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Rhododendron

Folkert Siemens Folkert Siemens

Rhododendron is one of the most beautiful flowering shrubs for the garden, but also one of the most demanding. You can read about the correct way to plant and care for it here.

Origin

Rhododendron is also known as Alpine Rose and is without a doubt one of the most important flowering shrubs, as well as one of the most demanding. The genus belongs to the Heather (Ericaceae) family and consists of 1,000 species from a sub-arctic, 5.91 inch tall dwarf shrub to a 66 feet tall tree. Most of the species which are interesting for the garden originate from East Asia, where they grow in species-rich deciduous or mixed forests on acidic, evenly moist raw humus soil. The numerous garden types and hybrids from Asia are also often given the name alpine rose. The previously independent genus of azaleas is today also considered among the Rhododendrons due to the large similarities. Incidentally, the botanical name Rhododendron originates from the Greek and translated it means ‘Rose Tree’.

In a nutshell: Planting and caring for Rhododendron

Rhododendrons and azaleas are demanding flowering shrubs. In addition to varieties that remain small, there are also those that grow into 787.4 inches high trees. In the ideal location and with the right care, rhododendrons thrive magnificently and bloom - usually in April and May - lushly and in a wide variety of colors. The rhododendron loves lime-free, loose soil that is rich in humus and evenly moist. Regular fertilizing in spring and the removal of wilted flowers are among other important care measures.

Appearance and Growth

A Rhododendron generally grows very slowly and usually has alternating leaves which are arranged radially around the branches. The foliage is predominantly evergreen - rarely deciduous - and can look very different from species to species. Most have ovate to lanceolate, fully margined leaves and the margins are often a little curved downwards. When budding, the foliage in some species is covered with a white, yellow or rusty-red felt. This protects against sunburn and disappears in the course of the summer. The deciduous azaleas have a partly yellow-orange foliage color in the fall.

The spectrum of flower colors for rhododendrons is even wider than for Roses, as there are even blue flowering species such as Rhododendron impeditum. The flower buds are located on the tips and already form in the spring then open on most species and varieties in April and May. One of the earliest Rhododendrons is the pre-spring Rhododendron, which already flowers in March before growing leaves. Some large flowering hybrids such as ‘Autumn fire’ or ‘Autumn joy’ give a taste of next year’s flower show at the end of the season.

Flowering Rhododendron

Rhododendrons are some of the most beautiful flowering trees. They can grow to be extremely old and develop into stately specimens in a suitable location

Location and Soil

Rhododendrons are extremely demanding where soil is concerned. Non-calciferous, extremely loose and humus rich soil is essential; it should be kept evenly moist. Most species grow in cool, damp light shade under trees which are not too dominant. An ideal shade provider for the is the Scots pine (Pinus sylvestris). Yakushimanum hybrids and some wild species of Rhododendron also tolerate sunnier locations, provided the soil is sufficiently moist.

Planting

It is important to prepare the soil well before planting a Rhododendron, as the soil in most regions is not ideal for the shrubs. Sandy substrates must be generously improved with foliage compost and bark humus. Working in mature cow dung has also proven advantageous. Never plant the shrubs too deep and cover the soil with a mulch layer of foliage or composted bark. A drainage layer of non-calciferous gravel is often also recommended for loamy soils which tend to become compacted. In such locations, preference should also be given to Inkarho Rhododendrons. These are varieties of large-flowered hybrids and Yakushimanum hybrids which are grafted to the loamy and lime-tolerant Inkarho base.

Azaleas

Japanese azalea transform into compact, vibrant balls of color during flowering

Care Tips

Only water the plants with rainwater, even in the garden - unless your tap or ground water is extremely low in lime. With all large-flowered Rhododendron, you should break off withered inflorescenses, in order to promote the formation of new flower buds. Regularly fertilizing Rhododendrons early in the spring is also important for the flower to form dark green foliage and put out lots of flower buds. For this, spread horn shavings or organic Rhododendron fertilizer in the root area. If you have mulched the plants, then carefully remove this from the soil before fertilizing and replace the mulch layer again when you have finished.

Pruning

Even though it is not generally necessary: Well rooted Rhododendrons tolerate shaping and also heavy cutting back to the old wood. However, the fine, sensitive root system hardly penetrates into the surrounding earth in heavy soils. These plants often never grow shoots again if they are heavily pruned. So you should check before any pruning whether your Rhododendron is actually well rooted. On the other hand, pruning is advisable for older Rhododendrons.

Overwintering and Winter Protection

Evergreen Rhododendron species are able to role up their leaves during a frost, to minimize the evaporation of valuable water from the leaves. Nonetheless, plants can be damaged by dry east winds and winter sun. You should therefore protect Rhododendrons in a heavily exposed position in the winter with a net or fleece as a precautionary measure.

Utilization

The Rhododendron divides the gardening world. For some, it stirs a collector’s passion, for others it seems too artificial and exotic. However, using these plants requires finesse: The sensitive bushes do not tolerate lots of root competition and should therefore only be combined with tolerant shrubs such as Hosta or groundcover plants such as Foam flowers (Tiarella). Suitable tree partners include Kousa dogwood (Cornus kousa), Witch hazel (Hamamelis) or various species of Viburnum. You could, for example, place the flowering shrubs in a forest garden individually or in groups under slightly shady pines, however, lots of lower growing species are also happy in semi-shady . Japanese azalea (Rhododendron obtusum) are often planted in groups in Japanese style gardens.

Most Rhododendron only have limited suitability as container plants, as they need extremely evenly moist soil. The evergreen foliage means incorrect watering is only noticeable late on, but deforms the bush for a long time when it is pruned. Indoor azaleas are a little tricky to care for: They must be regularly and extensively watered with lime-free rain water.

Important Species and Varieties

Rhododendron variety ‘Polaris’

‘Polaris’ is one of the Yakushimanum hybrids. The Rhododendron species group grows compact and tolerates the sun

Propagation

Small growing species and varieties are largely propagated through cuttings, however, this is quite difficult without a greenhouse and the appropriate technical equipment. Most large flowering and Yakushimanum hybrids today are grafted onto unrooted cuttings of the ‘Cunningham’s White’ species or on a lime-tolerant Inkarho basis. For hobby gardeners, propagation through offsets (the offsets of the shoots near the soil) is the easiest way to breed new rhododendrons.

Diseases and Pests

Phytophtora is a wilting disease which causes individual branch parts or the entire plant to die. It often occurs on impermeable or waterlogged soils. Further, generally harmless, fungal infections are leaf spot diseases and - in deciduous species and varieties - powdery mildew. The most common pest is the. It lays its eggs in the flower buds of the Rhododendron and at the same time transfers a fungus which triggers bud death. If the leaf margins have been eaten, this is usually down to vine weevils. Gray marbled leaf surfaces are generally a sure sign of a case of Rhododendron lace bugs.

If your Rhododendron has yellow leaves with green leaf veins it is suffering from lime chlorosis. Too much lime in the soil disrupts the iron supply as the bushes can only absorb this nutrient in sufficient amounts at low pH values. The phenomenon is generally accompanied by growth disruptions. It helps to replant the Rhododendron in a spot with more suitable soil.

If your rhododendron shows yellow leaves with green leaf veins, it is suffering from a condition called lime chlorosis. Too much lime in the soil interferes with the iron supply, as the shrubs can only absorb this nutrient in sufficient quantities when the pH is low. Usually, the phenomenon is accompanied by growth disorders. You can remedy this by transplanting the rhododendron to a place with more suitable soil.

Frequently Asked Questions

When can I prune Rhododendrons?

You can prune Rhododendrons in February and March and from July to November. 

When is the best time to fertilize Rhododendrons?

After the initial fertilizing directly when planting, the Rhododendron is fertilized once a year in March or April . It is possible to re-fertilize until the end of June.

When should Rhododendrons be planted?

are best planted between the end of April and the start of May. This is also the best time for re-planting.

What kind of soil is suitable for a Rhododendron?

The Rhododendron needs extremely loose, humus-rich and lime-free soil, which should be as evenly moist as possible. The pH value should ideally be between 4.5 and 5.

How often should Rhododendron be watered?

As a fibrous root plant, the Rhododendron tends to prefer even soil moisture. You should water it regularly in the first year after planting and in persistent dry spells. It is best to water in the morning.

What can I do if the Rhododendron leaves turn yellow?

One of the most frequent causes of yellow leaves on Rhododendrons is lime chlorosis, which can occur if the soil is too alkaline. It is advisable in this case to replant in lime-free, loose, humus-rich soil. Alternatively, the pH value of the soil can be adjusted with a fertilizing agent.

How much water does a Rhododendron need?

The Rhododendron is a fibrous root plant and must be regularly watered in persistent dry weather in particular, in order to keep the soil evenly moist. Only water the plant with lime-free rain water.

How big does a Rhododendron grow?

The size of Rhododendrons varies depending on the species and variety. Large flowering hybrids can grow 10 to 13 feet tall, while the height of Dwarf Rhododendrons is generally between 12 and 39 inches.

How fast does a Rhododendron grow?

Rhododendrons generally grow extremely slowly. The annual growth is often only 3.94 inches. However, there are also fast growing varieties.

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