Azaleas: planting, growing, pruning and care

Description and botany

Azaleas belong to large family Ericaceae, like the Heathers, Blueberries, Japanese andromedas, Strawberry trees and of course Rhododendrons. There are more than 50 wild species of Azalea, such as Rhododendron luteum, R. canescens and nearly 8,000 cultivated varieties developed worldwide.

Botanists sensibly place Azaleas in genus Rhododendron, as their genetic origins and organs are very close. Growers and gardeners, more pragmatic, prefer to call them Azalea. Azaleas are generally smaller, differently shaped, sometimes deciduous and often fragrant. Compared with Rhododendrons, Azaleas have foliage more often lanceolate, ending in a point. Their flowers usually have 5 to 7 stamens compared with 10 in Rhododendron. Finally, they are used differently in the garden.

Azaleas are grouped into subgenera within genus Rhododendron. Evergreen Azaleas mainly come from subgenus Tsutsusi and deciduous ones from subgenus Pentanthera.

1. Chinese Azaleas or Azalea mollis: deciduous, they don warm autumn colours and their flowering often releases a delightful fragrance. Colours range from white through various shades of yellow, red and flaming orange. They are vigorous and very cold-hardy. Most originate from North American species, except Rhododendron luteum, the pontic Azalea found in the Caucasus between Turkey and Eastern Europe. Modern varieties mostly descend from Rhododendron luteum, japonicum, molle, viscosum, occidentale, atlanticum, calendulaceum, flammeum, periclymenoides and arborescens.
2. Japanese Azaleas or Azalea japonica: evergreen, they form small, rounded, compact shrubs. There are pink, red, white and mauve to purplish Azaleas, but never yellow. Original species are exclusively Asian, mainly Japanese. Parents of garden-cultivated Azaleas are generally Rhododendron kiusianum, kaempferi or yedoense. Indoor Azaleas, or florist’s Azaleas, derive from subtropical species such as Rhododendron simsii, indicum and mucronatum.

Azalea: in bud and in flower

Main species and varieties of garden azaleas

Our favourite Japanese azaleas

Some reliable choices among deciduous azaleas

Precious or original azaleas to discover

Azalea japonica Adonis

A superb variety with large pure-white flowers that cover the whole plant in spring.

• Flowering time May
• Height at maturity 60 cm

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Rhododendron kaempferi Amoena - Japanese Azalea

Forms a compact ball covered with small pink-lilac flowers. Very hardy variety.

• Flowering time April, May
• Height at maturity 80 cm

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Azalea japonica Blue Danube

Hardy and easy to grow, this plant has been awarded for its generous flowering in pink with bluish tints.

• Flowering time June
• Height at maturity 80 cm

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Azalea japonica Kirin

A pretty bush that displays, from March, a multitude of small satin-pink flowers with a darker centre.

• Flowering time April, May
• Height at maturity 1 m

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Azalea japonica Schneewittchen

Very elegant, this late variety offers abundant white flowering in late spring.

• Flowering time June, July
• Height at maturity 60 cm

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Rhododendron luteum

It is the most fragrant of the wild species. Its foliage takes on a beautiful red-orange hue in autumn.

• Flowering time June, July
• Height at maturity 2,50 m

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Rhododendron Berryrose - Western Azalea

An excellent compact variety with large scented flowers.

• Flowering time May, June
• Height at maturity 1,20 m

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Azalea mollis Glowing Embers

An azalea remarkable for the warm colour of its scented flowers.

• Flowering time June, July
• Height at maturity 1,50 m

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Azalea Jolie Madame

A compact variety with upright habit offering scented flowering in pink with orange markings.

• Flowering time May, June
• Height at maturity 1,50 m

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Azalea mollis Persil

An award-winning variety for its abundant, bright and scented flowering.

• Flowering time June
• Height at maturity 1,25 m

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Rhododendron macrosepalum Koromo-shikibu - Large-sepaled Azalea

A unique azalea with finely cut flowers in a rare lavender colour.

• Flowering time April, May
• Height at maturity 60 cm

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Rhododendron kaempferi Peggy Ann - Japanese Azalea

A simply delightful variety with bicoloured flowers! Its compact habit allows it to be grown in a container or at the front of borders.

• Flowering time June
• Height at maturity 60 cm

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Azalea Cannons Double

A cultivar distinguished by its astonishing double flowering in nested corollas, where white, yellow and pink blend together.

• Flowering time May, June
• Height at maturity 1,20 m

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Planting azaleas

When to plant azaleas?

A bush grown in a container can be planted almost all year round, except during periods of frost or heatwave. It is nevertheless preferable to plant Azaleas in spring, around April, or in early autumn, in September–October. Planting while in flower also allows testing colour combinations in the garden before final installation.

Where to plant?

To succeed with Azalea culture, choose a bright but non-scorching situation and plant in non-calcareous soil.

Azaleas thrive in semi-shaded areas sheltered from wind, protected from the burning midday sun. Japanese Azaleas tolerate sun better in regions with temperate summers. In pots on terraces, they will do best in shade, or in a sunnier spot if you are diligent with watering.

A north or even west aspect is generally the best choice. Ideally, for the luckiest, plant under the light shade of large trees.

However avoid too dense shade, as good light encourages flowering. Also avoid placing a Japanese Azalea facing east, because morning sun can cause leaf and bud burn in winter after a sudden thaw.

If your soil is very calcareous, or you live in a very hot, dry summer region, choose a different type of plants, better adapted. Otherwise you would spend all your time watering for rather disappointing results. Azaleas are happiest in oceanic, temperate continental or mid-mountain climates.

How to plant an azalea correctly?

Planting an Azalea is easier in acidic to neutral soil that is fairly porate and well aerated. If your soil is very heavy, clayey, follow the adapted advice below.

1. Dig a hole about 40 to 60 cm deep, three times wider than the rootball.
2. Mix in thirds non-calcareous soil, potting compost and ericaceous soil (based on acidic turf with high water retention). Add pumice to the mix and place some at the bottom of the hole so that water drains well. Indeed, Azaleas need soil with an acidic tendency to have healthy, green leaves. Acidity is measured by pH and their ideal for the genus Azalea lies between 5 and 6. They also dread lime, much less so if it is magnesium-based, such as dolomite. Finally, drainage is important, especially in winter so roots remain healthy and plants in excellent condition. In heavy, poorly draining clay soil, it is advisable to plant your bed on a small mound. This should be formed with the mix described above so that water drains away easily.
3. To improve rooting, gently untangle the felt of small roots (rootlets) that may have formed around the plant if compact. Spray the edge of the rootball with a jet of water, then brush the rootlets to fluff them out and break the regular cylindrical shape of the rootball.
4. Then soak the rootball for 10–15 minutes, then let it drain a little.
5. Plant your azalea, preferably on a slightly raised bed so that water runs off well.
6. Cover the rootball lightly with the prepared soil mix and tamp down gently.
7. Create a slight depression with soil around the plant so you can water abundantly at its base without runoff.
8. Water with about 10–15 litres of water, applied in two stages to finish consolidating the soil and to eliminate air pockets. It is essential that rootball and soil are in perfect contact. The hollow will then be covered with a mulch.

How to grow an azalea in a pot?

Japanese azaleas are well suited to container cultivation on balconies and terraces.

My tips :

• Choose a shaded spot during the hottest hours of the day to limit heating and evaporation of water.
• Provide a draining layer at the base of the pot and a layer of mulch on the surface. Heathers planted at its base can serve the same purpose, while flowering through the long winter months. Spray its foliage in dry weather to prevent attacks by red spider mites.

Note: indoor or florist azaleas are different varieties and should be brought indoors for winter into a cool room at 5–15°C.

Potted azalea

Caring for azaleas

Maintaining Azaleas is fairly simple when they are well planted and in a location that suits them.

A good layer of mulch!

To reduce maintenance of your Azaleas, it is important to cover soil with a thick plant-based litter layer. This mulch is preferably made from pine bark, pine needles, or fern fronds. As it decomposes over time, top up mulch at the start of autumn, whenever necessary. Ideally maintain a layer about 8–15 cm thick, 15–20 cm if using fallen leaves. Mulch can be light during first years to encourage Azaleas to develop deep roots.

If soil is poor and plants are struggling, the most durable and beneficial solution is to mix into the mulch one-third well-rotted manure, compost or leaf mould. Otherwise, applying a fertiliser for ericaceous plants can also stimulate growth, but it will not boost soil life or long-term fertility.

→ to read on the subject: “Mulching: why? How?”

Watering

Azaleas need fairly constant moisture around roots. They should therefore be watered regularly in dry weather. Water needs are greater in spring, when growth resumes. Best practice is to allow soil surface to dry out between two waterings, encouraging Azaleas to send roots deeper.

Prefer watering with non-calcareous rainwater. Hard water may, over time, cause yellowing of leaves, a sign of chlorosis.

In summer, water in the evening if needed, also mist foliage to cool surroundings and increase air humidity.

A useful tip is to space out waterings in August–September; this encourages formation of flower buds for following spring.

In winter, remember to check whether Japanese Azaleas need water, especially after thaw. Their evergreen foliage needs water all year, transpires and can warm up in low winter sun.

Pruning Azaleas

Pruning Azaleas can be carried out after flowering.

Japanese Azaleas can be pruned into a ball, cushion shape or left natural. Shorten green shoots with shears, taking care not to cut into leafless wood.

Pruning of deciduous Azaleas is only done to rejuvenate an old plant or to encourage branching. Shorten a few overly long branches by cutting above a branching point. Remove some old branches at their base if younger ones can take over.

Potential diseases and pests

Your Azaleas will have very few health problems if grown in good conditions.

• If your Azalea’s leaves yellow around the veins, it suffers from chlorosis. This iron deficiency is caused by calcareous soil or irrigation water. Water the plant with a dilution of commercial anti-chlorosis product. Or prepare yourself a 10% dilution of nettle manure, or a 20% dilution of horsetail manure.
• Red spider mites can bite leaves of your azaleas located in a dry, warm spot. As prevention, regularly mist azalea foliage with water. As repellent, if infestation occurs, spray a garlic decoction onto foliage.
• Aphids may appear on shoots of your azaleas. Spray a 10% dilution of liquid black soap.
• Otiorhynchus weevils are beetles that notch leaf margins in a very characteristic way. Presence of ground beetles in garden helps regulate their population. Damage is more unsightly than dangerous, but a natural treatment using nematodes (Heterorhabditis) is possible. Diluted and watered into soil, it attacks buried larvae of these weevils and must be repeated until eradication, for several years if necessary.
• Azalea moth caterpillars sometimes devour their shoots. Treat only if very numerous, with organic insecticidal Bacillus thuringiensis (BT). Male moths can also be caught with a pheromone trap.
• Two major fungi able to kill garden azaleas are honey-coloured Armillaria and Phytophthora. Their appearance is due to root asphyxiation from excess water in winter and excessive soil compaction. Burn affected plants. Good drainage is therefore essential, with a bed of pumice at bottom of planting hole in heavy soil. Preparing a porous, well-aerated soil is also key.
• Powdery mildew sometimes whitens leaves of azaleas. At first there are white spots, then the fungus can cover entire leaf. As prevention, space plants sufficiently apart to encourage air circulation and avoid over-enriching soil with manure or nitrogenous fertilisers. Water at base of plant rather than on foliage. You can spray a horsetail or nettle manure onto foliage.

Propagation: layering, cutting

Propagation of azaleas is not among the easiest but can be attempted by the most daring, or those with a greenhouse or cold frame.

• Layering is by far the simplest method of propagation, although fairly slow. Use a low branch and bury it in June–July to make it root. Remove leaves from the part to be buried while keeping the leafy tip. Cut a thin strip of bark on the lower part of the stem. Dig a furrow about 7 cm deep and lay the branch down, keeping the incised lower part in contact with soil using a bent wire staple. Cover with soil while leaving the leafy tip above ground. It will take one and a half to two years before the layer can be separated from the parent plant. Replant in April–May or September–October.
• Cuttings can be planted in August–September, ideally under cover with good humidity. Gather early in morning thin lateral shoots of 9 to 12 cm from lower parts of plant. Leave top 4 leaves. Cut a thin vertical strip of bark and use plant hormone or willow water (maceration of willow shoots). Plant cuttings 5 cm deep, fairly close together, in turf mixed with a little sand. Ideally grow them under mist or enclosed under plastic, ventilating regularly and lightly, which is a delicate operation. Rooting can occur in about 4 months. Pot on in following spring, after removing terminal bud to encourage rooting and branching.

Uses and companion planting in the garden

Japanese azaleas can be placed on the edge of a terrace, at the front of a border, or planted in containers. Pruned into balls or cushions, they sit perfectly in the hummocks lining paths in Japanese gardens. Of montane origin, they are also comfortable in a cool rock garden, for example north-facing, accompanied by conifers and other rock-garden plants.

Japanese azaleas at Maulévrier Oriental Garden (49)

Deciduous azaleas, larger and more erect, are placed at the back of a bed.

Some planting ideas

Ericaceous plants are ideal partners for azaleas. You can therefore plant with them Japanese pieris, Enkianthus, Japanese maples, Magnolias, Mahonia, Aucuba, Camellias, mountain laurels (Kalmia), Sarcococca, etc. And why not plant heathers and callunas at their feet, thereby reducing maintenance and keeping soil cooler for longer.

To create a natural look, ferns are very useful. See Alexandra’s article: 9 easy ideas for combining ferns successfully. Grasses and related plants fulfil the same role. Think of Carex, Hakonechloa, Luzula, Ophiopogon.

Perennial plants complete the picture perfectly. For example Ajuga, columbines (Aquilegia), anemones, Brunnera, bleeding-heart (Dicentra), comfrey, Corydalis, Hostas, Euphorbia griffithii ‘Dixter’ (polychrome) or Euphorbia amygdaloides ‘Purpurea’, Heucheras or Tiarellas, Hellebores, pulmonarias (lungwort), Tricyrtis or toad lily.

Among bulbs, you can plant without hesitation: Camassia, Spanish bluebell (Hyacinthoides hispanica), narcissus, Fritillaria.

And finally some annuals to sow such as forget-me-not (Myosotis).

For example, imagine a Rhododendron luteum and an Azalea ‘Schneewittchen’ surrounded by a carpet of Spanish bluebells!

An example of a combination: Rhododendron luteum, Azalea ‘Schneewittchen’ and Hyacinthoides hispanica

Did you know?

The origin of these plants’ name has had several twists, sometimes called Azalea, sometimes Rhododendron, as botanists disputed. In antiquity, it was the oleander that was called Rhododendron (Tree of roses in Latin). It was the Italian Andreas Caesalpino who first attempted, in 1583, to include these ericaceous plants in that genus.

Father of modern botany, Carl Von Linne, included nine of these plants in his famous foundational work, Species Plantarum, in 1753. He separated five plants that he considered of different character, naming them Azalea. Azalea is in Ancient Greek the feminine of Azaleos, which means “dry”. This refers to their relatively greater resistance to dry periods compared with Rhododendron.

Azaleas were reintegrated into the genus Rhododendron by botanists, following demonstrations by English botanist Richard Salisbury in 1805 and 1808.

But let us not forget what French writer and horticulturist Alphonse Karr said: “Botany is the art of drying plants between sheets of blotting paper and insulting them in Greek and Latin.” Far from these botanical quarrels, gardeners and nurserymen continue to distinguish this distinct group by calling it Azalea, even if this means blaspheming the sacred botanical scriptures!