Flowering Shrubs – Rhododendrons and Azaleas
Of all flowering shrubs, with the solitary exception of roses, rhododendrons make the greatest contribution to the flower colour of gardens. At their peak in May and June, they are terrific and people flock to enjoy the display made by such famous collections as those at the Saville Gardens and Valley Gardens, Windsor Great Park, the Isabella Plantation, Richmond Park, the two fine woodland gardens in Bushey Park, Hampton Court, and such gardens as Exbury Gardens in Hampshire and RHS Gardens, Wisley in Surrey.
The family is immense, including all those plants commonly known as azaleas; varieties vary enormously in size, habit, colour and appearance, many being so different from the popular conception of a rhododendron that most people without specialist knowledge would not recognize them as such. There are something like six hundred known species growing in the wild besides many thousands of garden varieties either selected from these species or made by hybridization between them Many are easy to grow but all dislike lime, though the degree of dislike varies and the common purple Rhododendron ponticum, which has naturalized itself in many parts of Britain, will actually survive in soils that are neutral or even very slightly alkaline.
Most prefer to grow in semi-shade, preferably the dappled or intermittent shade cast by trees fairly widely spaced. They are the most popular shrubs for planting in woodland gardens; indeed you could even say that in Britain the woodland garden owes its existence to the great increase of interest in rhododendrons during the past hundred years. Yet some kinds will grow very well in full sun and these include some of the small-leaved mountain rhododendrons that are popular rock-garden shrubs as well as the very different hardy hybrids and the deciduous azaleas.
The hardy hybrids are what most people think of as typical rhododendrons, but in fact they are not natural species but man-made hybrids — the result of a long continued effort on the part of rhododendron enthusiasts to perfect and extend a race of evergreen rhododendrons capable of flowering regularly and reliably outdoors in all parts of Britain. Since it is the open flowers that are most at risk, this has meant producing rhododendrons which will flower between mid-May and late June when the risk of severe and sustained frost is not great. The hardy hybrids mostly have fairly large leaves and dome-shaped clusters of large flowers. The colour range is wide, including pink, rose, red, carmine, crimson, purple, blue, light yellow and white, and most of them grow fairly slowly into large bushes. They are among the easiest to grow, surpassed only in this respect by R. ponticum which lacks their colour range and quality of bloom.
The hardy hybrids are of very mixed parentage whereas there are many other hybrids which are the result of crossing two species. These are often given distinguishing names, eg. all hybrids between R. fortunei and R. griffithianum are known as R. loderi. Since they will not all be of similar merit, particularly good forms are increased vegetatively, by, or , each selected form being given an additional distinguishing name; for example ‘King George’ is a particularly fine pink and white form (a clone) of R. loderi.
Rhododendrons differ enormously in size, habit and leaf as well as in their flowers. At one extreme are prostrate species, such as the scarlet-flowered R. forrestii, at the other tree-like shrubs, such as the red, pink or white R. arboreum. There are kinds with tiny leaves and small flowers, such as the purplish-blue R. impeditum, and others with enormous leaves and flower trusses to match, such as R. sinogrande. Some are completely hardy, some are only satisfactory in the milder maritime localities and some require greenhouse protection. In addition to the familiar type of bell-shaped flower packed into quite dense clusters there are rhododendrons with tubular flowers, almost flat flowers, funnel-shaped flowers and many other variations.
The azaleas can be split between evergreen and deciduous kinds. The hybrid evergreens are all fairly low-growing but rather wide-spreading shrubs with small evergreen leaves and small to medium sized flowers produced very freely, mainly in May and early June. Many are sufficiently hardy to be grown in all but the coldest parts of Britain. All prefer some shade and protection from searing winds.
The hardy deciduous azaleas are medium-sized shrubs, usually rather open-branched, with mainly medium-sized flowers in May and June. They will tolerate more sunshine and exposure than theand can be grown outdoors virtually throughout the British Isles provided the is acid. The leaves of many colour well before they fall in autumn. These azaleas can also be purchased in excellent mixtures of colours such as the Knap Hill and Exbury strains
To bring some kind of order into all this, botanists have divided the genus into groups, which they call ‘series’, the members of which have obvious characteristics in common The Royal Horticultural Society’s Rhododendron Group has published several useful handbooks on the species and hybrids. These use a system for assessing hardiness based on four grades: H-4 for a rhododendron that is hardy anywhere in Britain; H-3 for those hardy in the South and West in maritime areas and sheltered gardens inland; H-2 for those requiring protection even in the most sheltered gardens, and H-1 for rhododendrons normally requiring greenhouse protection. Some nurserymen use this classification in their catalogues but, in general, only varieties in classes H-4 and H-3 are readily available.
All are easy to grow provided the soil is not alkaline or liable to dry out badly at any time. Rhododendrons enjoy humus and benefit from generous annual mulches of leafmould or peat, but peat is not essential and excellent plants can be seen on acid clay soils. They benefit from spring feeding with a high-nitrogen compound fertilizer preferably including iron, magnesium and manganese as well as the usual phosphorus and potash. If they become overgrown they can be thinned or cut back in May or June, preferably immediately after flowering, but will not then flower the following year.
Although the peak flowering season for rhododendrons is May-June, some kinds flower as early as January in very sheltered places and a few do not start flowering until July.