How To Grow Rhododendrons / RHS Gardening

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Growing guide

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.

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Quick facts

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

Before you get started

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, 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,
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.
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 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

Planting

Plant in October or March/April
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

Container cultivation

Peat-free 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.

Planting, pruning and caring for a Rhododendron

Ongoing care

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

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.

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 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

Propagating

Only species rhododendrons come true from seed. Named cultivars must be propagated from semi-ripe cuttings, grafting or layering.

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, scatter the seed over the compost and ‘plough’ in using a pencil. Place the pots in a heated propagator

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) are 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

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Tag » How To Care For Rhododendrons

How To Grow Rhododendrons / RHS Gardening

Quick facts

All you need to know

Site selection

Container cultivation

Planting, pruning and caring for a Rhododendron

Watering

Feeding

Non-flowering and bud drop

Leaf drop

Leaf droop

Other problems

Discover rhododendrons

Get involved

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