Author: Ing. Uljana Blažková
Genus Rhododendron (Rhododendron)
Name rhododendron comes from Greek words “rhodos” = pink, purple, reddish and “dendron” = tree. They are evergreen, semi deciduous (keeps foliage in winter) or deciduous shrubs, which are very variable in look and growing conditions according to their original habitat. This genus belong to one of the most decorative and rich ones; literature talks about 1000-1400 species. They are northern hemisphere plants except for tropical species growing in Malaysia, Indonesia and New Guinea and one Australian specimen Rhododendron lochae. The biggest numbers of species grow in east and south-east Asia and also in New Guinea (about 170). Most of them belong to alpine flora, undergrowth of mountain forests, or they form extensive populations above forest line. Some of them are part of tropical jungles.
The richest areas are Himalayas and western China (a few hundreds species). Rhododendrons grow here in great variability from subtle shrubs to trees reaching up to 30m like Big Tree Rhododendron (R. giganteum) or Tree Rhododendron (R. arboreum) – a national flower of Nepal. Distribution area continues to east coast and further to south from Malaysia, Borneo, Indonesia, Sund Islands to north Australia. These tropical species are attractive for breeding due to their colors and flower size, but for conditions of Central Europe they’re more or less useless without appropriate greenhouse. But north-east Asia including Russia, Korea and further Japan is a very interesting area for introduction to European climate.
European area includes also species growing in Caucasus and Asia Minor. There are 9 species of rhododendrons. Hairy Alpenrose (R. hirsutum) and Rusty-leaved Alpenrose (R. ferrugineum) are typical alpine species growing in Alps, Pyrenees and Apennines. There is a hybrid of these two growing in the Alps known as Alpenrose (R. x intermedium). Its extension is restricted to contact areas of the two species mentioned. In Carpathians there is only one rhododendron (R. kotschyi) and 5 species grow in Caucasus: Georgian Snow Rose (R. caucasicum), Yellow Azalea or Honeysuckle Azalea (R. luteum), Pontic Rhododendron (R. ponticum), Smirnov Rhododendron (R. smirnowi)i and Ungern's Rhododendron (R. ungernii). Yellow Azalea grows also in Belorussia, Ukraine and rarely in Poland or in former Yugoslavia areas. Pontic Rhododendron could be found in Pyrenees and Balkan Peninsula as far as Asia Minor, the same for Georgian Snow Rose and Yellow Azalea. The last European species is Lapland rosebay (R. lapponicum) from Scandinavia, Finland and Greenland. It grows in North America, Labrador Peninsula and north-east Asia, too.
North American Species
There are about 30 species cited in literature in North America. They grow there in humid mountain areas on waterside locations. The most of them are from East Coast, some from the West Coast or the North. The center of the North American continent is too dry for rhododendrons.
The classification of the genus is still in progress, as well as other genera, especially with a new knowledge in molecular genetics. Today’s botanical system divides the genus into eight sub-genera (sometimes nine – Vireya section from a sub-genus Rhododendron is sometimes mentioned as single sub-genus).
It has the most species (about 500, only in section Vireya, there are roughly 170 species of tropical rhododendron from Borneo and New Guinea). They are mostly evergreen, rarely deciduous with smaller foliage.
This is the second largest sub-genus (about 300 species) of evergreen vigorous shrubs with large leaves. But also subtle shrubs, almost creeping, belong in here.
There are about 25 species in this smaller sub-genus of deciduous rhododendrons.
These smaller semi deciduous shrubs gather about 110 species.
Literature mentions from 5 to 30 species in this sub-genus of evergreen shrubs or smaller trees.
Only Cascade azalea (R. albiflorum) belong in here, it’s a smaller deciduous shrub.
Also only one species is listed in this sub-genus. R. semibarbatum is deciduous shrub of smaller size and very tiny discreet flowers.
R. camtschaticum is only one in here: subtle, ground, deciduous shrub.
All sub-genera are further divided into sections and sub-sections.
Rhododendrons grow mostly in rich slightly acidic soil, only very few species tolerates lime base. Standing in shade is important mostly for evergreen species. Semi deciduous and deciduous azaleas can be planted without shading, but they need sufficient water supply. Enough irrigation is essential for all species, but they can’t stand waterlogged lands (except for R. canadense). That’s why drainage is recommended when planting. Evergreen rhododendrons require good watering before winter season. Growing standpoint should be sheltered and on the leeward side.
Some species contain in leaves, flowers and partially in roots poisonous substances (flavonoids, saponins, phenolic compounds) with can cause intoxication to animals and people. There are known cases of honey poisoning from alpine Rusty-leaved Alpenrose. Effects are mostly laxative or hallucinogenic, severe poisoning leads to slow heart beat and cardiac arrest. On the other hand, these substances have medical uses. E.g. in folk medicine they’re used for fever, rheumatism, kidney or heart regulations, to make stimulant potions or to help with some intoxications. Essential oils from Yellow Azalea flowers were used in Russian cosmetics.
It is also interesting that some of the rhododendrons can be labeled as invasive plants. Like Pontic Rhododendron in Great Britain. It produces seeds with good germination in vast numbers, plus it can propagate by layering, which causes overgrowth. Local vegetation is displaced and grazing animals are endangered by poisonings.
Translation : Marketa Machackova