- Common names: rhododendron, azalea
- Botanical name: Rhododendron
- Family: heather (Ericaceae)
- Type: evergreen and deciduous shrubs and trees
- Flowers: spring and summer
- Planting time: spring and autumn
- Height: 60cm to 30m (2 to 98ft)
- Width: 60cm to 30m (2 to 98ft)
- Light: depends on your location
- Hardiness: H4 to H6
- Difficulty: easy to average
Straddling the seasons, rhododendrons are the buxom queens of spring, leading us into the floriferous summer months without any subtlety. They can grow into vast shrubs that, in bloom, are wondrous clouds of colour formed of handful-sized flowerheads.
Hailing from the cool, moist forests of the Himalayas, rhodos like the drizzly British climate and have been part of our gardens since they were introduced here in the 18th and 19th centuries. In the wilds of Nepal, they form dreamy forests of pink and red flowers, while in the large rhododendron gardens of the UK, they are planted in a kaleidoscope cacophony of clashing colour, with scarlet, purple, tangerine, yellow, white, and brash magentas and pinks growing side by side.
There are 1,089 species, native to much of the northern hemisphere, especially the Sino-Himalayan region, and over 25,000 hybrids. These bloom at different times: mostly April, May, June, or July, but there are forms that flower earlier and later. They are woody shrubs or trees and, with the exception of most azaleas (which are a form of rhododendron), they are evergreen. Being mountain forest plants, they enjoy sloping ground and good drainage, as well as shelter and a cool environment: either a leafy woodland garden in the south or a more open site in the colder north.
In the right conditions, they are low-maintenance fuss-free stalwarts that provide evergreen foliage and shelter for wildlife and – in spring and summer – cheer us when they transform into mounds of unapologetically gaudy colour.
Which rhododendrons to grow
If you like the bold colour of rhododendrons, 'Cynthia' is a fabulous old variety with bougainvillea-pink flowers and 'Biskra' has bell flowers in a stylish shade of soft pink-orange; 'Tortoiseshell Orange' is also a good choice for attractive warm-copper flowers. Or for cooler colour, R. augustinii 'Electra' is a enchanting mass of blue in full flower.
The wonderful Loderi rhododendrons were bred by Sir Edmund Loder, prior to 1920, at his garden, Leondardslee, in West Sussex, which has recently been restored. Two of the best for scent and blousy pink-white flowers are 'Sir Edmund' and 'King George'.
All of these medium-sized rhododendrons will grow to around 2 metres in ten years, but can grow to 4 metres over time.
For compact colour, 'Elizabeth' is a reliable letterbox-red gem, 'Sneezy' is a carmine-pink award winner, and 'Gristede' is a twilight-purple dwarf cultivar.
For gentler colouring, 'Silberwolke' (silver cloud) has frilly shell-pink and white flowers, and 'Centennial Gold' blooms primrose-yellow, while adorable 'Tinkerbird' has snow-white flowers from pink-tinted buds and is rare in being a semi-dwarf rhodo that is both scented and hardy.
How to plant a rhododendron
Choose a sheltered site with free-draining, retentive acid soil. Gently fork in ericaceous compost or leafmould. Do not plant too deeply – the roots should just be covered by a thin layer of soil.
How to grow rhododendrons
Buy from a specialist supplier (such as Burncoose, Loder, or Millais) to ensure good quality plants and to ask for advice about the best varieties for your site because rhododendrons vary dramatically in size and hardiness.
Rhodos relish acid soil that is well-drained, retentive, and rich in humus. They dislike light extremes (i.e. intense hot sun and deep shade). Many rhododendrons also hate strong wind, but the hardier small-leaved forms that flower later are more wind tolerant, so speak to your supplier. Rhododendron yakushimanum and its hybrids are very hardy.
The standard advice is that rhododendrons prefer dappled shade, but that isn't always the case and insufficient light will result in straggly plants that refuse to flower well. In the cooler regions of the UK, rhododendrons can enjoy a much more open, sunny position. In the milder parts of the country, the dappled shade of a woodland garden is fine, as is a sheltered site in semi-shade. However, rhododendrons must receive good light, so a dense tree canopy above won't work; planting them near trees that don't cast as much shade, such as Japanese maples and Himalayan birches, is better.
The soil should be pH 4.5 to 6. If your soil is neutral or alkaline and you don't want to grow in containers, you could try a plant grafted onto the pH-tolerant Inkarho rootstock (such as 'Dufthecke White', 'Anuschka', or Bloombux), which copes with soil up to pH 7.5. This originated from a rhododendron found growing – by some miracle – in a lime quarry in Germany.
In early spring, put down mulch (for instance leaf mould, pine needles, or ericaceous compost) around the plant, ensuring it isn't near the stems and does not densely cover the rootball. Feed with slow-release fertiliser in spring. Established plants being naturally nourished by leaf matter from surrounding trees may not require feeding. If growing an Inkharo plant, feed and mulch annually.
Rhododendrons hail from cool, damp forests. Water plants regularly in hot, dry weather, especially if they receive several hours of sun a day or if they are young. Rainwater is best, particularly in hard water areas.
In early to mid spring, pinch out the elongating growth of terminal single buds to encourage branching.
Pruning is not essential and can be damaging. When necessary, do it in March; this may mean a year or two without flowers, but will benefit the plant in the long run. Never prune rhododendrons with smooth or peeling bark.
For the first two winters, a young plant is best shielded from winter weather (for example, with horticultural fleece), especially if the site is exposed or the rhododendron is tender.
How to grow rhododendrons in pots
Compact rhododendrons are happy in containers, and in gardens that don't have acid soil, it is one of the best ways to grow them. It also allows tender forms to be overwintered under cover.
Choose a pot with drainage holes and a dwarf or compact rhododendron (such as 'Blue Tit' or 'Hydon Dawn'). Place the pot in dappled shade or semi-shade in milder regions or in open sun or semi-shade in cooler regions. Plant with a mix of loam-based peat-free John Innes ericaceous compost and grit or bark.
Keep the compost moist but not saturated. Water when necessary, using rainwater if possible (this is especially important if you live in a hard water area). In early spring, replace the upper layer of compost with fresh ericaceous compost. Every few years, when your rhododendron doesn't look as healthy, repot into a slightly bigger container.
