Rhododendron

Cultivation notes

Site selection

Choose a site with dappled shade in sheltered conditions. Avoid deep shade beneath other trees. Most rhododendrons will tolerate a more open site if sheltered from cold, dry winds. Dwarf alpine species will tolerate full sun provided the soil does not dry out. Avoid frost pockets and sites exposed to early morning sun.

• Rhododendrons need moist but well-drained, acid soil between pH 5.0 and 6.0 that is rich in organic matter.
• If you have alkaline soil, grow rhododendrons as container plants.
• Dwarf alpine rhododendrons are effective in a rock garden.
• Larger rhododendrons are excellent for woodland gardens.
• Compact hybrids are ideal for containers on shaded patios.

Planting

• Before planting, dig in plenty of neutral or acidic organic matter: composted tree bark, leafmould, decomposing pine or spruce needles, or composted chopped bracken.
• Do not plant too deeply - all rhododendrons are surface-rooting and the roots should be just covered.

Watering

• Rhododendrons grow best in areas of high rainfall. Even on suitably acid soils they are more difficult to grow successfully in the drier parts of the country.
• Tap water, especially in hard water districts, contains too much calcium for rhododendrons, reducing acidity around their roots.
• Use rain water for watering rhododendrons, but if rain water runs out, tap water is satisfactory for a month or two in summer.

Feeding

In spring, an application of ericaceous fertiliser, controlled release fertiliser pellets, or ‘Vitax Q4’ at a rate of 70gms per m2 (2oz per sq yd) in cool, moist conditions is beneficial.

Container cultivation

• John Innes ericaceous loam-based compost makes a good compost choice, generally providing easier management of watering and feeding.
• RHS research has found that soil-less potting media including ericaceous peat-based and peat-free potting composts are suitable for rhododendron growing. However they can lose their structure over time leading to poor drainage and an airless root environment, causing leaves to brown and die back. Re-potting every other year into fresh potting compost in early spring as soon as signs of growth are seen is recommended. In the intervening year, replace the top 5cm (2in) of compost. You can re-pot back into the same pot if you trim off up to a third of the roots to make room for fresh potting compost.

Links

Tree and shrub planting
Shrubs
Peat-free growing media
Soil: understanding pH and testing
Soil types
Acidifying soil

Pruning and training

Rhododendrons don’t require much pruning other than the removal of dead wood and deadheading of spent flowers if practical.

• If pruning is necessary to restrict size, follow the guidelines in our profile on pruning evergreen shrubs
• Many rhododendrons respond well to hard cutting back; response is best from deciduous azaleas and rough-barked rhododendrons such as R. ponticum, rather than from smooth-barked kinds. 

Propagation

Only species rhododendrons come true from seed. Named cultivars must be propagated from cuttings, grafting or layering. Rhododendron cuttings are taken with a heel and wounded to encourage rooting. See our profiles on layering and semi-ripe cuttings for further information.

• Grafting is used where cuttings cannot be rooted. Rootstocks are taken from Rhododendron ‘Cunningham's White’. Saddle grafting is done in January to March or side veneer grafting in spring.
• Seed propagation: sow into pots in January and leave the seed uncovered on the surface of the compost. Alternatively, top off the pots with a 2cm (3/4in) layer of damp perlite, scatter the seed over the perlite and ‘plough’ in using a pencil. Place the pots in a heated propagator.

Cultivar Selection

The following should be available in plant centres and nurseries:

Rhododendron ‘Cynthia’ AGM: Grows to a height and spread of 6m (20ft) with trusses of rose-pink flowers in late spring.
Rhododendron dauricum ‘Midwinter’ AGM: Very hardy, deciduous or semi-evergreen variety with a height and spread of 1.5m (5ft) and rose-purple flowers in early spring.
Rhododendron ‘Dopey’ AGM: Evergreen variety with a height and spread of 2m (6ft) and long-lasting red flowers in late spring.
Rhododendron macabeanum AGM:  Evergreen tree rhododendron with huge trusses of creamy to deep yellow flowers in mid-spring and large leaves to 30cm (12in) long; eventual height of 15m (50ft) and spread of 6m (20ft).
Rhododendron luteum AGM: Deciduous type for full sun with a height and spread of 4m (12ft) and strongly scented yellow flowers in late spring.
Rhododendron yakushimanum ‘Koichiro Wada’ AGM: Dome-shaped rhododendron to 2m (6ft) across and high, with rose-pink buds opening to pale pink flowers in late spring.

Links

RHS Plant selector
Find a stockist with Plant Finder
AGM recommended varieties
RHS Plant Bulletins

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Rhododendrons can easily be grown in containers, as long as they are watered regularly.

Bold drifts, whether on a small or grand scale, can be impressive in spring.

Rhododendron 'Patricia's Day'

Rhododendron williamsianum

Rhododendron 'Rwain'

Rhododendron 'Caroline Allbrook'

Rhododendron leutum.

Rhododendron 'Martha Isaacson'

Rhododendron rex subsp. fictolacteum

Deadheading rhododendrons encourages better flowering.

Problems

Non-flowering and bud drop

• Rhododendron flower buds start forming in late summer. A short period of dry conditions at this time may cause flower buds to fail to form, or buds may only partially form, drying up and dropping unopened the following spring
• Prevent this by mulching and watering thoroughly and regularly during dry periods from July onwards

Leaf drop

• Leaf drop can occur following a period of drought and is usually preceded by drooping and rolling of the leaves. The oldest leaves are the first to drop. Higher leaves may show browning at the leaf tip or edge. Leaf drop can also occur following extended periods of waterlogging
• As with all evergreen shrubs, each year a proportion of the foliage (mainly older leaves) is shed in spring and summer. This is normal and not a cause for concern

Leaf droop

• Rhododendron leaves may droop in severe cold, but usually recover with a return to milder conditions

Other problems

• Windy, cold or wet weather can cause leaf scorch and flowers can be damaged by rain and frost
• Nutrient deficiencies may cause yellowing foliage
• Vine weevil can be troublesome (especially for container-grown rhododendrons), as can rhododendron leafhopper and the diseases bud blast, azalea gall and honey fungus

Links

Non flowering woody plants
Scale insects
Rhododendron diseases