The rhododendron is one of the most beautiful, but also most demanding flowering shrubs for the garden. Here you can read how to plant and care for it properly.
- Top articles
- Appearance and growth
- Location and ground
- Care tips
- Overwintering and winter protection
- Important types and varieties
- Diseases and pests
- Frequently asked questions
- More articles
Planting rhododendron correctly
Propagating rhododendrons through cuttings
How to fertilize your rhododendron
The rhododendron is also known as the alpine rose and is undoubtedly one of the most important, but also most demanding flowering shrubs. The genus belongs to the heather family (Ericaceae) and consists of over 1.000 species: from the subarctic 15 centimeter high dwarf shrub to the 20 meter high tree. Most of the species of interest for the garden originate from East Asia. There they grow in species-rich deciduous or mixed forests on acidic, uniformly moist raw humus soils. Two species are native to the German Alpine region: the rusty-leaved alpine rose (Rhododendron ferrugineum) and the ciliated alpine rose (Rhododendron hirsutum). It is also found in other southern and eastern European mountains in sparse coniferous forests and shrubberies on humus-rich soils.
The numerous garden forms and hybrids from Asia are also often given the German name ‘Alpenrose’. The formerly independent genus of azaleas is now also counted among the rhododendrons because of the great similarities. The botanical name Rhododendron comes from the Greek and literally means "rose tree".
Rhododendrons and azaleas are demanding flowering shrubs. In addition to small remaining varieties, there are also those that grow to 20 meter high trees. In the ideal location and with the right care, rhododendrons thrive magnificently and bloom – usually in April and May – profusely and in a wide variety of colors. The rhododendron loves lime-free, loose soil, which is rich in humus and evenly moist. Regular fertilizing in spring as well as the removal of withered blossoms are, among other things, important care measures.
Appearance and growth
A rhododendron usually grows quite slowly and usually bears alternate leaves arranged radially around the branches. The foliage of the plant is mainly evergreen – rarely deciduous – and varies greatly depending on the species. Most bear ovate to oblong, entire-edged leaves with leaf margins that often curve slightly downward. In budding, the foliage of some species is covered with a white, yellow or rust-colored felt. It protects against sunburn and disappears during the summer. A deciduous azalea sometimes shows a yellow-orange foliage coloration in autumn.
The color spectrum of the blossoms is even larger with the Rhododendron than with roses, because there are also blue-blossoming kinds as for example Rhododendron impeditum. The terminal flower buds are formed in the previous year and open in most species and varieties in April and May. One of the earliest rhododendrons is the early spring rhododendron (Rhododendron ‘Praecox’), which presents its flowers in March, before the leaves are out. Some large-flowered hybrids such as ‘Herbstfeuer’ or ‘Herbstfreude’ already show a foretaste of next year’s blooms at the end of the season.
Rhododendrons are among the most beautiful flowering shrubs. They can become quite old and grow into stately specimens in the right location
Location and soil
As for the soil, rhododendrons are quite demanding. Important is lime-free, very loose and humus-rich soil, which should be as evenly moist as possible. Most species prefer a cool, moist location and grow best in light shade under trees that are not too dominant. The native Scots pine (Pinus sylvestris) is an ideal shade provider for the evergreen plants. Yakushimanum hybrids and some wild species of rhododendron can tolerate sunnier locations if the soil is sufficiently moist.
Japanese azaleas turn into compact, bright balls of color when in bloom
It is important to prepare the soil well before planting rhododendrons, as the soil in most regions is not ideal for the shrubs. Our tips: Improve sandy substrates with plenty of leaf compost and bark humus. The incorporation of rotted cow manure has also proven to be effective. With just a few ingredients you can also mix your own rhododendron soil without peat. Be sure not to plant shrubs too deep and cover the ground with a mulch layer of leaves or composted bark. In loamy soils that tend to compact, a drainage layer of lime-free gravel is often also recommended. In addition, you should prefer so-called Inkarho rhododendrons in these locations. These are varieties of the large-flowered hybrids and yakushimanum hybrids grafted onto the more clay- and lime-tolerant Inkarho rootstock.
Tips for care
There are a few important points to keep in mind when caring for these demanding flowering shrubs: A common mistake in rhododendron care is watering with tap water. Water plants exclusively with rainwater, even in the garden – unless your tap or ground water is extremely low in lime. For any large-flowered specimens, remove the rhododendron’s faded flowers to encourage the formation of new flower buds. Regular fertilizing of the rhododendron in early spring is also an important part of care, so that the plant can form beautiful dark green foliage and set many flower buds. For this purpose, spread horn shavings or organic fertilizer for rhododendrons in the root zone. Once you have mulched the plants, carefully clear the soil before applying fertilizer. Then reapply the mulch layer.
You cherish it, but your rhododendron just won’t bloom? One-sided fertilization as well as the wrong place in the garden can be the cause, among other things.
Pruning rhododendrons properly
Even if it is usually not necessary: Well-rooted rhododendrons tolerate pruning and also stronger pruning down to the old wood. On heavy soils, however, the fine, sensitive root system hardly penetrates the surrounding soil, even after years of growth. These plants often do not sprout again if they are cut back severely. Therefore, before any pruning, check whether your rhododendron is really well-rooted. On the other hand, pruning is advisable for older rhododendrons. Since rhododendrons are poisonous, it is advisable to wear gloves when carrying out maintenance work.
Actually, you do not have to cut a rhododendron. However, if the shrub gets a little out of shape, small maintenance cuts can do no harm. MEIN SCHoNER GARTEN editor Dieke van Dieken shows you in this video how to do it right.
Credit: MSG/Camera+editing: Marc Wilhelm/Sound: Annika Gnadig
Wintering and winter protection
Evergreen rhododendron species are able to curl their leaves in frost to minimize evaporation of valuable water through the foliage. Nonetheless, plants can be damaged by dry easterly winds and winter sun. Therefore, shade very exposed rhododendrons in winter with a net or fleece as a precaution.
Use in the garden
The rhododendron divides the gardening world. Some people are passionate about collecting rhododendrons, while others find them too artificial and alien. However, if you have ever been to the rhododendron bloom in the Ammerland region of Lower Saxony (Westerstede district), you will hardly be able to escape its fascination. Last but not least, there are beautiful rhododendron gardens to visit in Germany, where you can be enchanted by the pretty plants.
Nevertheless, tact is required when using them: The sensitive shrubs do not tolerate strong root competition and should therefore only be combined with compatible plants such as perennials, woody plants, ferns and ground covers. You can choose from a wide range of suitable companion plants for the rhododendron garden: suitable perennials are, for example, Hosta and Bergenia. Ferns include the peacock wheel fern (Adiantum pedatum) and an ideal groundcover would include the foam flower (Tiarella). Suitable woody plants are, for example, flowering dogwood (Cornus kousa), witch hazel (Hamamelis) or various types of snowball (Viburnum). The list of plants that go with rhododendrons is long and really has something for everyone.
If the green foliage of the plants after the magnificent bloom is not spectacular enough, you may find what you are looking for among newer cultivars: Among them are rhododendrons with great foliage decoration, which enrich the design of flower beds.
Flowering shrubs can be planted singly or in groups under light-shaded pines in woodland gardens, for example, but many lower species also feel at home in cool, semi-shaded rock gardens. Japanese azaleas (Rhododendron obtusum) are popular for planting in groups in Japanese-style gardens. Vigorous varieties of the large-flowered hybrids such as ‘Catawbiense Grandiflorum’ or ‘Cunningham’s White’ are occasionally planted in northern Germany as wide, semicircular hedges.
As potted plants, most rhododendrons are only suitable to a limited extent, as they require very even soil moisture. Due to the evergreen foliage, watering errors become noticeable only late, but disfigure the shrubs for a long time if they are not cut back. But with cultivars such as the dwarf rhododendron ‘Bloombux’ (Rhododendron micranthum ‘Bloombux’), balconies and terraces can also be decorated. Indoor azaleas are also somewhat tricky to care for: They need to be watered very regularly and well-dosed with lime-free rainwater.
Important species and varieties
The choice of rhododendron varieties is diverse. The largest and most important groups of varieties for the garden are the evergreen large-flowered hybrids, which are often several meters tall, and the somewhat stockier growing Yakushimanum hybrids. The most popular azaleas are the deciduous Knap Hill hybrids, which are about two meters tall, and the wintergreen Japanese azaleas (Rhododendron obtusum). In addition, the third large group are the non-hardy indoor azaleas (Rhododendron Simsii hybrids). In addition, there are other rhododendron cultivars, such as the Williamsianum hybrids with large, bell-shaped flowers and the mostly red-flowering compact Repens hybrids.