Fl. Amer. Sept. 1: 329. 1813.
Shrubs, 15–40 dm. Stems erect to ascending, green, weathering brown, gray, or stramineous, glabrous or sparsely strigose (especially at nodes); bark deciduous, exfoliating or flaking, reddish; axillary buds hidden in pouches, sometimes apex exposed, especially on vigorous sprout-shoots. Leaves: petiole 1–6 mm; blade broadly lanceolate to broadly ovate, or narrowly to broadly elliptic, 1.5–10 × 1–5 cm, larger blades usually less than 6 × 2.5 cm, base narrowly cuneate to rounded or cordate, margins entire or irregularly to regularly serrate, crenate, or dentate, plane or slightly revolute, abaxial surface glabrous or sparsely strigose, hairs usually appressed-ascending, not twisted, main vein axils often densely strigose-tomentose, main veins sometimes sparsely strigose, secondary and tertiary veins rarely sparsely strigose, adaxial surface glabrous or sparsely to moderately strigose, especially near base and margins. Inflorescences usually cymose racemes or cymose panicles, sometimes flowers solitary, (1–)7–49-flowered, proximal 2, 4, or 6 flowers often in axils of nearly normal to much reduced (bracteal) leaves. Pedicels 3–8 mm, glabrous or moderately strigose. Flowers: hypanthium glabrous or sparsely strigose, hairs scattered or concentrated on veins; sepals ovate, ovate-lanceolate, or triangular, 5–8 × 3–5 mm, apex acute to acuminate, abaxial surface glabrous or sparsely strigose, adaxial surface glabrous except densely villous distally; petals white, oblong, obovate, or orbiculate, 5–20(–25) × 4–15 mm; stamens 25–40(–50); filaments distinct, 5–11 mm; anthers 2 × 1.5 mm; styles 4, cylindric, 4–8 mm, connate proximally, lobes 1–4 × 0.4–0.5 mm; stigmatic surfaces 1–3.5 mm. Capsules obconic to obovoid, 7–11 × 5–7 mm. Seeds caudate, 3 mm. 2n = 26.
Phenology: Flowering May–Jul; fruiting Jun–Sep.
Habitat: Cliffs, rock outcrops, slopes in pine woodlands and forests, stream banks, talus, seasonally dry ravines.
Elevation: 0–2500 m.
Alta., B.C., Calif., Idaho, Mont., Oreg., Wash.
Philadelphus lewisii, the state flower of Idaho, is rarely cultivated beyond its native range; there is no evidence that it is naturalized in other parts of North America.
A broadly defined Philadelphus lewisii is here recognized with some reluctance; it includes all previously named northwestern entities of subg. Philadelphus. Hu S. Y. (1954–1956) recognized 16 entities at varietal and specific rank, arrayed in two subgenera. C. L. Hitchcock et al. (1955–1969, vol. 3) described P. lewisii as being extremely variable in both vegetative and floral characters and further stated that most ecological or geographic races do not merit taxonomic recognition. Most recent floristic treatments have followed Hitchcock et al. or in some cases recognized two or three entities. The treatment of Hitchcock et al. is followed here, and there does not appear to be a case for the recognition of the two possible exceptions they mentioned (vars. gordonianus and parvifolius). Philadelphus californicus, P. cordifolius, and P. insignis were separated by Hu from P. lewisii at a subgeneric level based primarily on exposed buds and many-flowered inflorescences. The exposed buds are seen sporadically in P. lewisii, including outside the distribution accorded P. californicus, and are not evolutionarily cognate with the exposed buds of subg. Deutzioides; based on examination of herbarium material, they seem to be an expression of extra-vigorous growth, but additional study of plants in the field is needed. Some plants from the Sierra Nevada in California, considered by Hu to be occupied solely by P. californicus, are indistinguishable from P. lewisii in flower number, bud exposure, or any other character. The elaborated inflorescences of some plants in this part of California are striking; for now P. californicus and relatives are treated as part of a broadly defined P. lewisii. In her phylogenetic analysis based on ITS sequence, A. E. Weakley (2002) also found that P. californicus was not distinguished from P. lewisii.