Rhododendrons and azaleas – Soil conditions and planting in containers

How do you like your rhododendrons? Do you like the giant forms, the medium ones perhaps, or the dwarf kinds? Attraction to plants is often linked to early associations. I personally like rhododendrons semi-wild and high above my head. But some of the smallest varieties also capture my attention.

My earliest memories of rhododendrons date back to childhood. Suddenly, when walking among shrubs, I came upon tall branches of leathery-looking, large leaves, topped with magnificent flowers – the most bold and impressive blooms that I had ever seen. I set about picking a bunch to take home to my mother. The flowers of the brightest colours were few and difficult to reach. They tended to be on spindly twigs, threaded through more powerful and vigorous branches carrying mauve flowers. These were spotted in their throats with deeper markings of luscious hues. I became utterly engrossed in the flowers – they gleamed against the dark leaves, like brightly coloured foxgloves gathered on giant inflorescences.

Name that Rhodon!

The name Rhododendron comes from rhodon (rose) and dendron (tree). Some gardeners enjoy growing all the unusual species and cultivars; others delight in showing their plants, yet others gain pleasure in becoming expert on the numerous botanical divisions and the geographical distribution of the species.

There is a huge range of species – more than 700. Besides the giants, there are many medium and low-growing species, including the evergreen and deciduous azaleas - which also belong to the genus Rhododendron. In Britain flowers appear mostly in spring. And their inflorescences vary greatly, from single bell shaped blooms, to enormous trusses of scented flowers. Flower colours include white, cream, yellow, orange, pink, red, dark red, mauve, near blue and many shades in between.

Deciduous azaleas can be seen in all their glory in May. Look out for Ghent hybrids, Exbury hybrids, American hybrids, and Mollis hybrids. There are many flower colour combinations to choose from among the hybrid varieties. A moist peaty soil is their main requirement. But it is still possible to grow azaleas if you live in an area with alkaline soil. You can grow them in containers of peaty soil. Remember to top-dress pots annually with a peat, or peat substitute, compost. More names of azalea varieties coming soon.

 Evergreen azaleas

The hybrid evergreen azalea varieties were mostly derived from Japanese plants. Popular as houseplants, and chosen for growing in containers on patios in small gardens, most of these spring-flowering shrubs thrive in semi shade or sun - so long as the soil is never allowed to dry out. Modern cultivars are available in most colours: white through creams, yellow, orange, pink, crimson, red-purple, or mauve.

Hardy hybrid rhododendrons are numerous – varying in foliage, flowers and forms, as nurserymen continuously seek new varieties to introduce. Dark evergreen leaves are present all year. The variety ‘Pink Pearl’ is an old favourite. Even if you live in an area where the soil is predominantly alkaline, where it is considered difficult to grow rhododendrons, it is sometimes possible to gain some success by treating the soil around the plants four times a year with Miracid.

Dwarf evergreen varieties

Dwarf evergreen rhododendrons are low-growing shrubs that flower in spring. Numerous flower colours and flower forms are available from specialist nurseries. The species R. forrestii f. repens from China is unusual: crimson flowers are borne singly, or in pairs, on plants less than 12 in tall. Larger, low-growing hybrid varieties include ‘Elisabeth’, vermilion red flowers. No pruning is necessary. Plant shallowly in spring or autumn, avoiding deep shade under trees. Choose an acid soil. The small varieties are very effective when planted in rock gardens; alternatively they are a good choice for patios, where there is some shade.

Medium-sized evergreen varieties

These are evergreen rhododendron varieties reaching a height of between five and ten feet tall. Their dark green foliage forms a bold contrast to the flowers. Astonishingly beautiful, heavily scented blooms open white on variety ‘Fragrantissimum’ (R.edgewworthii x R. formosum). This delightful shrub grows well in dappled shade; but it is wise to take precautions in cold areas by mulching, or grow in a container that can be moved under cover during winter. R. ‘May Day’ - a variety 3’-5’ in height, bearing scarlet flowers - will tolerate a sunnier site.

Large evergreen varieties

These rhododendrons grow tall and are suited to woodland settings. The leaves are dark green or they can be brownish in colour. The flowers appear mostly in spring. Highly spectacular are varieties R. Older ‘King George’ (pink buds fade to white with a green basal blotch), R. 'Sapporo’ – a striking variety with white flowers and dark purple markings - ‘Purple Splendour’ purple with black blotch, and R. ‘Susan’ a delicate lilac shade. All these varieties grow to a height of between ten and twelve feet.

Soil Requirements

To grow well rhododendrons require acid compost (i.e. one that is not alkaline, chalky or limy). It should also be rich in organic matter and not compact. Peaty compost is suitable or sandy loam.

Rhododendrons and azaleas in containers

One of the advantages of growing shrubs in containers is that it is a way of growing plants that would not tolerate the soil that exists in the gardens of some areas. For example, if they are given lime-free water and ericaceous compost, rhododendrons and azaleas can be grown in large pots even in chalky districts. Many rhododendrons are scented which definitely adds to their appeal as pot plants. Azaleas may be brought into a garden room to provide colour in spring.

Tips on rhododendron cultivation

• Many rhododendrons grow best with some shade during the day – this makes them suitable for woodland gardens. Hardy varieties can be grown in an unheated shady greenhouse.
• Some species and varieties are slightly tender, making shelter in a greenhouse during inclement months a sensible option.
• In June, remember to remove unwanted rhododendron seed heads and mulch the ground around the shrubs.
• Don’t stand rhododendrons and azaleas in pots in water-filled saucers.
• Remember to provide an acid soil, and that tap water may have the same effect as alkaline soil over a period of time.