Rhododendron diseaseRhododendrons delight us in spring with a multitude of bright blooms. The varied species and cultivated forms of the mostly evergreen shrubs have long since conquered the hearts of many amateur gardeners. We showcase beautiful rhododendrons, give tips on planting and care, and tell you what to do about pests and diseases.
Azaleas like this Knap Hill hybrid used to form their own genus, but were botanically assigned to the genus Rhododendron several decades ago.
– About the history of the rhododendron – The most beautiful rhododendron varieties – The seven rhododendron groups – The perfect location for rhododendron in the garden – The optimal soil for rhododendron – Planting rhododendron properly – Fertilizing rhododendron – Pests and diseases of rhododendron
On the history of the rhododendron
The natural distribution center of the genus Rhododendron, also called alpine rose in German, is located in East Asia. Most species grow in West-. Central China with the Himalayas. They are found both in the subtropical jungle and up to heights of up to 5.000 meters. They produce a wide variety of growth forms, ranging from dwarf shrubs only a few centimeters in size to trees nearly 30 meters high. From there, from the middle of the 19th century, plant hunters brought the best rhododendrons. and beginning of the 20. In the early twentieth century, countless species were introduced into the botanical institutes. Since then, intensive breeding and crossing of the various rhododendron wild species has resulted in an almost unmanageable abundance of garden varieties.
Good to knowThere are also two Rhododendron species that are at home in the German Alps: the Rusty-leaved Alpine Rose (Rhododendron ferrugineum) and the Creeping Alpine Rose (Rhododendron hirsutum).
The most beautiful rhododendron varieties
Due to its origin, the rhododendron initially had difficulty adapting to the local climate. In addition to winter hardiness, which is the main goal of German nurseries, the main aim is to achieve a compact, stocky growth. After all, the ornamental shrub should fit today's garden plots, while the old varieties were mostly still created for an existence in spacious parks. The love of strikingly marked flowers is common to both old and new cultivars. But beautiful foliage is also in demand: after all, the alpine roses should also look good outside the flowering season from late April to mid-June.
The double, pink rhododendron variety 'Homebush' is a real eye-catcher thanks to its unusual flower shape.
The rhododendron hybrid 'Helen Martin' grows very compact and blooms from the beginning of May.
Rhododendron luteum 'Persil' is one of the very few deciduous species that shed their foliage in autumn.
The play of colors of the rhododendron hybrid 'Brasilia' is spectacular: it changes from orange to pink to a subtle cream-orange during the blooming period.
The combination of yellow and pink rhododendrons achieves great effect even from a distance.
The spherical growing Rhododendron 'Catawbiense Grandiflorum' is covered with countless large flowers in purple from the end of May to mid June.
The rhododendron cultivar 'Professor Wolters' was created from a cross between the species Rhododendron simsii and Rhododendron mucronatum. It is not yet very well known in Germany.
Unusual but real: Rhododendron 'Saphirblau' really blooms light blue with a violet touch. The variety grows very compact. Is similar to the Japanese azaleas.
The new rhododendron cultivar 'Pushy Purple' is colorful and uncomplicated – it needs no special soil and thrives in almost any location.
The new cultivar Rhododendron Inkarho, which is particularly robust and easy to care for, is a convincing choice as a scented hedge.
The special flower shape led to the name of this species: Rhododendron jasminflorum. Hardly known in this country.
To see the Rhododendron lochiae x brookeanum, you have to go to special rhododendron parks (here: Botanika Bremen) or to an English flower show.
The breeders of the rhododendron 'Bloombux' wanted to find a substitute for the box tree – they really succeeded with this feast for the eyes.
The flowers of Rhododendron spinuliferum are unusually shaped: Thimble-like, they open only a few centimeters.
The seven rhododendron groups
Rhododendrons on the market today are classified into seven major groups according to their parentage and outward appearance:
Large-flowered rhododendron hybrids: Overman-high shrubs. They grow the fastest of all rhododendron species. Huge flowers in all colors. Requires a lot of space, but there are now also newer, compact varieties.
Yakushimanum hybrids: Small-growing to about 130 centimeters in height. Broadly spherical, compact growth. Blooming already at a young age. Ideal for most gardens as relatively sun tolerant.
Williamsianum hybrids: Compact shrubs up to 150 centimeters with graceful, downward sloping calyxes. Also known as "bell rhododendrons". Early flowering from the end of April.
Repens hybrids: Small-growing, very undemanding. Flower only in red, form attractive brown-red buds. Bloom as early as April if the weather is good.
Wild species and their hybrids: Very heterogeneous group of very different rhododendrons, which have been relatively little changed in terms of cultivation. Among them there are many dwarf garden and varieties, but also stately specimens with leaves up to 25 centimeters long.
Deciduous Azaleas: Broadly upright shrubs up to 1.20 meters tall. Relatively small leaves that fall off in winter. Especially colorful. The so-called Knap Hill hybrids are robust.
Japanese azaleas: They are descended from Japanese wild species. Rather flat, broad-rounded growth up to 1.50 meters high. Very rich flowering. Especially for boxes. Suitable for containers as well as for the foreground of borders. Especially suitable for boxes and tubs as well as for the foreground of borders. A particularly richly flowering dwarf form, which descends from the Japanese azaleas, are the so-called diamond azaleas.
A visit to a rhododendron park is definitely worthwhile from May to June. These can be found in many German regions.
Famous birthplaces of many rhododendron species were or still are the English nurseries Walter C. Slocock, Anthony Waterer (Knap Hill Nursery), Exbury Gardens and Peter A. Cox. Here in Germany T. J. Seidel near Dresden and Dietrich G. Hobbie, G. D. Bohlje and J. Bruns in the Oldenburg area, Hans Hachmann in Barmstedt near Hamburg and Georg Arends in Wuppertal.
Often the growers are also in connection with a rhododendron park, which attracts in May and June with the most beautiful impressions. Meanwhile there are several parks in each federal state – a visit is worthwhile in any case.
The perfect location for rhododendron in the garden
In our gardens, the evergreen or deciduous shrubs need a wind-protected, semi-shaded place, for example, in front of a hedge or against a house wall. If the soil and air are sufficiently moist, the rhododendron can also tolerate sunlight. In wind-exposed locations, it dries out too much, especially in winter.
The flowering shrubs are best in the light shade under high, old house trees, which in turn increase the air humidity. The rhododendron has a very shallow root system with a high proportion of fine roots – thus only woody plants with deep roots, such as pine, oak, acacia, ginkgo, bald cypress, laburnum, tree hazel or amber tree are suitable as natural shade trees. With its root system, the rhododendron has adapted perfectly to its natural habitat. It grows mainly in forests, where it takes root in the thick, airy humus layer formed over centuries from the autumn leaves of trees and other plant debris.
Good to knowRhododendrons are not compatible with birch, Norway maple and horse chestnut, as these trees have shallow roots and can dry out the topsoil, especially in spring and early summer.
Rhododendrons were once created primarily for parks. Today, more likely to affect private individuals and their gardens, i.e., smaller.
The optimal soil for rhododendron
Rhododendron belongs to the bog plants, but this name is actually misleading, because rhododendrons do not originate from the bog, but are mostly at home in the mountain forests. This is also indicated by the German name Alpenrose. The term bog bedding plant was coined mainly by the gardening trade, as the large cultivation companies in Germany are mainly located in regions with extensive, drained bog areas. Rhododendron grows optimally on the moist and airy, lime-free peat soils.
Plant rhododendron correctly
To ensure that the rhododendron also grows well in your garden, you should make it as native to it as possible. So when planting, place it in acidic and lime-free soil, preferably rich in humus. If the soil conditions are unfavorable, you should prepare the soil. Dig a round hole about half a meter deep and 1.5 meters wide for each plant. To avoid waterlogging, apply a ten centimeter thick drainage layer of coarse sand. Then fill the rest with a 50/50 mixture of sand and bark or leaf compost. Well-rotted cow manure is also a proven soil amendment for rhododendron. On loamy soils with higher pH, the so-called INKARHO rhododendrons have proved successful. These are special refinements, which have a higher tolerance to lime than the classic rhododendron.
Be careful not to place the rhododendron too deep in the planting hole. Otherwise it would die, because the sensitive root system would die off. If the hole is too deep, simply fill it back in a bit. To be on the safe side, the root ball should protrude one to two centimeters from the soil. After placing the root ball in the planting hole, tread it down carefully, water it, sprinkle the root area generously with horn shavings and cover everything with a layer of bark mulch about five centimeters thick.
When planting rhododendrons, ensure proper soil preparation, good depth and suitable distance from other plants in the bed. Without regular fertilizing the rhododendron will not grow very large. Also does not set as many flower buds. Horn shavings are recommended as fertilizer. Good slow release fertilizer. The best time to fertilize is in March or April. For smaller plants (up to 60 centimeters high), the optimal dosage is about 50 grams of slow-release fertilizer per square meter and 30 grams of horn shavings. For larger plants at about 90 grams of fertilizer and 60 grams of horn shavings. Older plants or plants from a size of 1.2 meters tolerate a fertilizer portion of 120 grams and 70 grams of horn shavings.
Re-fertilization is possible until July. But do not overdo it. Give a maximum of 30 grams per square meter. When choosing fertilizer, keep in mind that rhododendrons do not tolerate lime – check the label. If you are undecided, buy special fertilizer for rhododendron.
Coffee grounds have proven to be a good organic rhododendron fertilizer. In small amounts, it provides all the nutrients necessary for the plant, also acidifies the soil and enriches the soil with humus. Ideal therefore! Since the amount of nutrients is really manageable, you can spread coffee grounds in addition to other fertilizers.
Larger rhododendrons are perfect for demarcating the property with a hedge.
Pests and diseases of rhododendron
The rhododendron is usually a fairly robust plant that causes the amateur gardener little grief. If pests and diseases do occur, this is usually due to the wrong choice of location, poor soil conditions or nutrient deficiencies. If the rhododendron is too exposed to the sun and lacks adequate soil moisture, the whitefly or rhododendron skin bug can spread. Waterlogging and soil compaction favor infestation with fungal diseases.
Since the shrubs require a lot of nutrients for the formation of their large leaf mass and abundance of flowers, insufficient fertilization can lead to deficiency symptoms such as yellow leaves or lazy flowering. Fertilizing should not be done too late – after the summer month of July – otherwise the shoots will not be able to mature before the onset of winter and will suffer frost damage.
Skin or net bugs
The damage pattern for skin or net bugs includes yellowish mottled leaves and premature leaf drop. The cause of infestation is often too dry air. Lack of soil moisture. Control: From May to July, treat repeatedly with approved insecticides.
Infestation of net bugs.
The rhododendron cicada is a ten millimeter insect. Their larvae suck on the leaves in summer. Cause white speckles (similar to the red spider mite). More annoying is the fact that the insects lay their eggs in the new flower buds that form from June onwards. It transmits a fungus that causes the buds to die, so that they do not open in the following year – hobby gardeners often think that this phenomenon is due to frost damage. Control: only in case of heavy infestation, you should treat the plants with approved insecticides during the summer months. Another tedious but effective method is to break out and dispose of all dead buds during the winter months – this can significantly reduce infestations, as the new generation of pests cannot hatch in the spring.
The black vine weevil is a black beetle about one centimeter in size. From June onwards it is on the rampage. Eats the edges of the leaves in the shape of a cove. It does not affect the growth of the plants. Its white, brown-headed larvae are different: they gnaw at the roots in the soil, can severely weaken the plants, and in extreme cases can even kill them. Often the larvae are introduced in the potting soil. For control: The exclusively nocturnal weevil can only be caught with difficulty. It is better to get to the root of the problem: From mid-September to the end of November and from mid-April to mid-May – depending on the weather – you can fight the larvae with parasites (nematodes). They are mixed into lukewarm, stagnant water and are best applied to the root zone early in the morning or in the evening with a watering can.
The weevil eats the leaves of rhododendrons.
If the rhododendron suffers from iron deficiency, the leaves turn yellow. In the early stages, the leaf veins still remain green, but as the disease progresses, the plants wither and die. This can have several causes: Lime-containing watering, soil compaction, waterlogging or a generally too high pH value – the so-called lime chlorosis. In this way, the plants can obtain the iron necessary for the formation of green leaves. Do not absorb magnesium from the soil.
To counteract iron deficiency, foliar fertilization with appropriate iron fertilizers helps in the short term. In the long term, however, you can only solve the problem by transplanting the rhododendron and placing it in a new location with properly prepared soil. If in doubt, take a soil sample in the root zone before transplanting. Test them with a pH-test from the specialized trade. The pH value should not exceed 5.5. Regular mulching with spruce or pine needle litter can keep the soil pH in the acidic range over the long term.
twig and leaf dieback
Twig and leaf dieback manifests itself in leaves or twigs of the rhododendron that continue to lose their luster, turn brown and die off. Fungal pathogens of the genus Phytophtora are the cause. The only way to combat the disease is to rigorously remove the infected shoots. In this case, you should also critically examine the site and soil conditions, because it is mainly already weakened rhododendrons that are attacked. This is where it makes sense to prune the rhododendron.
Sooty dew fungi
Sooty dew fungi cause leaves and twig sections to look lackluster, turn brown, and dry up. The fungus usually occurs under trees infested by aphids. Their sugary excretions – the so-called honeydew – drip onto the leaves. Is colonized by a black fungal lawn within a short time. It is only a cosmetic problem, because the fungi do not attack the leaf tie. Nevertheless, you should rinse the soiled leaves with warm water and remove the honeydew with a soft brush, because the fungal coating reduces the photosynthetic capacity of the leaves.
In rhododendrons suffering from nitrogen deficiency, the leaves turn pale green to yellowish and the plants hardly grow at all. In the short term, the problem is best remedied with a pure nitrogen mineral fertilizer such as ammonium nitrate. Dissolve a handful of fertilizer pellets with lukewarm water in a watering can and apply the solution in the root area. At the same time, remove the bark mulch layer and fertilize the root area with plenty of horn shavings before reapplying the mulch layer.