Monogr. Epilobium, 276, plate 14, fig. 66. 1884.
Herbs with slender stolons to 18 cm with minute, rounded leaves. Stems erect or ascending, often loosely matted, often flushed purple distally, terete, 8–30(–40) cm, simple or sparsely branched from base, subglabrous. Leaves opposite proximal to inflorescence, alternate distally, subsessile; blade broadly elliptic proximally, narrowly elliptic or lanceolate to sublinear distally, 5–25 × 1–7 mm, longer than internodes proximally to much shorter distally, base cuneate to rounded, margins subentire, veins extremely faint, 3–5 per side, apex obtuse, surfaces subglabrous; bracts extremely reduced and linear. Inflorescences usually erect, sometimes nodding in bud, racemes, open, unbranched, sparsely strigillose and glandular puberulent. Flowers suberect or nodding; buds 2–3.5 × 1–1.5 mm, apex blunt; pedicel 2–7 mm; floral tube 0.8–1.8 × 1–2.1 mm, with faint ring of hairs at mouth inside; sepals often flushed purple, 2.5–4.5 × 1–1.6 mm; petals white to pink, 5–8 × 2.8–4 mm, apical notch 0.8–1.5 mm; filaments white, those of longer stamens 2.8–4.5 mm, those of shorter ones 2–3.8 mm; anthers yellow-cream, 0.8–1.2 × 0.4–0.5 mm; ovary green to purple, 8–14 mm, sparsely strigillose and glandular puberulent; style white, 3.8–4.8 mm, glabrous, stigma subcapitate, 1–1.4 × 1–1.2 mm, surrounded by longer anthers. Capsules slender, often purplish green, 21–40(–52) mm, surfaces subglabrous; pedicel 20–65 mm. Seeds narrowlyoblanceoloid or subfusiform, 1–1.4 × 0.4–0.6 mm, chalazal collar 0.1–0.2 mm, light brown, surface low papillose; coma persistent, whitish, 3–4 mm. 2n = 36.
Phenology: Flowering Jul–Aug.
Habitat: Montane to subalpine boggy or mossy areas, wet meadows, protected, semi-shaded stream banks.
Elevation: 1200–3000(–3500) m.
B.C., Ariz., Calif., Colo., Idaho, Mont., Nev., N.Mex., Oreg., Utah, Wash., Wyo.
Epilobium oregonense is a distinctive western North American endemic, found primarily throughout the Cascade–Sierra mountain complex barely into the Transverse Ranges of southern California, and very scattered through the Rocky Mountains. It is exceedingly rare in Arizona, Colorado, and New Mexico.
Even though Epilobium oregonense bears some similarity to E. anagallidifolium and other members of the Alpinae group, and often grows in close proximity to them, this species does not share the derived CC chromosomal arrangement with that group, instead having the more globally widespread BB arrangement. The similarities with E. anagallidifolium include the small stature, small, obtuse, and subentire leaves, and long pedicels in fruit; however, E. oregonense differs by its long, threadlike stolons, distal leaves extremely narrow and reduced in size relative to the internodes, and near complete absence of pubescence on the plant, including a lack of raised lines of hairs on the stems.
Another species with which Epilobium oregonense has been confused is E. hallianum, but that species always forms condensed basal turions, is more strictly erect, and generally has larger and more denticulate leaves. The distinctive and diagnostic stolons of E. oregonense are similar to those found in E. palustre and related species (all of which also have the BB chromosome arrangement), except that those of E. oregonense never terminate in a condensed turion, as found in those other species. The exact affinities of E. oregonense remain uncertain, but it appears to be most closely related to the E. palustre complex.
Some specimens of Epilobium oregonense grow as floating mats in cold streams; these specimens are notably large, with particularly strong development of basal stolons and larger, more lanceolate leaves. As evidenced by mixed herbarium collections, E. oregonense grows sympatrically with several congeners, including E. anagallidifolium, E. ciliatum subspp. ciliatum and glandulosum, E. hallianum, and E. hornemannii, and hybridizes occasionally, at least with E. ciliatum subsp. ciliatum and E. hornemannii.
Epilobium oregonense var. gracillimum Trelease, which pertains here, was not validly published, and other names based on it are also invalid.