Shrubs, 2–3 m tall, older branches glabrescent, branches stout; young shoots thinly floccose and sparsely glandular. Petiole cylindric, 10–20 mm, pubescent and glandular-hairy; leaf blade leathery, elliptic to ovate-elliptic, 7–12 × 2–5 cm; base rounded or cuneate; margin entire; apex obtuse or slightly cuspidate or acute to acuminate; abaxial surface indumentum thin, pale olive to fawn, veil-like; adaxial surface green, glabrous, midrib prominently raised abaxially; lateral veins 12–13-paired, slightly impressed. Inflorescence racemose-umbellate, 5–10-flowered; rachis 15–30 mm, pubescent and glandular. Pedicel stout, 1–1.5 cm, glandular; calyx discoid; lobes 5–7, minute, glandular and gland-fringed; corolla tubular-campanulate, pink, with crimson blotch, 3.5–5 cm, nectar pouches at base, glabrous within, lobes 5–7, suborbicular, 1.5–1.7 × 2–2.2 cm, emarginate; stamens 10–14, unequal, 2–3 cm, filaments pubescent at base; ovary cylindric-conoid, ca. 6 mm, glandular-hairy, with a few strigose hairs or tomentose; style stout, 2.5–4 cm, glandular to tip or glabrous; stigma slightly swollen, discoid. Capsule curved, ca. 30 × 10 mm. Fl. Apr–May, fr. Jul–Aug.

Rhododendron agastum is probably a hybrid between R. decorum and R. delavayi, as suggested by field observations at the type locality (Yangbi, near Dali in W Yunnan) and as mentioned by George Forrest in the field notes accompanying the type specimen. This entity would not, therefore, belong in R. subsect. Irrorata, as given in FRPS (57(2): 130. 1994) and here.

Also see Zhang, J.-L. et al. 2007. Natural hybridization origin of Rhododendron agastum (Ericaceae) in Yunnan, China inferred from morphological and molecular evidence. J. Plant Res. 120: 457-464.

Mixed forests, broad-leaved forests, valleys; 1900–3300 m. ?E Guizhou, N and W Yunnan [NE Myanmar].