The Old World Deciduous Azalea Species
One of the showiest groups of flowering shrubs are the deciduous azaleas. Not only do they produce a profusion of spring flowers, many also exhibit wonderful fall colours in shades from brilliant yellow, through vibrant orange, flaming red to deep burgundy. In the simplest terms, these azaleas are essentially deciduous rhododendrons. However, florally, their flowers differ in having 5 stamens per bloom while evergreen rhododendrons have 10.
Worldwide, deciduous azaleas are only found in temperate regions of the northern hemisphere. The main areas of concentration are eastern North America and China. North American species are flat-out winners when it comes to bright colours. It is hard to beat the sunset shades of Rhododendron calendulaceum, R. bakeri and R. flammeum. To learn more about the eastern American species, read the article by Marie Harrison (can2grow) on Native Azaleas for Florida and the Deep South. The focus of this companion article will be on the Old World deciduous azalea species.
Europe is quite poor in native deciduous azaleas. In fact, they only have a single native species, the Pontic azalea, R. luteum. This species is distributed from eastern Europe to the Caucasus Mountains. This species has fragrant yellow flowers on plants which may reach 4 m (2 to 3 m is more typical). The fall colour is yellow. This was historically an important species in the creation of many of the older azalea hybrids such as the Exbury and Knap Hill hybrids. One drawback of this species is its susceptibility to powdery mildew.
Details of the delightfully-fragrant Pontic azalea, R. luteum
The other Old World azaleas all hail from China, Japan and Korea. While there are several yellow, orange and red-flowered American species, eastern Asia has only one species with similar colours; R. molle. Rhododendron molle hails from eastern to central China and has yellow to golden-yellow blooms. The counterpart in Japan was once called R. japonicum but that species is now considered a subspecies of R. molle. Its flowers are shades of orange, pink to nearly red. Among the Asian deciduous azaleas, this species is relatively compact with plants rarely exceeding 2 m. Like the Pontic azalea, R. molle also features highly in many of today's modern deciduous azalea hybrids including the Northern Lights azaleas.
Rhododendron molle and R. molle subsp. japonicum
The remaining Asian deciduous azaleas all have flowers that are either white or shades of pink. Due to genetic incompatibility, these remaining species cannot be hybridized with the American species, R. luteum or R. molle. This remaining group have unique foliage with leaves arranged in attractive whorls of 5 leaves. These include R. pentaphyllum, R. quinquefolium, R. albrechtii and R. schlippenbachii. The former species is native to central and southern Japan. Shrubs may reach a towering 8 m but are generally under 4 m. They have has bubblegum-pink flowers. Rhododendron quinquefolium, from central Japan, can reach similar dimensions but has white flowers. Rhododendron albrechtii is quite dwarf, only 1-2 m with dark, purplish-red flowers. It is also native to Japan. The last species, R. schlippenbachii , hails from Korea and is commonly called the royal azalea. It also has whorled leaves but the leaf shape is more reminiscent of English oak than azaleas. It produces pale pink flowers on shrubs 2-5 m in height. The fall colour is a mix of orange shades.
Rhododendron pentaphyllum, R. albrechtii and R. quinquefolium
Flowers and fall colour on R. schlippenbachii
Cultivation for these Old World azaleas is the same as for azaleas in general; full sun in the north, part shade in the south and organic-rich, well-drained soil that is acidic in nature. All of the above species are hardy in my zone 5b. In fact, R. schlippenbachii is considered the hardiest of the Old World azaleas, rated for zone 4. These Asian species are not as easy to find as the American, but are well worth the search.
I would like to thank growin (R. molle and R. schlippenbachii) and rebecca (R. molle subsp. japonicum) for the use of their pictures.
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