Common Name: Rhododendron

General Information

Rhododendron ponticum is an evergreen shrub or small tree that can grow from 2 - 8 metres tall. A compact shrub when growing in open areas, it adopts a larger spread in the shade of a woodland[].
The plant is harvested from the wild for local use as a medicine and source of materials. It is often grown as an ornamental, where it can be used as an effective hedge.
Rhododendron ponticum has often been cultivated outside its native range as an ornamental and, in many areas of Europe, it has escaped from cultivation and become naturalized. It is particularly problematic in Britain, where plants are self-sowing aggressively in woodlands, often out-competing native trees by filling the understorey and preventing natural regeneration. Left untreated, the plant can end up producing a virtual monoculture. The species is considered to be a pernicious weed by many environmentalists[].

Known Hazards

Although no specific mention of toxicity has been seen for this species, all parts of Rhododendron species (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

Southern Europe to western Asia - Portugal, Spain, Bulgaria, Turkey, Lebanon and the Caucasus

Habitat

Rich forests under Fagus, Picea and Abies species, it is also found above the tree line[]. Sandy and peaty soils in woods and open places in Britain[].

Properties

Medicinal Rating
Other Uses Rating
Habit	Evergreen Shrub
Height	5.00 m
Pollinators	Insects
Cultivation Status	Ornamental, Wild

Cultivation Details

Rhododendron ponticum is a very cold-tolerant plant, when fully dormant it can withstand temperatures down to about -15°c[]. 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 full sun, but is also very tolerant of deep shade, even almost closed canopy conditions do not prevent this species from growing[]. Prefers a moist but well-drained, humus-rich lime-free soil. Prefers a peaty or well-drained sandy loam[]. A pH between 4.5 and 5.5 is ideal[]. Succeeds in sun or shade, the warmer the climate the more shade a plant requires[, ].
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[].
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[].
Plants in this genus are notably susceptible to honey fungus[].

Edible Uses

None known

Medicinal

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

A medicine made from the plant is used to treat heart and circulation malfunctions, but it should not be used without expert supervision[].

Agroforestry Uses:

Plants are being grown as a hedge at the RHS gardens in Wisley, Surrey[]. It tolerates quite hard clipping[].

Other Uses

The reddish wood is firm, hard, beautifully patterned. It is suitable for turning[].

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[].