Rhododendron: planting & care tips for your garden

Rhododendrons are real eye-catchers in any garden. However, you need to know about their requirements so that they also bloom magnificently.

The rhododendron (Rhododendron) is a versatile plant that ranges geographically over a large area. Its flowers shine in many bright colours and delight many amateur gardeners. This deciduous, semi-evergreen or evergreen shrub from the heather family (Ericaceae) is a great addition to any garden. We reveal how you can also make the rhododendron native to your garden and what you should keep in mind.

Rhododendron species and varieties

The rhododendron genus is one of the largest shrub genera in the world, with over 1,000 species. That is why it is not surprising that there are countless species and varieties for the home and garden. We have put together a small selection of the most popular species and varieties here.

Rhododendron subgenus Azaleastrum

Shrubby varieties with tall growth (up to 5 m); flowering time between March and April; mainly white flowers; hardy down to -12 °C

Rhododendron subgenus Hymenanthes

Tree-like growth habit; rather unpopular in German gardens; partly hardy down to -15 °C; blooms between January and May; mostly pink to strong red flowers

Rhododendron subgenus Therorhodion

Dwarf shrub species (between 30 and 40 cm); some species hardy down to -30 °C; flowering time between May and June; sometimes bicolour flowers (pink-purple; rarer white, pink or red)

Popular varieties of rhododendrons:

• ‘Cunningham White’: Large-flowered hybrid; large white flower; relatively site-tolerant; frequent grafting on INKARHO rootstock; grows to over 2 m tall
• ‘Nova Zembla’: Large-flowered rhododendron; hardy and hardy; blooms in soft pink to bright red; flowers have dark spotted patterns
• ‘Bloombux’: Hybrid; evergreen variety; offered as a boxwood alternative; very easy to care for and rich in flowers; blooms in April/May

Buying rhododendrons

You can get things wrong when buying your rhododendron specimen. The choice of plant sometimes determines whether the cultivation of rhododendron in your own garden will be successful or not. We explain how to choose the best plant for your garden with a few simple tips.

Buying a rhododendron: watch out for pests and diseases

The rhododendron is predisposed to attack by fungal diseases. Small yellow lesions and wilted leaves, for example, indicate the common leaf spot disease but Phytophtara wilt is also a serious disease in rhododendron. In addition, some animal pests because trouble for the rhododendron, including the rhododendron cicada. However, the cicada itself acts more as a vector for fungal diseases by laying eggs in the buds. Therefore, when buying your rhododendron make sure the plant is free of lesions and small critters. However, if you are buying from a professional the likelihood of disease infestation is very low.

Buying rhododendrons: examine the roots

For more than twenty years, cultivators have been working to plant the rhododendron in neutral and slightly alkaline sites as well. By grafting a wide range of varieties onto the rootstock of lime-tolerant cultivars, we have succeeded in breeding so-called INKARHO rhododendrons. These develop a stronger root system and can thus be planted in almost any soil. When buying, look for the “INKARHO” label so that you can also enjoy lush flowers in your garden.
Tip: even if you have slightly acidic soil in your own garden, an INKARHO variety is a good choice. Due to their strong, extensive root system, faster and more vigorous growth is often observed, which is also reflected in its magnificent flowering.

Rhododendron: the right location

Rhododendrons originally come from higher altitude regions. They experience special conditions there, to which today’s garden varieties are still well adapted. We reveal the most important things about choosing the right place for your rhododendron in a nutshell.

The rhododendron tolerates direct sunlight well under certain conditions. If the soil is of appropriate quality and given a continuous supply of water, your favourite will thrive even in hot climates. However, during very prolonged periods of heat associated with drought, rhododendron prefers partial shade in the shelter of larger trees and shrubs.

The right soil

Rhododendrons are accustomed to growing on shallow and rather acidic sites. These properties result from its origins at higher altitudes. For the perfect location in the garden, this means loosening the soil well and mixing it with plenty of humus. The humus slightly acidifies the soil and ensures good permeability and an optimal grain size spectrum. Since shallow soils in mountainous areas are often very permeable, it is advisable to incorporate a drainage layer of gravel in the planting hole. Here you can learn more about the right soil for rhododendrons.

Planting rhododendrons

Rhododendrons are undoubtedly one of the most widely grown evergreen garden perennials. This actually suggests that its planting and care requirements are comparatively low. On the contrary, however, there are some things to consider when planting rhododendrons. We explain the key factors in a step-by-step guide:

1. Dig the planting hole

Rhododendrons are a sensitive shallow rooter that like well-drained and slightly acidic soils. Therefore, dig a wide planting hole (about three times the diameter of the root ball) and enrich the soil with peat or special rhododendron compost (and coffee grounds if necessary) to acidify the soil. Furthermore, in heavier soils, a drainage layer of gravel should be placed at the bottom of the planting hole, or at least half of the filler soil should be replaced with sand. This increases the grain size spectrum and prevents waterlogging.

2. Planting rhododendrons

Rhododendrons are usually sold in a pot with a well-formed root ball. Pull the plant, including the root ball, out of the pot and place it in the planting hole. Fill the planting hole with the sand-humus soil mixture until 2 cm of the root ball still protrudes from the soil and lightly press the loose substance. This will prevent the rhododendron from being planted too deep and possibly causing the root system to die.

Detailed instructions for planting a new rhododendron can be found here.

Rhododendron in a pot for balcony or terrace

Even if you do not have a large garden, it does not mean that you have to go without rhododendrons. Certain species such as Japanese azaleas and dwarf varieties like Rhododendron ‘Yakushima’ are also suitable as container plants and can thus decorate the balcony or terrace with their flowering splendour. When choosing a pot for the rhododendron, you should take into account the following: the shape of the container should be wider rather than tall to accommodate root development. Good water drainage should also be ensured. Several holes and a drainage layer are ideal so that the root is never wet and rot is prevented.

You should also think ahead with regard to overwintering. After all, even though the variety of rhododendron is considered hardy, the cold should never directly affect the roots. It also should not be directly exposed to wind and sun during the winter. However, if you have considered these points, nothing will stand in the way of your potted rhododendron.

Rhododendron care

The genus of rhododendrons is generally more at home on acidic sites. If these conditions are not present in your garden, it is advisable to help things along a little. We reveal how you can easily create the optimum conditions for your rhododendron. More about rhododendron care can be found in our special article.

Watering rhododendrons

Rhododendrons prefer very well-drained and shallow soil. This also makes them very sensitive to waterlogging. Young plants should still be watered regularly, preferably daily, but without over-saturating the soil.

Tip: Several small waterings a day is always a better solution if you are not sure how much to water.

Fertilising rhododendrons

Unlike many other heathers, the rhododendron is a very hungry plant. Therefore, you should not let your precious plant starve by fertilising it regularly. However, it is important to choose the right dose at the right time. Experts advise to fertilise young plants planted in the spring with a slow-release fertiliser. For this purpose, special rhododendron fertilisers or also general purpose, mainly organic fertilisers with a slow release of nutrients are suitable. Many rhododendron experts swear by bone meal as an additional source of nutrients. Between 20 and 30 grams of bone meal per square metre is quite sufficient.

Tip: Our Plantura Hydrangea Food is a plant-based fertiliser with a long-term effect and is therefore ideal for rhododendrons. This fertiliser releases its nutrients slowly and gently over a period of three months, providing your rhododendron with iron.

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Older, established plants are also fertilised at the same time as the young rhododendrons planted in the spring. This involves applying a slow-release fertiliser such as our Plantura Hydrangea Food in March or April and topdressing with bone meal as needed. However, the dose of bone meal in this case should not exceed 120 grams per square metre.

Tip: If you like coffee and are also a fan of rhododendrons, you have found the perfect combination. Coffee grounds have proven to be an excellent fertiliser for rhododendrons because they are completely organic, provide a broad spectrum of nutrients and also slightly acidify the soil. To spread the coffee grounds, simply work them in at the surface so that they decompose as quickly as possible.

We have compiled more information about fertilising rhododendrons for you in our expert article.

Pruning

Rhododendrons impress with their abundant flowering and spreading branches. This opulent feast for the eyes does not require regular pruning. However, pruning should be carried out at irregular intervals to maintain the good shape of your precious plant in the long term. We reveal how this works.

When and how to prune?

The right time for pruning is always based on the reason for pruning. If you would like to perform a maintenance pruning to keep your rhododendron in shape, it is best to do so after flowering, between May and June. However, if there is a need for more radical pruning because the rhododendron is increasingly bare inside, it should be done in the autumn to spring. When pruning rhododendrons, the procedure must distinguish between two cases. First, there is radical rejuvenation pruning, and second, you can keep your rhododendron in shape with maintenance pruning. We explain what you need to pay attention to.

Procedure for maintenance pruning

To keep your rhododendron in a nice shape, it is recommended to perform a more restrained maintenance or topiary pruning. A suitable time for this is immediately after flowering. In this way, you can easily estimate the later shape of the bush, remove any wilted flowers and do not damage the bush too much so that its photosynthetic performance is not limited and enough energy can be provided for the coming flowering.

To proceed: using sharp rose shears, simply cut the shoots a few inches below the tip of the shoot so that you are satisfied with the visual cut. Tip: pruning also promotes branching and provides denser foliage and flowers the following year.

Procedure for rejuvenation pruning

The cold winter months, when the rhododendron remains in hibernation, is the right time for a comprehensive rejuvenation pruning. To do this, use sharp rose shears to cut back old shoots, exposing inner parts of the shrub so that light can stream in.

For maintenance pruning after flowering, remove diseased and dry shoots but be sure to leave shoots with buds. For both types of pruning, always cut the shoots at an angle to allow dew and rainwater to drain away.

More detailed instructions for pruning rhododendrons can be found here.

Tip: we always recommend wearing gloves when working on rhododendrons as the shrub is poisonous. For more information on poisonous rhododendrons click here.

Common diseases and pests

The rhododendron is often affected by fungal diseases but insects also affect the popular garden shrubs. We give a brief overview of the main diseases and pests of rhododendrons.

Rhododendron cicada

• Small, light-green species
• Causes entry point for fungal infestation through egg laying; this results in brown flower buds, which are covered with fungal growth in the spring
• Control cicada through preventive measures: good location, proper care and fertilisation as needed
• Use of chemical pesticides is possible but only against vector (Cicada), not against fungus
• Control the fungus transmitted by cicada by removing the dried buds

Black vine weevil

• Big black beetle
• Feeds at night on leaves of rhododendron and other evergreens
• Leaf margin damage
• Both beetles and larvae are dangerous; larvae feed on roots
• Control by laying out old, rotten boards that serve as shelter for the beetle; beetles can then be picked off
• Nematodes are effective as beneficial insects; chemical crop protection virtually futile

Leaf spot disease

• Common; easy to identify
• No clear damage pattern as many weak parasites can be triggers
• Brown lesions typical in leaf centre and on leaf margin; partial curling of leaves
• Control exclusively by preventive measures (location and care) and chemical preparations

Powdery mildew

• Infestation with powdery mildew manifests itself in white mycelium on upper surface of leaves
• Problems with powdery mildew often appear from mid-July
• Knap Hill azaleas particularly affected
• Powdery mildew does not directly because leaf dieback but can weaken rhododendrons through prolonged infestation
• Control by preventive measures before infestation and chemical preparations after first signs of infestation; use of chemical agents more effective the earlier the infestation stage

For more information on common diseases and pests of rhododendron and their control, click here.