Family: Ericaceae

This very large genus of 700-800 species of evergreen and deciduous shrubs and trees originates mainly in temperate areas of the north-ern hemisphere, especially Southeast Asia and the Himalayas. They are also found in the high-lands of New Guinea, and some originate from North America and Europe. Thousands of cultivars and hybrids have been bred since the 19th century, these being more popular than the species. Rhododendrons are among the most popular of all flowering shrubs. They can be divided into the following groups: the hardy, broad-leaf rhododendrons, the tender, Vireya rhododendrons and the hardy azaleas.

Varieties

Broad-leaf rhododendrons are generally evergreen shrubs that range from less than 1 m (3 ft) to tall, tree-like species, with the greatest range around 3-4 m (10-13 ft) in height. Most have rather leathery leaves, often somewhat hairy on the undersides. Flowers are borne in large, showy trusses in white, pink, red, blue, mauve, purple, cream, yellow and orange. Some are fragrant. An enormous range of species and cultivars is available. Broad-leaf rhododendrons are cool-climate plants. They may be grown as specimens but look spectacular when mass planted.

Vireya rhododendrons are evergreen shrubs, rarely growing more than 2 m (6 ft) high. Some are open in habit, others more compact and densely foliaged. The glossy leaves are mid-green. Flowering is variable and may be more than once a year. Colours range from white, yellow, apricot and salmon through to pinks and bright red. There are many cultivars, mostly in 'sunset' shades. Vireyas do well as understorey plants, but can be planted in a mixed shrub border. They are suitable for container growing and some also do well in hanging baskets. Azaleas, which are really types of rhododendron, can be divided into deciduous and evergreen types. Their showy clusters of flowers provide a reliable show of colour in spring, year after year. There are various groups of deciduous azaleas, best known being the Ghent hybrids, the Knap I Iill-Exbury Hybrids, and the Mollis hybrids. Their strongly coloured flowers come in red, yellow and orange shades, although some come in pastel shades, plus cream and white. Many cultivars are fragrant. The best-known evergreen hybrid azaleas are the Kaempferi and Kurume hybrids, which are generally dwarf to small plants in pink, red and also white.

Cultivation

Rhododendrons need a well-drained soil with a high organic content. Dig in copious quantities of well-decayed organic matter before planting and mulch well with organic matter. All must have acid or lime-free soil. They should not be planted too deeply as they are surface-rooting plants. Spread the roots, cover with soil and top with leaf litter, old manure or compost. Rhododendrons need shelter from strong wind and prefer morning sun and afternoon shade, or the light shade provided by tall trees. Rhododendrons do need some sun in order for the flower buds to form. They will not develop in permanent shade. Keep well watered in the warmer months and in dry, windy weather. Rhododendrons must never be allowed to dry out or they will suffer and drop their leaves. Broad-leaf rhododendrons may be in flower any time between late winter and late spring, depending on the variety. In their habitats, Vireyas flower from late summer through to early winter, but in cultivation, flowering time is variable and plants may flower more than once a year. Azaleas flower in the spring. The tall, deciduous kinds usually bloom in late spring and continue into early summer. Dwarf, evergreen azaleas such as the Kurume hybrids, bloom in mid to late spring. Rhododendrons do not generally need feeding and in any case, fertilizers which are alkaline, such as blood, fish and bone, should be avoided. If it is considered necessary to feed rhododendrons, apply a proprietary, slow-release fertilizer in spring. Little or no pruning is needed. If it is necessary to improve the shape of a plant, it should be done immediately after flowering. Dead flower heads can be removed by twisting them off. Propagate evergreen rhododendrons from semi-ripe cuttings in late summer, rooted with bottom heat. Hormone rooting powder increases the chances of rooting. Cuttings of many can be slow to form roots. Deciduous azaleas can be propagated from softwood cuttings in spring, again with bottom heat, but these are even more tricky. Far better for propagating rhododendrons is to layer young stems, in spring. This is a slow process, and may take a year, but is almost guaranteed to produce a new plant. Rhododendrons are susceptible to various pests and diseases, including root weevil larvae which eat the roots and can cause severe damage. The adults eat the edges of the leaves but this is not considered serious, only unsightly. Vireyas under glass may be attacked by scale insects and whitefly.

Climate

With such a vast range of species and hybrids, it is possible to give only general guide-lines as to suitable climates. The broad-leaf rhododendrons and azaleas vary in their hardiness, but they are suited only to temperate (cool temperate for the majority) and cold climates. In climates prone to frost, grow Vireya rhododendrons in a cool conservatory or green-house. They adapt well to pot culture.