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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Rhododendron caloxanthum' from the website Trees and Shrubs Online (treesandshrubsonline.
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An evergreen shrub 3 to 5 ft high; young shoots glandular. Leaves broadly oval to roundish, 11⁄2 to 21⁄2 in. long, not quite so wide, pale glaucous green beneath, glabrous or nearly so at maturity; stalk about 1⁄2 in. long, glandular. Flowers in trusses of four to nine opening in April and May. Corolla bell-shaped, 13⁄4 in. wide, scarcely so long, five-lobed, scarlet in bud, sulphur- to orange-yellow when fully open; stamens ten, up to 1 in. long, not downy; ovary densely furnished with stalked glands which extend to the lower third of the pistil. Calyx small and like the flower-stalk (which is about 1⁄2 in. long) thickly clad with glands. (s. Thomsonii ss. Campylocarpum)
R. caloxanthum was discovered by Farrer and Cox on the Hpimaw and Chimili passes, upper Burma, near the border with China, in 1919 and was introduced from there (Farrer 937). Farrer described the flowers as vermilion in bud, flushed with apricot and tipped with orange-scarlet as they open, finally clear citron yellow. In the following year Farrer found it farther north, on the Chawchi pass, where it is even more abundant, covering the open slopes and precipice ledges in dense masses of 2-3 ft jungle (Cox, Farrer’s Last Journey, pp. 225 and 239).
Plants raised from Farrer 937 agreed well with the wild parents in their flowers and this form received an Award of Merit when shown from Exbury on May 1, 1934. Farrer described the young shoots as ‘almost cobalt blue’, a character which did not show on the plants raised from his seeds, though there are forms of the species in cultivation which have the mature leaves distinctly glaucous. R. caloxanthum is perfectly hardy south of London, in light woodland. It is very closely allied to R. campylocarpum, and according to Cowan and Davidian the two species merge into one another.
This becomes a subspecies of R. camplyocarpum, with R. telopeum as a synonym.