Azaleas belong to the genus Rhododendron from a botanical point of view. The term Japanese azaleas (Rhododendron japonicum) refers to a group of plants with very similar characteristics and properties. These are various hardy and semi-evergreen, low-growing shrubs that are visually very similar to the indoor azaleas (Rhododendron simsii). Most of the original species of the Japanese azaleas originate, as the name suggests, from Japan. The most important parent species of today’s varieties are the wild species Rhododendron obtusum, its variety amoenum and Rhododendron kaempferi. Already more than 400 years ago, the first hybrids were created in Japan. The further development and breeding history of Japanese azaleas, in which new hybrids and garden forms were created from many different wild species and other forms, is, to say the least, confusing. An exact differentiation and classification is difficult even for experts.
Japanese azaleas have a particularly flat, densely fibrous root system, which spreads mainly in the upper humus layer. They form mostly roundish to expansive, bushy crowns with dense branching. The young shoots are very thin and have a reddish brown bark. The bark of the older branches is mostly light grey to reddish brown and peels off in thin, fibrous plates. The growth heights vary greatly depending on the variety and lie between 50 and 130 centimeters. Older plants are usually wider than tall.
The leaves of the Japanese azaleas are small and oval-elongated. Many varieties form a brown-red autumn and winter colour. In severe winters they usually lose all their foliage, but with some varieties, depending on temperatures, it remains until spring and is only then replaced by the new leaf shoot. However, most varieties already shed their coloured leaves in autumn and then carry only smaller, green winter leaves, which are only replaced in spring. This makes the Japanese azaleas a pretty plant for the garden even in winter. Botanists call such plants semi-evergreen or wintergreen.
An essential characteristic of Japanese azaleas is their abundant flowering. Partly the whole shrub is covered by a sea of flowers covering every leaf. The colours range from white, salmon, pink and pink to a deep red or violet. It blooms in April and May.
In the garden, a sunny to semi-shady place is best for Japanese azaleas, a certain amount of sunlight is required for the shrubs to form their typical, very dense flower pile. Make sure that the plants are somewhat protected and are not exposed to a sharp easterly wind in winter. In the ideal case, they stand wind-protected all year round.
Japanese azaleas prefer uniformly moist soils as bog bed plants. The substrate should be well permeable and loose and very rich in humus. The ideal pH value lies in the acid to weakly acid range between 4.5 and 5.5. Japanese azaleas, like all rhododendrons, react very sensitively to lime. They then show the typical lime chlorosis: a yellowish discoloration of the leaves, because the high lime content in the soil hinders iron uptake.
The ideal planting season for Japanese azaleas is early spring. Plants in containers can also be planted into the summer if you supply them well with water afterwards. Autumn is not an ideal time for planting because badly rooted plants can be badly damaged by frost. Keep a planting distance of 50 to 80 centimeters, depending on the final size of the varieties. Unfavourable soil must be well loosened, drained and enriched with plenty of humus before planting. Ideal is a base of rotten cow dung and leaf compost. In the case of loamy and clayey soils, replacement is necessary – otherwise the plants take care because they cannot take root in cohesive soil.
Japanese azaleas should be fertilized only weakly, if at all. Use commercially available rhododendron fertiliser for this purpose. It is better to cover the soil with a mixture of horn shavings and compost every spring. Attention: Never use garden compost – it contains too much lime. Do not mulch the plants until the soil has completely thawed out again after the winter months. Higher temperatures and stronger sunlight can otherwise cause severe frost damage, as the roots do not compensate for the higher evaporation. Thicker mulch layers as winter protection should be removed in time before the new sprouting. During their exceptionally rich flowering period, the plants need a lot of water, otherwise the pile withers quickly.
Especially in the Japanese Garden, azaleas often receive a spherical shape cut after flowering. However, regular pruning is not necessary for flowering. Shrubs with a gaping or one-sided crown can also be rebuilt with a more pronounced contour cut into the old wood. But beware: Like all rhododendrons, Japanese azaleas do not tolerate being placed on a stick immediately after transplanting. In most cases the root pressure is not sufficient for a new shoot from the old wood.
Although Japanese azaleas are visually an absolute asset to any garden, they are relatively seldom seen in gardens. This could possibly be due to the somewhat more elaborate winter protection that plants need in winter cold regions. In addition, the winter hardiness of the varieties can vary greatly – so find out when you buy them. What they all have in common, however, is that with increasing age they become more and more resistant to cold temperatures. Young plants, on the other hand, should always be provided with winter protection. They must not only be protected from the cold from the ground (for example with a leaf blanket). A fleece or sackcloth for the crown is also advisable. This also keeps off the winter sun, to which Japanese azaleas react very sensitively.
Japanese azaleas are a year-round ornament for every garden. Due to their compact growth, they are also suitable for small gardens and front gardens. They can be easily combined with other rhododendrons and forest shrubs with similar site requirements. The Aronense varieties and the so-called diamond azaleas show a strikingly beautiful flower decoration. Both remain very small, so you can put them even in beds of small size. Although Japanese azaleas are generally regarded as pure garden plants, there are some species that have been grown in pots, tubs or stone troughs in Japan for decades. An attempt, especially with the mentioned small species, is definitely worth it.
Japanese azaleas can be assigned to different hybrid groups according to their origin. Kurume hybrids, named after the Japanese city of Kurume, have a very compact growth and do not reach more than 80 centimeters in height. During the flowering season, the shrubs are covered over and over with countless tiny flowers whose colours range from a light pink to a dark red or even purple. Highly recommended for small gardens. Kaempferi or Malvatica hybrids, on the other hand, have relatively large orange or red flowers. Newer varieties of these Japanese azaleas also play into the violet. The breeding goals for the Vuykiana hybrids were very clear: to create new flower colours and shapes. The first results, however, were mixed and not very convincing, until further crosses like ‘Vuyk’s Scarlet’ or ‘Vuyk’s Rosyred’ produced varieties with very beautiful flowers – but in terms of winter hardiness rather less shiny. Antifreeze is essential for them, no matter at what age. Typical characteristics of this group are flowers with a diameter of five to seven centimeters and a stocky growth.
Arendsii hybrids originated from a cross with the very robust ‘Noordtiana’. This is the only variety of Mucronatum hybrid that has proven itself in the garden and is very hardy in winter. The flowers of the Arendsii hybrids are usually light purple, but can also be pink or fiery red in colour, and thanks to the ‘Noordtiana’ they cope well with the cold.Japanese azaleas are also partly named after the breeder or the place where they were bred. The varieties are then called for example Rhododendron kaempferi ‘Blue Danube’. The current Arendsii-, Arosense-, Kiusianum-Azaleas as well as the Diamant-Azaleas come from German breeders. This is a protected brand name for the particularly compact and flower-rich varieties of the East Frisian azalea breeder Carl Fleischmann, the azalea varieties ‘Adonis’, a Kurume hybrid in white with a frayed edge, the pink ‘Betty’ and ‘Favorite’, a Kaempferi hybrid with a crimped edge and early flowering. Modern breeds usually consist of so many different plants that they can no longer be assigned to a particular hybrid group.
Japanese azaleas can be propagated via cuttings. The chances of success that these root are highest from mid-July to early September. A tip: Cut the cuttings in dry weather. They should not be wet or damp at this time. Three to eight centimeter long shoots have proved their worth in terms of length. However, the success rate varies greatly depending on the variety and origin. A special cultivation station with floor heating is strongly recommended. In addition, use an acidic, lime-free growing substrate, ideally a mixture of two parts pure white peat and one part quartz sand.
Diseases and pests
Japanese azaleas are very resistant to diseases and pests. If the pH value is too high or the soil is too solid, lime chlorosis may occur, as already mentioned, because the plants cannot extract enough iron from the soil. Among the most common diseases are leaf spot infections and the so-called earlobe disease. More serious problems can be caused by fungi of the genus Phytophthora to the azalea friend. They are responsible for root and stem rot. One of the most frequent animal pests is the weevil, and spider mites can also occur in very air-dry locations.
Whether fruit, vegetable and ornamental plants in the garden or indoor plants in the house: spider mites can infest and damage many different plants. Here René Wadas, a herbalist, will tell you his tips on how you can effectively fight the arachnids.Credits: Production: Folkert Siemens; Camera: Fabian Primsch; Editing: Dennis Fuhro, Photos: Flora Press/FLPA, GWI
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I am Don Burke, one of the authors at My Garden Guide. I am a horticulturist that cultivates, grows, and cares for plants, ranging from shrubs and fruits to flowers. I do it in my own garden and in my nursery. I show you how to take care of your garden and how to perform garden landscaping in an easy way, step by step.I am originally from Sydney and I wrote in local magazines. Later on, I have decided, more than two decades ago, to create my own blog. My area of specialization is related to orchid care, succulent care, and the study of the substrate and the soil. Therefore, you will see many articles dedicated to these disciplines. I also provide advice about how to improve the landscape design of your garden.