm, beak to 0.5 mm, central column persistent, surfaces densely villous; subsessile or pedicel 1–2.5 mm. Seeds 3–8 per locule, irregularly angular-fusiform, 1.2–1.6(–1.9) × 0.4–1 mm, without a chalazal collar, light brown, surface irregularly reticulate with raised cells. 2n = 20.
Phenology: Flowering May–Oct.
Habitat: Vernally wet places, moist pastures, woodlands, meadows, along streams and ditches, alluvial valleys, often on low ground in volcanic or sandy soils.
Elevation: 0–2600 m.
B.C., Ariz., Calif., Idaho, Mont., Nev., Oreg., Utah, Wash., Mexico (Baja California).
Epilobium densiflorum is an extremely variable species that changes its aspect through the flowering season. Collections made very early in the season may include only well-spaced, narrowly lanceolate leaves, the proximal ones usually opposite, and a short, sparse, somewhat open inflorescence. A late-season collection, even from the same population, may entirely lack cauline leaves, and consist instead of bare, peeling stems topped by dense, tightly imbricate-bracted inflorescences, with each broad bract enclosing a capsule or flower.
Boisduvalia douglasii Spach is an illegitimate substitute for Oenothera densiflora Lindley and pertains here.