Difference Between Rhododendrons and Azaleas

Kingdom, phylum, class, order, family, genus, species.

This may sound like something you remember from your 7th grade Science class that you never thought you would hear again, but we are going to put it to use right now.

This is known as the taxonomy of a biological organism, also known as its classification rank.

Many plants or animals share the same taxonomy properties until reaching the family or genus classification. For example, dogs and cats are two very different types of animals, but they share all of the same classifications until reaching the family rank.

The family classification of domestic dogs is canine, where the family classification of domestic cats is feline. Dogs and cats are then each classified by genus and then finally species.

The same taxonomy classification applies to plants.

The genus of a plant contains a variety of different species, which brings us to rhododendrons vs azaleas.

The Rhododendron Genus

Azaleas and rhododendrons have the same kingdom, phylum, class, order, and family. Both are in the Plantae kingdom and share the same classifications all the way down to the Ericaceae family. The Ericaceae family of plants are flowering plants that contain over 4,000 species, making it one of the largest families of plants in the world.

These are among the most beautiful of all flowering shrubs and trees and it is unfortunate that their requirements are so specific that they can be grown successfully in only a limited number of regions.

Botanically, azaleas are included in the genus rhododendron, but, gardeners continue to refer to them as azaleas.

Rhododendrons have foliage which is usually handsome—often larger than that of azaleas, sometimes leathery in texture, dark green on the upper surface and pale on the underside, and carried in whorls. Many of the evergreen species are decorative throughout the year.

The large round heads of flowers make a glorious show from late winter to late spring. Rhododendrons grow magnificently in many parts of England but they are not native to that country. Most plants in England and other parts of Europe are derived from the first plants imported at the beginning of the nineteenth century.

Some of these were from America and Spain but most of them were from the East—the Himalayas, China and Tibet. It is recorded that there are now something like 500 species in cultivation and thousands of named cultivars. Each year sees the introduction of more of them, and keen growers are advised, therefore, to consult nurserymen who will know the names of those likely to thrive in their area, or to study the Rhododendron Year Book.

Rhododendrons Size

Some grow to tree size, spreading across 3 m (10 ft) whilst others are less than 1 m (3 ft) in height and spread. The form and colours of the flowers and leaves vary, too.

Culture of Rhododendrons

Plant rhododendrons in acid soil rich in leaf mould and in filtered shade, except in regions with mist, where they can be grown in the open. If hot dry winds prevail, shelter plants and water them abundantly to keep the air and soil moist. They enjoy moderate to severe frost.

The plants have a fibrous root system and it is advisable to mulch the soil during dry periods of the year to retain moisture and to prevent the ground from heating up. Old oak leaves, decomposed pine needles and peat are excellent for this purpose as they are acid in nature.

In regions where the water is likely to change the nature of the soil from acid to alkaline, apply a monthly sprinkling of aluminium sulphate, sulphur or iron sulphate. A tablespoonful spread on the soil around the plant and watered in will usually correct a tendency to alkalinity.

The smaller rhododendrons will flower well also when grown in large containers filled with peat and acid compost. Before planting, soak the peat thoroughly as otherwise it absorbs moisture from the plant. They need cool, moist conditions for their best development.

Some Rhododendron Varieties

The following are the names of a few of the many cultivars available. These are likely to do well in the right conditions in Australia, South Africa, New Zealand and parts of Southern California.

The flowering season varies from late winter to the end of spring. Early means late winter and early spring; mid-season is mid-spring, and late is the end of spring.

‘Alice’

2 m (6 ft). Lovely rose-pink flowers which turn pale as they age. Mid-season.

‘Anah Kruschke’

1,5 m (5 ft). Lavender flowers on sturdy plants. Mid-season.

‘Betty Wormald’

2-3 m (6-10 ft). Carmine flowers streaked with purple. Mid-season.

‘Blue Diamond’

1 m (3 ft) with lavender-blue flowers. Stands sunshine. Early to mid-season.

‘Blue Peter’

1,5 m (5 ft). Sturdy plant with lavender-blue flowers blotched with purple. Mid-season.

‘Bow Bells’

Rounded plant with height and spread of 1 m (3 ft). Pink, bell-shaped flowers. Mid-season.

‘Britannia’

A compact plant to 1 m (3 ft) with loose clusters of gloxinia-like crimson flowers. Late.

‘C.B. van Nes’

Robust plant 2 m (6 ft) tall with good foliage and lovely crimson flowers. Early.

‘Christmas Cheer’

A dwarf plant with a mass of frilled flowers of blush-pink. Early.

‘Cor nubia’

Grows to 2-3 m (6-10 ft) and has crimson flowers in large, handsome clusters. Early.

‘Corry Koster’

Reaches 2 m (6 ft) and has frilled pale pink flowers flushed with rose to crimson in the throat. Mid-season.

‘Countess of Athlone’

Has silvery-lilac flowers on plants 2 m (6 ft) tall. Early to mid-season.

‘Countess of Haddington’

A neat, compact plant with tubular, white to pink scented flowers. Early.

‘Countess of Sefton’

Grows to 1 m (3 ft) or more and has white, scented flowers flushed with pink. Mid-season.

‘Cynthia’

Bears large trusses of rose-coloured flowers on plants 2-3 m (6-10 ft) tall. Mid-season.

‘Decorum’

A strong-growing plant to 2 m (6 ft). It has lightly scented white to pale pink flowers. Mid-season.

‘Dido’

A robust plant which grows slowly to 3 m (10 ft). The flowers are creamy-pink suffused with orange. Late.

‘Elizabeth’

A splendid small one growing to 1 m (3 ft). It bears clusters of bright red waxy, trumpet-shaped flowers. Mid-season.

‘Fastuosum Flore Pleno’

A reliable old favourite with lavender-blue flowers on plants 2 m (6 ft) tall. Mid-season.

‘Fragrantissimum’

Grows to 2 m (6 ft) or more but will flower well if trimmed. Produces white and pinkish-white fragrant flowers. Late.

‘Gill’s Crimson’

A robust plant to 2 m (6 ft) or more with lovely trusses of waxy crimson flowers. Early to mid-season.

‘Jan Dekens’

A handsome plant with rich pink frilled flowers in large trusses. Mid-season. Height 2 m (6 ft).

‘Lady Roseberry’

Grows to 2 m (6 ft) and has charming clusters of tubular, rose-coloured flowers. Late.

‘Loder’s White’

Grows to 2 m (6 ft) and has large trusses of white flowers with frilled edges. Late.

‘Pink Pearl’

An old favourite of robust growth to 3 m (10 ft). It has large round clusters of shell-pink flowers. Late.

‘Purple Splendour’

A handsome small plant only 1 m (3 ft) high and wide. It has flowers of a deep violet. Late.

‘Sappho’

A tall plant with white flowers marked with dark purple in the throat. Mid-season.

‘Unique’

A compact plant to 1 m (3 ft) with apricot buds opening to creamy-yellow flowers. Mid-season.

‘Unknown Warrior’

Grows to 2 m (6 ft) and bears fine clusters of rose-pink to coral flowers. Early.

‘Van Nes Sensation’

A robust plant to almost 2 m (6 ft) with large trusses of pale lilac flowers. Mid-season.

Rhododendron is the genus that is assigned to azalea species. Azalea is just one out of the more than a thousand different species that are found under the rhododendron genus. This means that all azaleas are rhododendrons, but not all rhododendrons are azaleas.

If you were to go to a nursery and asked a specialist for a rhododendron, they would likely respond by asking which species you want. You may be looking for hybrids or particular varieties, like the vireya rhododendrons for instance. Rhododendrons contain over 1,000 different species of flowering bushes, shrubs, and small trees, many of which are the azalea.

Azaleas Are Now Known Botanically as Rhododendrons

But since gardeners are likely to continue to call them by the more familiar name of azalea, let’s call them azaleas!

Types of Azalea

A large number of different species of azalea were introduced into Europe from the East, and numerous very lovely cultivars have been developed from these. For the sake of simplicity it is convenient to divide azaleas into two main groups:—Evergreen types which include the Indicas, Kaempferi, Kurumes and Macrantha; and the deciduous ones which include Knap Hill-Exbury, Mollis and Occidentale hybrids.

All azaleas are beautiful in the garden, in containers on a shady terrace or patio, and in arrangements. They flower from late winter to mid-spring.

Evergreen Azaleas

Azalea Indica

Most of the evergreen azaleas popular in gardens in different countries of the world belong to this group. They include many single and double-flowered cultivars. The plants vary considerably in size, from shrubs 1 m (3 ft) tall and wide, to those which are 2 m (6 ft) or more in height and spread.

The leaves are usually soft and pointed, and the flowers are funnel-shaped. Where soil and climatic conditions suit them these are amongst the loveliest of all flowering shrubs, producing an abundance of flowers for a long period. There are now hundreds of named cultivars. The colours include white, pastel shades and brilliant tones of colour.

This class of azalea will stand 10 degrees of frost but the flowers are apt to be spoiled where frosts are more severe.

Kaempferi Hybrids

These beautiful azaleas are hardy in freezing conditions. Where the cold is severe they may lose most of their leaves. The funnel-shaped flowers are shades of salmon to red, mauve and pink, and the plants are often large. They tend to flower later than other species.

Kurume Azaleas

These Japanese azaleas are hardier than the Indicas. They are also smaller in growth with smaller flowers, but what they lack in size they make up for in the great profusion of flowers they carry. Very often the leaves are completely hidden by the abundance of flowers.

Some of the flowers bear one flower inside the other. These are referred to as ‘hose in hose’ varieties. The small-growing varieties are excel-lent for small gardens and rockeries. They are also fine plants for growing in containers in the house or on a shady terrace or patio.

There are many named cultivars available in all countries of the world. The colours include shades of pink, mauve, cyclamen, vermilion and crimson.

Macrantha Azaleas

These are pretty plants for small gardens and rockeries. They are generally compact and low-growing. The flowers may be small or large. These are also sometimes referred to as Gumpo azaleas.

Deciduous Azaleas

These are most rewarding plants with flowers of glowing shades of yellow, apricot, orange, flame, red, pink and white.

The three main groups are Knap Hill-Exbury, Mollis and Occidentale hybrids.

Knap Hill-Exbury Hybrids

Include spreading and upright plants to 2 m (6 ft) in height. The flowers are large and carried in clusters. They may have a scent and some have ruffled margins to the petals. These are also referred to as ‘ ‘Rothschild” azaleas because many of the hybrids were created on the Rothschild estate in England.

There are many named cultivars of these. The colours include charming shades of yellow, salmon, orange and red.

Mollis Hybrids

Are upright in growth to 2 m (6 ft) with large flowers in clusters of seven or more. These azaleas are spectacular plants when in flower. The colours vary from yellow through orange to tangerine-red. The brightest of them is known as ‘Roster’s Brilliant’. Some of them have leaves which turn pleasing shades in autumn.

Occidentale Hybrids

These often have scented flowers of the same size as the Mollis hybrids, but the plants are generally taller—often to 2,5 m (8 ft) in height and they tend to open later in spring than the Mollis azaleas. The colours are white and shades of pink and rose. The foliage of many of them assumes pretty tints in autumn.

Culture of Azaleas

Azaleas are not adaptable as to soil and situation as is the case with many other shrubs. They will not grow unless they have the kind of soil they like. They require a loose acid soil with a pH rating of 4,5 to 5,5. There must be no lime in the soil and it should not dry out quickly.

If acid compost is not available, work peat into the soil in which they are to be planted. In areas where the soil is alkaline plant them in containers above the ground or sunk into the ground, filling the containers with acid soil and compost, or with this mixed with peat.

Yellowing Azalea Leaves

Should the leaves of azaleas show signs of yellowing (chlorosis) it is an indication that soil conditions are not acid enough, and the plants may succumb quickly if action is not taken to change the nature of the soil. This can be done by sprinkling a tablespoonful of aluminium sulphate, sulphate of iron, or sulphur over the soil and watering it in, and by applying iron chelates to the plants or the soil, according to the directions given on the package. These commodities can be obtained from shops dealing in garden supplies.

Where to Plant Azaleas

Plant azaleas in dappled shade. They will grow in the open in gardens near the coast, but inland the intensity of the sunlight is too much for them. They make a charming picture if planted under tall trees which shade them but, at the same time, allow light to filter through onto them.

The roots of azaleas are fibrous and remain near the surface of the soil. To prevent them from drying out put a mulch of compost or of old oak leaves or decomposed pine needles over them. If these are not available, use a mulch of straw or peat.

They should never be given ordinary fertilizer. Fertilizers specially prepared for plants which require acid soil may be applied from time to time. Most beautiful azaleas are grown without any artificial fertilizer or manure. Water the plants liberally and regularly, particularly in areas where hot dry winds prevail.

Most azaleas are resistant to sharp frost and do better in a cool climate than a subtropical region. The deciduous azaleas and the Kaempferi and Kurume azaleas are particularly hardy.

The Indicas may suffer some frost damage in a very cold garden. These also generally tolerate more warmth and will do better therefore in gardens where winters are mild than the others will.

Azaleas seldom suffer from insect pests. In some areas leaf-eating grubs, thrips, leaf miner or red spider may cause damage in spring. If such insects appear to be damaging the plants apply a systemic insecticide according to directions given on the package.

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