Rhododendron yedoense is the name given by Maximowicz in 1886 to a double-flowered garden form of an azalea that grows wild in Korea. Unfortunately this azalea was not described as a species until many years later and must therefore be treated as a variety of its own cultivated offspring. The name of the wild form is therefore Rhododendron yedoense var. poukhanense (H.Lév.) Nakai[].

Common Name:

General Information

Rhododendron yedoense is a compact, densely-branched deciduous shrub growing 100 - 200cm tall and more wide[, ]. The plant can be evergreen in areas with mild winters[].
The plant is harvested from the wild and also cultivated for medicinal use. It is an ingredient of commercial cosmetic preparations and is also grown as an ornamental in gardens[].

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

E. Asia - southern Japan (Kyushu), Korea

Habitat

Stony mountain slopes, on rocks along streams; at elevations up to 1,800 metres[].

Properties

Medicinal Rating
Other Uses Rating
Habit	Deciduous Shrub
Height	1.50 m
Pollinators	Bees, Insects
Cultivation Status	Cultivated, Ornamental, Wild

Cultivation Details

Rhododendron yedoense is native to the temperate zone of Korea, where it can be found at elevations up to1,800 metres. A very cold-tolerant plant, when fully dormant it can withstand temperatures down to about -30°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[].
Rhododendron species generally succeed when grown in a non-compacted, humus rich lime free soil and a position with some shade, preferably light woodland shade. They strongly dislike soils of a dry arid nature, heavy soils or clays[]. 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[].
Rhododendron species are mainly woodland species that grow well in the dappled shade and shelter given by the other woodland plants. They are surface-rooting species with a fibrous root system, however, and do not grow well close to trees that are also surface-rooting, nor do they do well with ground cover or other small plants growing over or into their roots[, ].
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[].
This species is often used in breeding programmes to develop ornamental garden forms[].

Edible Uses

None known

Medicinal

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

The plant is used medicinally[]. No more information is given.

Other Uses

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

Propagation

Seed - surface sow in a shady part of the greenhouse in late winter or early spring[, ]. Another report says that the seed is best sown in the autumn as soon as it is ripe[]. Germination is variable and can be quite slow. Prick out the seedlings into individual pots as soon as they are large enough to handle and grow the pots on in a shady frame for 18 months before planting them out into their permanent positions[].
Cuttings of half-ripe wood, 5 - 8cm with a heel, mid summer in a frame. Plant out in spring. Fair percentage[].
Cuttings of mature wood, late autumn in a frame[].
Layering in the autumn. Takes 12 months[].
Division.