Common Name:

General Information

Rhododendron calendulaceum is an erect, much-branched deciduous shrub or small tree, usually growing up to 4 metres tall, occasionally reaching 10 metres This species does not usually have a rhizomatous rootstock[].
The plant is sometimes harvested from the wild for local use as a food and a medicine. It is grown as an ornamental in gardens, valued especially for its colourful flowers[].

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

Eastern N. America - Ohio to Pennsylvania, south to Tennessee, Georgia and South Carolina

Habitat

Mixed deciduous forests; at elevations from 200 - 1,000 metres[].

Properties

Edibility Rating
Medicinal Rating
Habit	Deciduous Shrub
Height	3.00 m
Cultivation Status	Ornamental, Wild

Cultivation Details

Rhododendron calendulaceum is very cold-hardy, tolerating temperatures down to around -25°c when fully dormant. 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[].
Plants can be cut back to the base and will generally resprout from the root crown[].
Considered one of the finest ornamental native shrubs growing in the United States the plant is the source of many ornamental cultivars[, ].
The acrid-scented flowers open before or as the leaves expand in spring[].

Edible Uses

A swelling ('apple') formed on the stems as a result of a fungus is eaten to alleviate thirst[]. This 'apple' could possibly be an insect gall

Medicinal

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

The twigs are peeled and then boil and rubbed on rheumatic joints to bring relief[].

An infusion of the plant (part not specified) is used by women as a gynaecological aid[].

Other Uses

None known

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[].