Fl. N. Amer. 1: 488. 1840.
Herbs with short, fleshy shoots from woody caudex, often extending 20+ cm underground; proximal epidermis peeling. Stems several–many, ascending to erect, terete, 10–25 cm, simple or well-branched, ± densely strigillose. Leaves often crowded, opposite and sometimes with fascicles of very small leaves at proximal nodes, subsessile or attenuate to broad petiole 0.5–1.5 mm, blade light grayish green, narrowly lanceolate to elliptic, 1–2.5 × 0.2–0.7 cm, often exceeding internodes, base cuneate to attenuate, margins entire or ± denticulate, 4–6 low teeth per side, lateral veins inconspicuous, apex blunt proximally to subacute, surfaces ± densely short-strigillose; bracts not much reduced in size. Inflorescences erect racemes or panicles, ± densely strigillose. Flowers slightly nodding; buds 4–8 × 1.5–3.5 mm, apiculate; floral tube funnelform to obconic, 1.8–3 × 1.9–2.6 mm, ring of spreading hairs 1–2.5 mm from base inside; sepals 3–6.5 × 1–2.6 mm, often apiculate, abaxial surface densely strigillose; petals cream to light yellow, obcordate, 5–9.3 × 2–3.8 mm, slightly unequal with upper 2 longer, apical notch 1–2.3 mm; filaments cream, slightly inflated at base, those of longer stamens 6–10 mm, those of shorter ones 4.5–8 mm; anthers cream-yellow, 1.4–2.2 × 0.6–1.1 mm; ovary 4–9 mm, densely white-canescent; style declined below main plane of flower, cream, 7.8–14.5 mm, glabrous, stigma deeply 4-lobed, 0.8–1.2 × 1.8–2.8 mm, lobes spreading-recurved 0.9–1.2 mm, exserted beyond longer anthers, often prematurely exserted and protogynous. Capsules often curved, fusiform-clavate, 10–30 mm, surfaces finely strigillose; pedicel 4.5–13 mm. Seeds narrowly obovoid to oblanceoloid, with constriction 0.7–1.3 mm from micropylar end, 2.1–3 × 0.7–1.1 mm, very inconspicuous chalazal collar, light brown, surface low-papillose; coma easily detached, tawny, 7–9.5 mm, with unusually dense hairs. 2n = 30.
Phenology: Flowering (Jun–)Jul–Aug.
Habitat: Gravel bars along rivers and streams, moist stabilized talus, moraines, other rocky places.
Elevation: 700–3000 m.
Idaho, Mont., Utah, Wyo.
Epilobium suffruticosum shares its unusual cream-yellow flower color only with E. luteum, a distantly related species in sect. Epilobium. Both species have relatively large flowers with 4-lobed stigmas and are visited quite intensively by bees and other insect pollinators. Nevertheless, these species differ dramatically in habit, leaves, seeds, and many other characters, do not overlap at all in distribution, and are never confused with one another; the similar floral features must have been derived independently.
The flowers of Epilobium suffruticosum are also slightly zygomorphic, which is relatively rare in the genus. In the field and on many herbarium specimens of E. suffruticosum, the stigmas are clearly exserted even before the flowers are fully open. The label for Raven 26451 (Wyoming, Park County, MO) notes: “protogynous; in late bloom, most flowers male-sterile.” Several flowers from this collection have undeveloped anthers, suggesting that the flowers are functionally pistillate. However, these plants are not sterile since they have apparently fertile capsules with fully developed seeds.
The distribution of Epilobium suffruticosum consists of two clusters of fairly common occurrence—in northwestern Wyoming around Yellowstone and Teton national parks, and in south-central Idaho mainly in the drainages of the Boise and Payette rivers—with more scattered collections in western Montana north to Flathead County, and a single collection to the south in Weber County, northern Utah. There are no obvious morphological discontinuities among these specimens, nor any obvious explanation for the gaps in distribution; it may be due to collecting bias. This species is commonly found on gravel/sand bars of cold montane streams and rivers, in a stable association despite the apparent ephemeral nature of these habitats. It would appear that the plants have deep, woody roots by which they anchor themselves; in the spring flood stages of these rivers, they must experience complete inundation and considerable scouring, yet persist, often in moderately large colonies.
The exact locality of the type collection (streams east of Wallawallah, plains of the Upper Columbia River, Oregon) is problematic, since the closest known localities are at least 250 km southeast of the town of Walla Walla, Washington. Whether this is a matter of the historical accuracy of the locality by Nuttall or of the local extinction of this species from a locality in eastern Oregon cannot be determined at present. A collection by Hayden in 1859 (Powder River, Wyoming) is far outside the range of E. suffruticosum and may have been mislabeled.