How to grow rhododendrons
Commonly classed as either rhododendrons or azaleas, these popular woodland shrubs put on spectacular flowering displays from spring to early summer. Rhododendrons are usually medium or large evergreen shrubs, while azaleas tend to be smaller and may be evergreen or deciduous. Azalea flowers are usually smaller, but come in a more vivid array of shades, and are sometimes gloriously fragrant. Most rhododendrons and azaleas require acid soils.
- Group: Evergreen and deciduous trees and shrubs
- Flowering time: Mainly spring
- Planting time: Autumn or spring
- Height and spread: Variable
- Aspect: Generally light dappled shade
- Hardiness: Mostly fully hardy
All you need to know
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, drying 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,
between pH 5.0 and 6.0 that is rich in organic matter acid soil
The acidity/alkalinity of soil is measured on a pH scale, which runs from 0 to 14. Neutral soil has a pH of 7. Acid (or ericaceous) soil has a pH of less than 7, and the lower the pH, the more acidic the soil. You can measure your soil pH using a simple testing kit. Acid-loving (ericaceous) plants include rhododendrons, camellias, heathers and blueberries. Adding lime to soil will reduce its acidity. A slightly acidic soil of pH 6.5 is the best general-purpose pH for gardens, allowing a wide range of plants to grow.
- If you have alkaline soil, grow rhododendrons as container plants or see our page on more tolerant species and cultivars. Reducing soil pH is not simple
- 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
- Plant in October or March/April
- Before planting, dig-in plenty of neutral or acidic organic matter: composted tree bark, leafmould, decomposing pine or spruce needles, or composted chopped bracken. Avoid simply lining the planting hole; mix well into the soil
- Do not plant too deeply - all rhododendrons are surface-rooting and the roots should be just covered
- Apply at least an 7.5cm (3in) mulch of chipped conifer bark or other acidic material. The mulch should be well-aerated, not packed or firmed down
- Renew or replenish the mulch each spring when the soil is still moist
John Innes ericaceous loam-based potting 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-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.
- 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
When grown in decent, acid soil, rhododendrons are unlikely to need additional feeding. In soil, or potting compost, where the pH level isn't sufficiently low, your rhododendron may suffer from magnesium, manganese or iron deficiencies. See our advice page on nutrient deficiencies for information on identifying the symptoms and how to remedy them. Plants in tubs or other containers will beneift from a slow release, granular fertiliser added in spring, or use of a liquid feed formulated for ericaceous plants. Take care not to exceed the manufacturer's recommended amounts, as the roots and leaves of rhododendrons can burn if over fertilised.
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 Rhododendron ponticum, rather than from smooth-barked kinds. After cutting back, mulch, feed if soil conditions are poor, and keep well-watered to encourage new growth
- Rhododendron cuttings are taken from the current year’s growth during late summer or autumn, once the shoots have completed their seasonal growth and the bud is fully developed. Take cuttings with a heel, and then wound the stem to encourage rooting
- Cuttings of evergreen azaleas are taken in mid- to late-summer. Deciduous azaleas are harder to propagate from cuttings; commercial growers take cuttings in early spring, and grow plants on in frames for up to two seasons before planting out
- Grafting is used where cuttings cannot be rooted. Rootstocks are taken from R. ‘Cunningham's White’. Saddle grafting (which involves cutting an inverted V in the scion stem) is done from January to March, and spliced grafting is done in spring
- Layering can take 15 months to two years (some smooth-barked rhododendrons may take three years until rooted). Check layered shoots in the autumn of the second year. If not well-rooted, partially sever and lift in the following spring as growth begins
- 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
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 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) are shed in spring and summer. This is normal and not a cause for concern
- Rhododendron leaves may droop in severe cold, but usually recover with a return to milder conditions
- Windy, cold or wet weather can cause leaf scorch and flowers can be damaged by rain and frost
- Nutrient deficiencies may cause yellowing foliage known as chlorosis
- Vine weevil can be troublesome (especially for container-grown rhododendrons), as can rhododendron leafhopper, scale insects and the diseases that affect rhododendron such as bud blast, azalea gall and honey fungus
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