Common Name: Tree Rhododendron

General Information

Rhododendron arboreum is a much-branched evergreen shrub or a tree with a wide crown; it usually grows from 5 - 20 metres tall, though can range from 1 - 30 metres. The bole is generally short[, ].
The plant is often harvested from the wild for local use as a food, medicine and source of materials - the flowers and bark are ingredients of commercial cosmetic preparations. The flowers are sold in local markets and used as offerings in temples[]. The plant can be used as a pioneer species for restoring native woodland and is often grown as an ornamental in gardens.
Rhododendron arboreum has a very wide distribution, large population, is not currently experiencing any major threats and no significant future threats have been identified. The plant is classified as 'Least Concern' in the IUCN Red List of Threatened Species(2018)[].

Known Hazards

All parts of the plant (including the leaves, flowers and pollen) contain greater or lesser amounts of the toxic compound andromedotoxin (also known as grayanotoxin) [, , , ]. Rarely lethal to humans (and used medicinally in some herbal disciplines), this compound causes dose-dependant overstimulation of the central nervous system with symptoms including various cardiovascular effects (mainly low blood pressure and cardiac rhythm disorders); nausea and vomiting; and a change in consciousness. The effects commence shortly after ingestion and last around two days. These effects are also transferred to honey made from the nectar of the flowers. In some parts of the world bees are used to deliberately produce a honey rich in andromedotoxin which is then eaten for its supposed medicinal, hallucinogenic and aphrodisiac effects.
In contrast to humans, many other creatures are more susceptible to the toxin and it has sometimes proved lethal to grazing animals and household pets. Some forms of honeybees are also killed by the toxin (resistant forms of the bee are used for honey production). Bumblebees are not affected, however, and are also more efficient in pollinating rhododendron flowers, so one theory is that the toxin is produced by the plant in order to favour the bumblebee and improve fertilization rates[].

Botanical References

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Range

E. Asia - Himalayan regions of China, India, Nepal, Bhutan and Myanmar; also further south in mountainous regions of India, Sri Lanka and Thailand

Habitat

Forests and shrubberies with Pieris ovalifolia and Quercus incana; at elevations from 1,500 - 3,600 metres[].

Properties

Conservation Status	Least Concern
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Evergreen Shrub
Height	12.00 m
Pollinators	Insects
Cultivation Status	Ornamental, Wild

Cultivation Details

Rhododendron arboreum is moderately cold-hardy, tolerating temperatures down to between -10 and -15°c when fully dormant. The pink flowered form is hardier than the red[]. Although cold-tolerant when dormant, the flowers and young growth of Rhododendrons are very susceptible to damage by late frosts in regions where these are likely to occur after new growth has commenced in the spring[].
Succeeds in most humus rich lime free soils except those of a dry arid nature or heavy or clayey[]. Prefers a peaty or well-drained sandy loam[]. Succeeds in sun or shade, the warmer the climate the more shade a plant requires[]. Requires a pH between 4.5 and 5.5[].
Succeeds in a woodland though, because of its surface-rooting habit[], it does not compete well with surface-rooting trees[]. Plants need to be kept well weeded, they dislike other plants growing over or into their root system, in particular they grow badly with ground cover plants, herbaceous plants and heathers[].
There are many named varieties selected for their ornamental value[].
Plants form a root ball and are very tolerant of being transplanted, even when quite large, so long as the root ball is kept intact[].
A very ornamental plant, it is the national flower of Nepal[, , ]. The flowers are sold in local markets and used as offerings in the temples[, ].
Plants in this genus are notably susceptible to honey fungus[].

Edible Uses

Flowers - raw or cooked[, ]. A sweet-sour taste, they are said to make a good sub-acid jelly[]. Th flowers are sometimes simply pickled by adding salt and chili[].
Some caution is advised, large quantities of the flowers can cause intoxication[, ].

A wine made from the flowers, known as 'Guranse', is commonly drunk in parts of the Himalayas - it is said to be very pleasant and also to be very efficacious in the prevention of altitude sickness[].

The tender young leaves are used as a cooked vegetable[, , ]. Caution is advised, see the notes above on toxicity.

Medicinal

Some caution is advised in the medicinal usage of this plant - see the notes above on toxicity.

The young leaves are astringent and poultice[]. They are made into a paste and then applied to the forehead in the treatment of headaches and high fevers[, , ].

The juice of the bark is used in the treatment of coughs, diarrhoea and dysentery[].

The flowers are used in the treatment of diarrhoea, dysentery and dyspepsia[]. A decoction of the fresh flowers is used to check a tendency to vomit, especially if there is also a loss of appetite[]. The juice of the flowers is used in the treatment of menstrual disorders[]. The petals are eaten to assist the removal of any animal bones that have become stuck in the throat[].

Agroforestry Uses:

This species often self-sows prolifically on disturbed soils and in grassland within its native range. It makes an excellent nurse tree for deodar plantations[]. This implies that it is a good pioneer species for use in restoring native woodland[].

Other Uses

The juice of the leaves is spread over cots and beds to get rid of bed lice[].

An extract of the flowers is used as an ingredient in commercial cosmetic preparations as a skin conditioner[].
An extract of the bark is used as an ingredient in commercial cosmetic preparations as a skin conditioner[].

The soft, reddish-white to reddish-brown wood is close and even-grained, though it is apt to warp and shrink. It is used in turnery, it can also be used to make charcoal and for fuel[, ]. The wood is much employed in Nepal where it is used for making household implements, building small houses and fences. Planks of the wood are carved to make boxes, cupboards and other furniture[, ].
It is a very good fuel according to some reports, burning well with a long-lasting heat, and is also used to make charcoal[, , ]. Other reports ay that it is a poor fuel, apt to smoulder instead of burning with a flame and makes a poor charcoal[].

Overcollection of the wod for fuel, and also for making charcoal, has become a cause for conservation concern in Nepal[].

Propagation

Seed - best sown in a greenhouse as soon as it is ripe in the autumn and given artificial light. Alternatively sow the seed in a lightly shaded part of the warm greenhouse in late winter or in a cold greenhouse in mid spring. Surface-sow the seed and do not allow the compost to become dry[]. Pot up the seedlings when they are large enough to handle and grow on in a greenhouse for at least the first winter.
Layering in late July. Takes 15 - 24 months[].
Cuttings of half-ripe wood, August in a frame. Difficult[].