Description from Flora of China
Shrubs or trees, 1–7(–12) m tall; bark grayish, exfoliating into thin and small irregular flakes; young shoots stout, white-floccose, glabrescent. Winter buds ovoid, greenish, 6–7 mm, slightly tomentose. Petiole cylindric, 7–20 mm, glabrous; leaf blade leathery, long-lanceolate, 7–15 × 1–5 cm; base tapering; margin revolute; apex obtuse or acute; abaxial surface with indumentum 1-layered, spongy or somewhat agglutinated, whitish to fawn, glabrescent; adaxial surface light or dark green, ± rugose, glabrous at maturity, midrib very prominent abaxially, deeply grooved adaxially; lateral veins 14–20-paired, slightly raised abaxially, impressed adaxially; reticulate veins evident. Inflorescence spherical, umbellate, 10–20-flowered; rachis ca. 10 mm, densely rufous-tomentose. Pedicel stout, 0.8–1 cm, densely fawn-tomentose; calyx lobes 5, ca. 2 mm, triangular; corolla campanulate, fleshy, deep crimson to carmine, 3–5 × 3–4 cm, with 5 basal nectar pouches; lobes 5, ca. 1 × 1.3 cm, apex emarginate; stamens 10, unequal, 1.6–4 cm, filaments glabrous, anthers oblong, 2–2.8 mm; ovary conoid, ca. 8 mm, 10-locular, densely fawn-tomentose; style ca. 2.8 cm, glabrous or floccose to tip, stigma capitate. Capsule oblong, 18–20 × ca. 8 mm, blackish and roughened with the remains of the rusty tomentum. Fl. May, fr. Dec.
Rhododendron delavayi intergrades with R. arboreum in Bhutan and NE India.
Mixed forests, evergreen broad-leaved forests, forest margins, thickets, hills, rocky slopes; 1200–3200 m. NW Guangxi, W Guizhou, SW Sichuan, SE Xizang, Yunnan [Bhutan, NE India, Myanmar, Thailand, N Vietnam].