Description from Flora of China
Dwarf creeping shrubs, 20–60(–90) m tall; young shoots ca. 2 mm in diam. below inflorescence; sparsely tomentose and glandular; bud scales persistent. Petiole 4–8 mm, glandular-hairy and sparsely floccose-tomentose; leaf blade leathery, obovate to orbicular, 1.3–2.8 × 0.8–1.7 cm; base broadly cuneate, narrowly decurrent; apex rounded or retuse, sometimes mucronate; abaxial surface pale purple or green, whitish puberulent with a few glandular hairs, or glaucous and papillate when mature; adaxial surface green, glabrous; midrib grooved adaxially; lateral veins 6–7-paired, impressed adaxially. Inflorescence 1(or 2)-flowered. Pedicel 1(–2.5) cm, densely glandular-hairy and puberulent; calyx saucer-shaped, fleshy; lobes 5, 1–3 mm, margin glandular-ciliate; corolla tubular-campanulate, crimson, 2.9–3.8 cm, with 5 basal nectar pouches; lobes 5, suborbicular, 0.9–1 × ca. 1.5 cm, emarginate; stamens 10, 1.5–1.8 cm, filaments glabrous; ovary conoid, ca. 4.5 mm, densely glandular-hairy and tomentose; style glabrous; stigma small. Capsule cylindric, ca. 10 × 6–7 mm. Fl. May–Jul, fr. Oct–Nov.
Rhododendron forrestii var. tumescens Cowan & Davidian (Rhododendron Year Book 6: 69. 1951), described from Xizang, Yunnan, and Myanmar, is intermediate between subsp. forrestii and subsp. papillatum and does not merit formal recognition.
Rhododendron ×pyrrhoanthum I. B. Balfour (Notes Roy. Bot. Gard. Edinburgh 12: 154. 1920) was described from a rogue hybrid in a batch of R. forrestii raised from seed collected by George Forrest in Yunnan. It is known only in cultivation.
Moist stony pastures, moist stony slopes, rocky slopes, cliffs; 3000–4200 m. SE Xizang, NW Yunnan [NE Myanmar].