The Royal Botanic Garden Edinburgh has been the major centre for Rhododendron studies since the late nineteenth century. Together, the collections in our four Gardens - Edinburgh, Benmore, Dawyck and Logan - comprise the world's richest assemblage of species rhododendrons. About half of all the 1,000 known species are cultivated in the Gardens and include most of the temperate species and over a third of those known in the tropics.
The Edinburgh collection is planted largely according to subsections. This gives visitors a good visual impression of the different groups, and allows rhododendron specialists to locate and compare closely-related plants easily.
More than 250 species, 100 subspecies and a further 300 hybrids of Rhododendron grow at Benmore. It also holds an outstanding collection of Hobbie hybrids, named after the German grower who crossed R. forrestii var. repens and R. williamsianum with existing hybrids to produce a series of dwarfed, hardy rhododendrons.
Dawyck has a unique and extremely valuable rhododendron collection. Rhododendron Walk and Scrape Glen burst with colour between April and June, as does the flamboyant Azalea Terrace.
At Logan, members of Rhododendron subsection Maddenia flourish in the Walled Garden. This group of rhododendrons is characterised by fragrant cream coloured blooms such as those of R. johnstoneanum, which grows in the shelter of the castle just inside the Woodland Garden. Rhododendron edgeworthii, one of the more tender species from the Indo-Himalayas and south-west China, also thrives at Logan, as does the hybrid R. 'Fragrantissimum' (subsection Maddenia), which is usually treated as a conservatory plant.
Classification of rhododendrons
Rhododendrons are divided into seven or eight subgenera. Two of these - including the subgenus Vireya - encompass the scaly-leaved or lepidote rhododendrons, numbering over 500 species. The remaining subgenera are non-scaly or elepidote. These include three further significant subgenera: the ‘Deciduous Azaleas' of Section Pentathera, of which R. luteum is an example (15 species), the ‘Evergreen Azaleas', including the ‘Pot and the Kurume Azaleas' (80-90 species) and a subgenus of around 300 mostly larger species, of which Rhododendron ponticum is an example. Species within the first and last subgenera are grouped into subsections.
About a third of all rhododendrons, over 300 species, belong to subgenus Vireya. Vireyas show a huge range of flower, colour, size and shape. Most are epiphytes, growing on trees in their natural habitats. Rhododendron polyanthemum from northern Sarawak has fragrant orange-red flowers pollinated by butterflies, while Rhododendron loranthiflorum is a scented, white-flowered species pollinated by moths.
RBGE has the largest cultivated collection of Vireya rhododendrons in the world. As natives of the mountains of south-east Asia, most like a cool, moist, frost-free environment. These plants thrive indoors in the Montane Tropics glasshouse at Edinburgh, benefiting from the long hours of daylight and cool temperatures of the summer growing season in Scotland.