Rhododendron (Azalea)

Also known as Rhododendron. The rhododendrons are a spectacular genus of around 800 evergreen semi-deciduous shrubs and trees. Although largely confined to the Northern hemisphere with the majority of species in southern China and the Himalayan region, many are found in North America, Japan and Southeast Asia, and the tropical branch of the genus extends the range of New Guinea and the far Northeast of Australia. They are woody stemmed plants grown mainly for their massed display of flowers, which vary greatly in size from tiny thimbles to 8 inch trumpets. They are usually clustered in inflorescences (trusses) at the branch tips and occur in every color except a true bright blue. The flowers may be self or multi-colored, often with a contrasting throat blotch (flare) or spotting. Some species have scented flowers. Temperate-climate rhododendrons tend to bloom from late winter to early summer, but the tropical rhododendrons are far less seasonal and bloom throughout the year, usually reaching a peak in early fall. Plant size ranges from near prostrate through to trees over 50 ft. tall. Yet despite this enormous variety of species, and the thousands of garden hybrids and cultivars, rhododendrons are all remarkably similar in general appearance and unlikely to be mistaken for any other plants. They are mainly broad leafed with roughly elliptical, usually deep green leaves and their 5-petalled (or more) flowers are flat, funnel-, bell- or trumpet-shaped. Rhododendrons also have beautiful; foliage. The leaves of the evergreens are usually thick and leathery, often deep green and lustrous. The deciduous and semi-evergreen forms tend to have hairy leaves and their foliage may change color in fall and winter. Leaf size varies enormously. Many of the alpines have tiny leaves under 1 in long, while the foliage of species from mild, high rainfall areas can exceed 24 in long. Rhododendrons are botanically divided by the presence (lepidote species) or absence (elepidote species) of scales on their leaves. While often barely noticeable, these scales sometimes take the form of felting (indumentum) in the foliage. This is usually confined to the leaf undersides and is a feature of many rhododendrons.

As far as gardeners are concerned there are 3 main categories of rhododendrons, namely: azaleas (deciduous and evergreen); tropical or Vireya rhododendrons; and the temperate climate plants we might call 'true' rhododendrons, which includes the small alpine or arctic rhododendrons. All have been extensively developed and hybridized, so in addition to those 800-odd species there now are countless garden forms. With such a huge variety of plants from widely differing backgrounds, it is easy to become confused. Consequently, some of the main hybrid groups are subdivided into smaller blocks of plants with similar characteristics or parentage.

Insider's Secret

As would be expected of such a large group, rhododendrons vary considerably in their climatic preferences. However, they all share the same general cultivation needs. This is because their roots lack the fine feeding hairs at the root tips that are found on most other plants; instead, the entire root ball is a mass of fine roots that serve the same function as root hairs. This lack of heavy, aggressive, spreading roots greatly influences the cultivation and uses of rhododendrons. A compact root ball and no tap roots make rhododendrons ideal for container cultivation and easy to transplant, but there are disadvantages. Fine roots dry out quickly in dry conditions. soon rot if kept waterlogged, suffer in compacted soils and cannot penetrate hard or rocky ground. Rhododendrons need loose, open, well-aerated, acidic soil with plenty of humus to retain moisture. They thrive with regular mulching that enables their predominantly surface roots to spread and develop. In wet areas or gardens with alkaline soil it is best to plant in raised beds filled with specially blended compost. Most rhododendrons are woodland plants that prefer dappled shade or at least protection from the hottest afternoon sun and strong winds They require little maintenance apart from a light trim after flowering. They are not greatly troubled by pests and diseases but are prone to infestation by thrips, two-spotted mites and spider mites and powdery mildew or rust in humid areas. Evergreen azaleas are sometimes disfigured by azalea leaf gall. Taller varieties make good woodland or shrubbery plants, even hedges, while the dwarf forms are perfect for mass plantings or rock gardens. Azaleas are usually at their best planted en masse. Vireya rhododendrons are superb plants for tropical or subtropical gardens or as potted plants in conservatories. Rhododendrons are propagated from seed or cuttings or by layering. the method and timing varying with the type of plant. Frost hardiness also varies greatly.