An evergreen shrub of lax, open growth up to 7 ft or more high; young shoots glandular-bristly and hairy. Winter-buds very distinct, being encased by bracts extending well beyond the bud and free at the ends. Leaves oblong-lanceolate, pointed, tapered at the base, 4 to 7 in. long, 1 to 13⁄4 in. wide; dull green above, covered with a pale buff wool beneath; stalk 3⁄4 to 11⁄2 in. long, glandular and hairy like the young shoots. Flowers opening in June, loosely arranged in trusses of five to twelve flowers. Calyx small and like the flower stalk, which is 11⁄4 to 11⁄2 in. long, viscous and downy. Corolla rosy scarlet, trumpet-shaped, narrowed to a fluted cylindrical tube at the base, five-lobed, 3 in. wide, 21⁄2 in. long, the lobes roundish ovate, 11⁄4 in. wide, with darker crimson lines running into the throat, very downy outside especially towards the base. Stamens ten, crimson, 11⁄2 to 2 in. long, slightly downy on the lower half; anthers very dark brown. Ovary covered with pale tawny felt; style crimson, downy at the base. Bot. Mag., t. 9195. (s. Griersonianum)
R. griersonianum was discovered by Forrest in June 1917 in W. Yunnan, near the Burma border, on the divide between the Salween and the Shweli, a tributary of the Irrawaddy, growing at 9,000 ft in open situations. He introduced it in the same year and sent seeds again in later years, all from this area, to which the species is apparently confined. In his field notes for 1924 he wrote: 'Extremely local; essentially a plant of the open, seldom, if ever, seen in the shade, though the glades where it is seen at its best are always more or less sheltered. In addition to the striking beauty of the flowers it is a most shapely shrub.' (F.24280)
R. griersonianum is one of the finest and most remarkable of Forrest's introductions. It was at first incongruously grouped with R. auriculatum, a species with which it really has nothing in common except the awl-shaped bud-scales. There it remained in the Auriculatum series, until 1964, when H. H. Davidian put matters right by placing R. griersonianum in a series of its own. It is taxonomically an isolated species, distinct from all others so far discovered in the combination of the awl-shaped bud-scales, the densely glandular inflorescence and ovary, and the uniquely coloured trumpet-shaped corolla. It is probably a relict species.
R. griersonianum is moderately hardy but needs the conditions suggested by Forrest's field note, i.e., a sheltered but fairly sunny position. It flowered at Caerhays, Cornwall, and Borde Hill, Sussex, in 1923, and received a First Class Certificate when shown jointly by Lionel de Rothschild and T. H. Lowinsky at Chelsea on May 27, 1924. No other species of Rhododendron has been so much used in hybridising – the R.H.S. Handbook lists about 150 hybrids with R. griersonianum as one parent, and of these almost one-third have received an Award of Merit or a First Class Certificate. Many of these are described in the section on hybrids.