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How to Grow Rhododendron/Azaleas

Rhododendrons, or azaleas, are popular woodland shrubs that produce stunning flowers from early spring to early summer. While azaleas are typically smaller and can either be evergreen or deciduous, rhododendrons are typically medium-sized or large-sized evergreen shrubs. Azalea flowers are typically smaller, but they can be found in a wider variety of vivid hues and can occasionally be gloriously fragrant. The majority of azaleas and rhododendrons prefer acidic soils.

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Common Names:Azalea
Botanical Name:Rhododendron
Plant Type:Evergreen and deciduous trees and shrubs
Sun Exposure:Light, dappled shade
Soil Type:Fertile, well-drained
Soil pH:Neutral-acidic
Flowering Time:Spring
Planting Time:Autumn or spring
Mature size:Dwarf: 40cm in 10 years. Tall: 4-6m in 10 years.
Hardiness:Mostly fully hardy
Difficulty:Easy-moderate
Toxicity:Harmful if eaten
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Flowers
Plant

Planting Conditions

Sunlight: Choose a protected location with dappled shade. Under other trees, steer clear of deep shade. As long as the soil does not dry out, dwarf alpine species will tolerate full sun.

Soil: Rhododendrons thrive in soil that is acidic between 5.0 and 6.0 pH, well-drained, and moist. Rhododendrons can be grown in containers if you have alkaline soil, or check out our page on species and cultivars that are more tolerant. It’s hard to lower the pH of the soil.

Watering: Young plants require regular, deep watering to ensure that the soil doesn’t dry out. As they become established, however, watering should be reduced as they are shallow-rooted plants which may become waterlogged. Water thoroughly when it is very hot outside and the soil has dried out. Curling leaves and drooping flower buds are signs that you may need to water.

Location: The majority of rhododendrons will tolerate a more open location if protected from dry, cold winds. Sites that are exposed to frost or early morning sun should be avoided.

In a rock garden, dwarf alpine rhododendrons are effective. Rhododendrons in larger sizes are ideal for woodland gardens. Containers with shaded patios work best with compact hybrids.

Planting

Planting from seeds:

Seeds can be started indoors or in a greenhouse or cold frame. Sow the seeds in compost – preferably seed-starting mix – in covered seed trays. Keep the soil moist and ensure the trays are getting plenty of light, either natural or from grow lights. Remove the lids of the trays once the seedlings have reached the top of their container. Tend the seedlings until they outgrow their space, and transfer them either to pots or into the ground. Only plant young plants in the ground when the soil is not frozen or waterlogged. Provide some protection such as netting or wire cages to avoid your young plants being attacked by animals. Keep seedlings and young plants well-watered while they establish.

Planting from potted plants:

When the ground is not frozen or saturated with water, plant in October or March to April. Dig in a lot of neutral or acidic organic matter before planting. Leaf-mould, decomposing pine or spruce needles, composted chopped bracken, and tree bark work well. Do not just line the planting hole; incorporate well into the soil.

Plant too deeply because all rhododendrons have surface roots that should only be covered. Apply a mulch of chipped conifer bark or another acidic material at least 7.5 cm (3 in) thick. Mulch should be loose and aerated rather than packed or compacted. Every spring, when the soil is still moist, renew or replenish the mulch.

John Innes ericaceous loam-based potting compost is a good choice for planting in a container because it generally makes feeding and watering easier to manage.

Propagation

Propagation by layering:

Layering is, perhaps, the simplest way to propagate a rhododendron/azalea. Simply hold a lower branch down on the soil and fasten it in place. Cut a slit into the branch, somewhere that it is touching the soil. For healthy root growth, apply a rooting hormone to the cut. Once roots have developed, the branch can be removed from the parent plant and grown as its own plant.

Propagation by seed:

Seeds can either be purchased or gathered from seed pods in the autumn. Gathered seeds should be dried and stored in a paper envelope until February-April, when they can be sowed. Sprinkle the seeds on top of your growing medium, which could be compost, seedling soil, or a sphagnum moss mixture. Make sure the seeds aren’t covered with the growing medium. Keep the seeds humid and water from the bottom to avoid washing the seeds away. Cover the container with a piece of clear glass or a clear plastic bag to hold in humidity. The seeds should take 4-8 weeks to germinate, at which point you should remove the glass or plastic.

As the plants grow, size up their pots to accommodate their root systems. It may take 2-3 years of growth in pots for them to be planted in the ground. Avoid planting them out when the sun is very intense, or be prepared to shade them for protection against scorching. Young seedlings are sensitive to overwatering, so be careful not to water them too much in damp periods.

Propagation by cuttings:

Not all rhododendrons/azaleas can be propagated by cuttings, but when it does work it is a very effective method.

  1. In early autumn, with a sterilised cutting tool, take a cutting from growth that has hardened off. It should be 2-4 inches long. Cut any large leaves in half to reduce the leaf area of the cutting.
  2. Create a wound roughly an inch on each side of the cutting, and cut the bottom tip off of the cutting just before dipping it in a rooting hormone.
  3. Place the cutting in a mixture of sphagnum moss and perlite. Bottom water the container and place it in a polyethylene bag in bright, indirect sunlight.
  4. Rotate the container weekly to evenly distribute light.

Once the cutting starts to root, continue to upsize the container to accommodate the root system.

Common Rhododendron/Azalea Issues: Pests and Diseases

Rhododendron leafhopper (Graphocephala fennahi):

These are sap sucking bugs that feed on rhododendrons. They are active from late spring-autumn, and are associated with (potentially causing) a fungal infection called bud blast.

Bud blast (Seifertia azaleae):

Bud blast is a fungal infection that kills developing flower buds. The fungus – associated with the activity of rhododendron leafhoppers – causes flower buds to turn brown and die.

Azalea leaf gall (Exobasidium japonicum):

This disease attacks rhododendrons, disfiguring – but not killing – them. It is spread by spores and causes pale green galls that turn white over time. The disease tends to replace leaves and flowers with these unsightly galls.

Ongoing Care

Rhododendrons do best in places with a lot of rain. In the drier regions of the country, it is more challenging to successfully grow them even on soils that are sufficiently acidic. Rhododendrons suffer from acidity around their roots because tap water, particularly in hard water districts, contains too much calcium. Rhododendrons can be watered with rainwater, but if that doesn’t work, tap water will do for a few weeks in the summer.

A general fertiliser application is beneficial late in the winter or early in the spring. Change the top 5 cm of potting compost every winter for plants in tubs or other containers, feed them in the summer with liquid fertiliser or add controlled-release fertiliser pellets to the potting medium.

Seasonal Care

Spring

  • Take cuttings of deciduous plants in early spring.
  • Deadhead spent flowers to encourage more blooms.

Summer

  • Take cuttings of evergreen plants in mid to late summer.

Autumn

  • Fertilise young plants or those growing in nutrient poor soil.

Winter

  • Sow seeds in January/February.

What to Plant Them With

Rhododendrons/azaleas are acid-loving plants, meaning they pair well with other ericaceous plants. Light compatibility is also important to consider, so pick plants that also enjoy filtered light. Here are some examples:

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Updated on April 17, 2024

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