Azalea and Rhododendron

Azaleas belong to the Rhododendron genus. They can be distinguished from real rhododendrons by their smaller, dark green leaves and tubular flowers. The shrubs are also smaller and wide branching.

There are many types of rhododendrons. They range from low-growing dwarf azaleas to huge evergreen shrubs that can grow very high. Rhododendrons vary in size from 15 cm to 10 m and everything in between.

Most rhododendrons really require acidic soil (pH levels from 4.5 to 5.6). Otherwise they do not grow any flowers. Fortunately, you can take the necessary precautions when planting your rhododendrons to ensure your plant will be absolutely fine for the first 20 years.

Rhododendrons are acid-loving plants. This means they need acidic soil and they certainly do not respond well to lime. When planting your rhododendrons, you can use a soil improver that makes the soil acidic and airy (DCM Vivimus for heathers, rhododendrons and all acid-loving plants or peat, for example). Peat also provides sufficient organic matter and nutrients to ensure a good start. The first year after planting rhododendrons, you will need to water them during a hot summer because of their shallow roots. From the second year, rhododendrons will have enough roots to get through a dry summer. The shallow roots can also get damaged quite quickly during hoeing.

Later you can apply a layer of peat around the plants every year to maintain the acidity level required for these acid-loving plants. This also prevents the soil from drying out and controls weeds.

Adding lime and sprinkling with hard water are obviously not recommended. You can add special fertiliser for hydrangeas and rhododendrons for good flowering and growth.

Not all rhododendrons prefer shade.

Rhododendrons do not do well near trees with shallow roots, such as birch and pine.

Pruning

Pruning rhododendrons is unnecessary and not advised.

If a shrub has grown too large and is in need of some reshaping or hard pruning into the old wood, the best month to do this is June. You can improve the flowering of young plants by removing any spent flowers. Carefully snap the flower from the plant without damaging the new flower. Old branches can be cut back.

  • The best time for deadheading is shortly after flowering. Gently snap the spent flower trusses from the plant between your thumb and index finger without damaging the underlying new buds.
  • You can also cut back branches that grew the year before to the underlying whorl of leaves.
  • Another limited pruning option is to pinch out the branches with one terminal bud. This encourages increased branching at the plant’s growing points.
  • Rigorous pruning is required for plants that have grown (far) too big, but be careful. Bear in mind that a large plant also has large root ball (circumference). This root ball gets the necessary shade and humus from falling leaves. If you drastically prune the branches all at once, these 2 benefits disappear. Drastic cuts may cause poor, scarce or no branching during a dry spring or summer. In that case, mulching is extremely important in order to keep the soil moist. There is no need to add a lot of fertiliser, as the large root ball would not be able to expend it all on just a few new leaves anyway. You simply have to wait patiently and hope that the new buds branch off. If it doesn’t happen this year, there is little hope of regrowth. Sometimes there is regrowth not from the branches, but from the plant’s base near the ground. The plant needs the energy to create new leaves and branches.

Dwarf rhododendrons

These dwarf varieties grow no taller than 50 cm (max. 1 m). They are great for mass planting, as ground cover, in smaller gardens or garden pots and as a single specimen in rock gardens.

In nature they grow in alpine conditions above the tree line.

Rhododendron ponticum

This wild rhododendron is common in larger gardens. It often grows up to 3 m high (or even higher) and was often planted to offer shelter and privacy in the past.

Rhododendron ponticum is a medium-sized, evergreen rhododendron with a bushy, broadly spread, domed habit. The leaves are narrow, elliptical and dark green with a creamy white to pale yellow edge. This type of rhododendron mainly blooms in May with bell-shaped, clustered, lilac to mauve flowers. The plant prefers filtered sunlight or partial shade, a shaded root area and some protection from harsh winds. It can withstand the midday sun, provided that the roots are shaded. Rhododendron ponticum requires acidic, moist, well-drained soil that should never dry out completely. It needs  little pruning and is very hardy.

The leaves are toxic to cattle.

Rhododendron hybrids

These species grow taller, sometimes up to 2 or 3 m high. They are designed to form wide hedges or ground cover for large areas. These plants are not suitable for small gardens.

The hybrid varieties include thousands of crossed species. The dwarf varieties are usually small-leaved with the underside of their aromatic leaves covered in scales. Their flowers are also smaller. The larger species tend to be broad-leaved with large flowers. Some leaves have a felt-like underside to prevent loss of moisture in the cold or sun.

Rhododendron yakushimanum

This species grows only a few centimetres per year and is not often taller than 60-100 cm. Like dwarf rhododendrons, they are suitable for mass planting, as ground cover and as a single specimen in rock gardens. They are a good choice for smaller garden, as they grow so slowly.

Japanese azalea

The Japanese azalea is a hardy, evergreen azalea. It keeps its leaves in mild winters. Some species’ leaves get a beautiful bronze or mahogany colour.

Evergreen azaleas are wonderful spring flowering shrubs that bloom in almost every colour. Small, compact types look stunning in a container on a shaded patio.

They prefer moist, but well-drained, humus-rich, acidic soil in partial shade. It is very important to ensure a shallow planting depth.

Rhododendron mucronatum

Rhododendron mucronatum is common in Japanese and Chinese gardens and was introduced in this country around 1825. The flowers are slightly fragrant and very hardy. After 10 years, the bush reaches a height of about 150 cm. Rhododendron mucronatum is very hardy, flowers in April and May and thrives in partial shade or sunlight.

Azalea mollis –Knaphill azalea

This is a hardy, deciduous shrub, which develops striking, lightly scented flowers in May and June. The flowers of the azalea mollis have the richest hues of bright yellow and pure orange. The plant’s foliage changes colour in the autumn from a magnificent bronze red to yellow. After that, it sheds all its leaves.

Azaleas, particularly the deciduous ones, prefer slightly more sunshine than most rhododendrons.

The shrubs grow between 70 and 300 cm high.

Azalea indica or azalea shrub

Azalea shrubs are widely grown in the Ghent region of Belgium. The rhododendron simsii or azalea indica is a typical Belgian product that is internationally successful. The plants can be kept in the living room at a moderate temperature of 15 to 22°C. Slightly cooler places, such as an entrance hall or corridor, are also suitable. Azaleas need a bright spot in the house, particularly these last few varieties. Direct sunlight should be avoided, as this will make the flowers wilt too quickly and make the plant lose too much water.

Problems

  • What if the rhododendron’s buds do not open or turn brown? These are signs of a fungal disease that is transmitted by the rhododendron leafhopper. The legs and underside of this beautiful insect are yellow and its wings are green with two red longitudinal stripes. The leafhoppers tend to appear under rhododendron leaves any time after April. They lay their eggs in the newly formed flower buds from September. This allows the fungus to penetrate and affect the plant throughout the winter.
    The fungal disease itself cannot be controlled, but the creatures can be. From June, the leaves – particularly the underside – can be treated with Edialux Okapi Garden, for example.
  • Yellowing leaves are usually caused by planting the rhododendrons too deep, by lime in the soil or by soil that is poorly drained. Transplant to suitable or improved soil and possibly add some rhododendron fertiliser.