Rhododendrons were among the earlier flowering plants to develop and the genus has since evolved into over 800 species with many subspecies and natural varieties. Most rhododendron species are native to an area bordered by Pakistan in the west, central China in the north, southern India, Vietnam and Burma in the south and Taiwan in the east. This includes the classic Himalayan rhododendron areas of northern India, Tibet, Nepal and southwestern China. Large groups also exist in Japan, Korea, and the rest of temperate Asia, on the east and west coasts of North America, the Caucasus, southern Europe and the tropical regions of Malaysia, Indonesia, New Guinea and northern Australia.
The countryside and general climatic conditions vary enormously throughout this vast range. Specimens can be found growing in all sorts of unlikely places as well as in the moist woodland conditions that we tend to regard as typical rhododendron country. Factors such as altitude, rainfall, temperature and forest cover are all-important influences. So if you are growing species it is a very good idea to thoroughly research the natural habitat of the plant for the vital clues that will help you to provide the ideal growing environment.
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Most rhododendron species have been identified within the last 200 years, but the history of rhododendron cultivation goes back much further. Chinese gardens have featured potted specimens for at least 2000 years, though it is unlikely that the Chinese practiced hybridization to any great extent. The evergreen azaleas, in particular those from Japan, probably have the longest history of hybridization: cultivated plants were first mentioned around AD 750.
Most of the species important to hybridizing and developing garden plants were introduced into Europe and North America during the period 1790-1935. Even though nurseries raised new hybrids through much of that period, it was the 20th century that saw the rhododendron explosion. The vast majority of the hybrids in modern gardens have been developed since 1900.
R. hirsutum was probably the first species widely cultivated by western European gardeners. Found in the European Alps it was followed in the mid-18th century by the large and shrubby R. ponticum, the deciduous azalea R. luteum, and the small R. ferrugineum, a close relative of R. hirsutum.
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Although we now acknowledge the greater Asian region as the main home of the genus, the first major influx of new plants to arrive in Europe came from among the 28 species native to North America. Many were deciduous azaleas, such as R. flammeum (1789), R. viscosum (c. 1734) and R. calendulaceum (1806), which, when crossed with R. luteum (a native of eastern Europe), produced the first hybrid deciduous azaleas. These plants enjoyed some popularity and the range of hybrids increased steadily, though they were scarcely a great sensation.
The first recorded European rhododendron hybrid is an azaleodendron. That is, a cross between an azalea (a plant of the Pentanthera or Tsutsutsi subgenera) and a rhododendron. Rhododendron periclymenoides, an American azalea, was accidentally crossed with R. ponticum to produce R. x hybridum.
There are not many true rhododendrons, as opposed to deciduous azaleas, native to North America but those that are generally show extreme hardiness. Rhododendron catawbiense, introduced in 1809, was undoubtedly the most important. Its great hardiness (-22°F/-30°C) and large trusses of flowers have had a considerable influence on many of the best hardy garden hybrids. Other American species, such as R. maximum, R. macrophyllum and R. minus, have also been used in hybridizing.
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As the 19th century progressed, the emphasis in exploration shifted from the Americas to Asia, particularly the area from northern India through southern China to Japan. Venturing into these botanically new areas dramatically increased the range of known rhododendrons. The first Western botanical explorers in the Himalayas found the area to be a vast storehouse of rhododendron species and without doubt the center of the genus. Sir Joseph Hooker's 1850 expedition to Sikkim alone discovered some 45 new species.
The first Himalayan species to arrive in Europe was probably R. arboreum. It was discovered in 1799. It was an influential plant for several reasons. First, the true species has bright red flowers and was the first large-growing species of this color to be used in hybridizing, bringing new brightness to rhododendron flowers. Second, there are varieties with flowers in shades of pink or white, which introduced an element of chance or genetic instability that has led to some strikingly marked plants. Third, despite only being hardy to about 10°F (-12°C) it tends not to transfer this tenderness to its progeny when crossed with hardier species, so it introduced the possibility of combining bright colors with hardiness.
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The next significant introduction was R. campanulatum around 1825. Although not greatly used in hybridization, R. campanulatum appears (in con, junction with R. ponticum) to have had a considerable influence in the development of mauve and purple cultivars.
The introduction of R. griffithianum in 1849 by Sir Joseph Hooker and of R. fortunei in 1856 by Robert Fortune (found while searching for new varieties of tea plant) were among the most important of the mid-19th century. The large-growing, fragrant R. griffithianum, a native of Sikkim and Bhutan, provided a boldness of foliage and flower that was previously unknown. However, it lacked extreme hardiness and was inclined to be an untidy grower. R. fortunei from eastern China is an extremely hardy, fragrant, neat, growing large bush or small tree with magnificent foliage.
Also around this time the first of the tropical vireya rhododendrons were arriving in Europe. These plants, also known as Malesian or Malaysian rhododendrons, have very strikingly shaped and colored flowers but will not tolerate frost, being greenhouse plants in most temperate gardens. Fortunately, greenhouse gardening was all the rage when they were first introduced and vireyas were an instant hit.
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Like many of the fashionable greenhouse plants of the 19th century, their popularity waned as hybridizers concentrated on hardy plants for outdoor use and as influential garden authorities emphasized naturalness in gardens. While rhododendron and deciduous azalea growers concentrated on producing hardy garden hybrids, the direction of evergreen azalea breeding was towards producing florists' plants for forcing into flower. The introduction of the tender R. simsii from sub-tropical Asia in 1808 initiated this development. Rhododendron simsii tends to produce double flowers, hybridizes freely and can easily be forced into early flowering by giving it greenhouse conditions.
Belgian flower growers were the chief producers of the early R. simsii hybrids. This led to the plants being known as Belgian Indica azaleas -indica meaning from, the Indies, not a reference to R. indicum -a point that has caused confusion ever since. The tenderness of the Belgian Indica azaleas meant that evergreen azaleas were largely ignored as garden plants in Europe. However, when they arrived in the United States in 1838, it was found that they were hardy enough to be cultivated outdoors in many of the southern states and they immediately became very popular.
The hardy, near-evergreen azalea R. kaempferi had originally been introduced to Europe as early as 1692 but it was largely ignored. Other hardy species from Korea, Japan and Taiwan, such as R. yedoense var. poukhanense, R. kiusianum and R. nakaharai, were much later introductions. They didn't arrive until the late 19th to mid-20th centuries, by which time the center of evergreen azalea breeding had shifted to the United States.
Introductions continued steadily throughout the rest of the 19th century and into the early 20th century. By 1930 most of the influential species were in cultivation, if not yet greatly used as hybrid parents.
The most significant 20th century introductions include R. griersonianum from Yunnan, China, in 1917 and R. yakushimanum from Japan in 1932. R. griersonianum is a somewhat leggy and tender species with loose trusses of bright orange-red flowers. It was extensively used in hybridizing for many years because its offspring tended to be very heavy flowering; unfortunately they also tended to become untidy growers with easily damaged foliage. Nevertheless many fine hybrids, such as 'Winsome', 'Anna Rose Whitney' and the 'Fabia' grex have R. griersonianum in their background.
R. yakushimanum has the kind of compact growth habit and heavy flowering tendencies that make it extremely well-suited to modern small gardens. Discovered as recently as 1920, it caused a minor sensation when first publicly exhibited at the 1948 Chelsea Flower Show. As well as being an almost perfectly formed dome-shaped plant that absolutely smothers itself in flowers, it also has very distinctive foliage. The young growth is covered allover with a soft beige indumentum (felt-like hairs) while the mature leaves are deep green above, slightly rolled at the edges with a thick felt-like white to warm brown indumentum on the undersides. In addition, it is hardy to at least -4°F (-20°C) and will tolerate some exposure to coastal conditions. Rhododendron yakushimanum quickly became the darling of species fanciers and hybridizers alike. Its influence can be seen in many hybrids and shows no sign of waning. And because it is also popular with species growers, several selected forms are now available.