The name, Rhododendron, derives from the Greek rhodon = rose and dendron = tree, or rose tree. Its origin is very ancient: fragments of this plant have been
recovered in China and the Caucasus; however the first written records date back to the sixteenth century.
In Europe it came only in the nineteenth century; English botanists began to produce hybrid species. A significant contribution was provided by
George Forrest, envoy of the Royal Horticultural Society, who during his long stay in Yunnan, discovered varieties not yet known. Because of the fragility of its flowers, al rhododendron The value of fragile enchantment is attributed; some believe it is the emblem of the first declaration of love.

The rhododendron

Rhododendron belongs to the Ericaceae family. It is generally a shrub or tree with persistent or deciduous leaves. It should be noted that initially some specimens were referred to as rhododendrons and others like azaleas. Later it was realized that between the two species there were no substantial differences and that they could be classified together. However, they are still referred to differently in the nursery. They are usually called rhododendrons evergreen specimens whose flowers have 10 stamens. Generally (but not a rule) they are medium-large sized specimens suitable mainly for cultivation in open ground.
The azaleas, on the other hand, are mostly deciduous and small in size. In this article we will talk at length about plants generally qualified as Rhododendrons and their cultivation in gardens. We will deepen the azaleas (and in general the cultivation of this type of plant in a container) in another passage.

Description and origins

The Rhododendron genus includes more than 750 species and is very diverse. Many are low, creeping shrubs, others are real trees. The areas of origin are many: from Europe to the Asian mountains to the boreal areas of the American continent. There are also spontaneous ones in Italy. In all the North the r. ferrugineum is popularly called rose of the Alps and is spread precisely from the Alps to the Pyrenees above 1400 meters. In the North-East it is instead common the R. hirsutum.


In general, rhododendrons are not difficult to grow if they are grown in their environment, that is in the woods or in the mountain prairies and consequently in the gardens at those altitudes. On the contrary, it is a rather delicate plant to grow in the lowlands because often there are very different soil and climate conditions. To be beautiful, their habitat must be faithfully reproduced. A big advantage though is the fact that, even if there are many species, they have almost all the same needs.


It is essential to prepare an acid and calcium-free substrate. The ideal pH fluctuates between 4 and 5.2. So let us cultivate them if our garden already has these characteristics. If instead we find ourselves in the presence of a clayey, calcareous and heavy soil we can bury large pots, perhaps in cement, filling them with a product suitable for acidophilic plants and periodically spreading on the surface of decomposing leaves or even better than pine needles. It may be useful in the month of October to spread on the surface a good dose of very mature bovine or horse fertilizer. It will enrich the soil and protect the superficial roots from winter frost (especially in the absence of snow. This, at height, helps to overcome the winter thanks to its insulating power). On the bottom of the hole it will also be absolutely necessary to create a good draining layer consisting of gravel and stones. This will avoid water stagnation, a frequent cause of root system rot.

Irrigation and humidity

Rhododendrons need a constantly moist soil, but absolutely no water. This is why it is essential to prepare an excellent drainage during planting.
It should also be noted that their roots may not always have easy access to water, even if present in the soil. What benefits them most is environmental humidity. They manage to live very well in sunny exposures if there is a high humidity in the air with a cooling effect on the aerial part. It is therefore appropriate to place the specimens in the coolest and most humid area of ​​our garden. It is certainly preferable to use demineralised or rainwater and any limestone-free water for both irrigation and possible vaporisations. This, in the course of time, would change the characteristics of the soil we had prepared for our bush.


As we said for these plants it is very important to live in a fresh and humid habitat. This significantly affects the exposure. In particular we can say that with increasing altitude the possibility of exposing the plant to sunlight increases. If we live in the plains or on the hills, it will be good to choose a shady place or at most half-shade. Usually the ideal location is under deciduous or coniferous trees. On the other hand, if we live above 800 meters we can place these plants in full sun because summers will never be too hot and there will almost always be rain and winds full of humidity giving them refreshment.


Most of the rhododendrons are very rustic and usually have no great problems with cold. They could, however, suffer damage if temperatures drop below -10 ° C in the absence of snow. This is because they are able to survive thanks to the insulation provided by the snow cover.
In these cases it is good to mulch the plant well with leaves and straw and the superficial roots should always be covered with a good layer of flour manure.


In addition to the soil improver, to be distributed every year in autumn at the foot of the specimens (very useful to protect them from the cold, but also to increase the amount of bacteria necessary for nutrient absorption) it is possible to administer a moderate amount of fertilizer for acidophilic plants in February , possibly a slow release granular.


Usually they do not need any intervention except the removal of the exhausted flowers to avoid them going to seed.


Sowing is possible, but it is not recommended because it is necessary to wait many years before seeing the flowering. The absolute most used method is the cutting: it is necessary to choose semi-woody branches. You can proceed in summer or autumn by taking a portion of at least 10 cm. The basal leaves must be eliminated and the cutting area must be immersed in a product that stimulates rooting. It is inserted into a mixture of forest sand and perlite which should always be kept moist. Everything must be kept in the shade and protected from currents until rooting has taken place (usually it takes about 2 months). It can then be transferred to more suitable soil. The multiplication can also be done by grafting, usually split or in a shield. It is also a rather fast method.

Parasites and rhododendron diseases

- If the leaves and flowers tend to dry or turn brown it is likely that there is a lack of environmental humidity and the plant is suffering from excessive heat. It is necessary to shade it more (perhaps with sheets) and vaporize it more often.
- The flowers wither on the plant without reaching maturity: excess in irrigation and probable beginning of root rot. Stop the administration of water. The cause could also be a late frost that caused the buds to abort.
- If the leaves are gnawed or engraved with circular motifs, the presence of oziorrinco is likely. A specific insecticide must be administered.
- Red spider mite: it is manifested by the yellowing of the leaves and, at an advanced stage, with the appearance of very fine cobwebs. You can notice its presence by looking at the lower page of the leaves: they are very small red or brown spots. To counteract it, the specimen must be shaded and the environmental humidity increased by vaporizing the leaves. If the infestation is severe it is advisable to spray the plant with a specific acaricide.


Rhododendron catawbiense originally from the USA. It is important because it has been used at crossings to give more resistance and rusticity to hybrids.
Rhododendro arboreum it can become a tree of considerable size, it blooms from March to May. It is native to the Himalayas. The flowers are large, bell-shaped and gathered in groups with colors ranging from white to bright red.
Rhododendron augustinii large shrub native to China, evergreen. The species has beautiful blue flowers in groups of 4.
Rhododendro ponticum comes from the Iberian peninsula. It blooms in mid-spring and is a rather large shrub. It is widely used for the creation of rootstocks since it grows very quickly from seed and is able to give great vigor to the graft.
Rhododendro lutescens coming from China, it reaches 2.5 meters. It has intense yellow flowers and has been used to convey this characteristic to modern cultivars.
Rhododendro auriculatum can reach 10 meters. Highly appreciated for its resistance and summer flowering. The inflorescences range from white to pink.
Rhododendron causicasicum a native of the Caucasus is a small shrub. Much used in the creation of modern hybrids.
Rhododendro griersonianum large shrub that blooms in summer. At the base of many modern hybrids with a late flowering.


The enthusiasm of the British nurserymen has led to the creation of many hybrids with the intention of creating plants with more beautiful flowers and above all greater vigor and resistance to non-ideal environments. In general, we can refer to the hybrids that derive from the Asian varieties and those that derive from the American, European and Himalayan varieties. However, they are all evergreen, very rustic, with abundant and very colorful flowers. There are also different habitats: from the very compact (even only 30 cm high) to cultivars that can become really impressive.

Rhododendron hirsutum

A rhododendron that deserves special attention even if it is not an ornamental plant is undoubtedly the rhododendron hirsutum. This variety of rhododendron is a spontaneous plant that grows on the Alps and from its name, hirsutum, one can easily guess one of its peculiar characteristics, namely the hair that covers the branches and leaves. This rhododendron variety, also known as the hairy rhododendron, is a small evergreen shrub that never exceeds 50-60 cm in height. It is an endemic species of the Alps that blooms between June and July with a slight delay compared to another rhododendron endemic to the Alps, the ferrugineum.
The rhododendron hirsutum is a plant that is typically found in calcareous soils and is a pioneer species, or a plant that colonizes landslides, cliffs and stones. It can be found at an altitude between 1000 and 1900 meters above sea level in sunny areas.

Rhododendron: Rhododendron ferrugineum

Just mentioned in the previous paragraph, the Rhododendron ferrugineum is the other rhododendron of the mountain. Autochthonous plant that grows on the slopes of the alpine mountainous areas, it gives an early flowering compared to that of the hirsutum. Unlike this rhododendron, ferrugineum prefers acid soils and owes its name, ferrugineum, due to the iron-red color of the underside of its leaves.
The flowering of this rhododendron is something very spectacular in the mountains because the different plants all bloom in the same period giving a very pleasant chromatic effect.
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Evergreen shrub or tree
Height From 1 to 30 meters

Difficult, especially if not in an ideal environment
Ground Acid (pH 4/5), absolutely free of calcium. The woodland is ideal
Irrigation Abundant, but without stagnation. Non-calcareous water.
Environmental humidity high
Exposure Half shade, shadow. Sun alone in the mountains
Playback Talea, layering, seed
colors White, pink, red, yellow, lilac, blue-lilac
Flowering All year, but every species in a specific period