A large evergreen shrubby tree in the wild, attaining a height of 40 to 50 ft; young stems coated with a woolly greyish tomentum. Leaves thick and leathery, clustered at the ends of the shoots, oblong-elliptic to broadly oval, up to 1 ft long, 6 to 9 in. wide, rounded and mucronate at the apex, narrowed or obtuse at the base, dark green above, undersurface whitish or greyish white, at first woolly from a coating of long-branched hairs which later fall away, leaving a suede-like indumentum of rosulate hairs; midrib and lateral veins prominent beneath; petiole stout, about 1 in. long, tomentose. Inflorescence a dense umbel of up to thirty flowers, opening in March or April; pedicels about 7⁄8 in. long, tomentose. Calyx rim-like. Corolla pale sulphur yellow to deep yellow, sometimes ivory-white, tubular-campanulate, pouched at the base, the upper pouches purple inside, almost 3 in. long, 2 to 21⁄4 in. wide, eight-lobed. Stamens sixteen, included in the corolla, with glabrous filaments. Ovary sixteen-chambered, tomentose; style glabrous, with a large crimson or pink stigma. Bot. Mag., n.s., t. 187.
Native of Assam in the Naga Hills and Manipur. It was discovered by Sir George Watt on Mt Japvo at 8,000 to 9,500 ft on March 9, 1882; a few weeks later he collected it again on the summit at 9,800 ft, where, as in neighbouring mountains at about the same altitude, it forms dwarf forests. In addition to making field notes, Sir George Watt drew up a description of this rhododendron, and tentatively gave it the status of a variety of R. falconeri. He named it after Mr M’Cabe, Deputy Commissioner for the Naga Hills, who helped him during his visit. But Watt’s account lay in manuscript until 1920, when Sir Isaac Bayley Balfour used it, and the specimens he collected, to make a formal description of this rhododendron, as a new species. Seven years later, Kingdon Ward collected seeds from the Japvo stand, and from these all the older plants in gardens are derived (KW 7724). He again gathered seeds in March 1935, this time from farther east, on a ridge at 9,000 ft below Mt Sarameti (KW 11175).
R. macabeanum is hardy in woodland near London and thrives and flowers beautifully in several gardens, notably at Nymans in Sussex. But it grows faster and attains its greatest size in the milder and rainier parts, where plants from the original introduction are now around 25 ft in height. At Kew, the climate is too dry for it, but it grows well at Edinburgh, where there is a fine specimen in the Rhododendron Walk. The yellow of its flowers varies much in depth, even in the plants from Kingdon Ward’s original collection. At Trengwainton, where Sir Edward Bolitho raised many plants from KW 7724, some are deep yellow and among the finest in the country. But the trusses shown by him in 1937 and 1938 were of pale yellow flowers, though still fine enough for the species to receive the Award of Merit on the first occasion and a First Class Certificate on the second. The famous plant at Trewithen, Cornwall, was a gift from Sir Edward Bolitho to George Johnstone and is one of the original seedlings. This has flowers of a very deep yellow and measures 23 ft in height and 35 ft in width (1974). It is portrayed in R.C.Y.B. 1962, fig. 14.