Common Name: Rosebay Rhododendron

General Information

Rhododendron maximum is an evergreen shrub or small tree that can be as small as 1 metres in the north of its range, but up to 12 metres in the south. Larger specimens can form a crooked trunk up to 30cm in diameter and heavy, contorted branches. Several stems can grow from the root crown[].
The plant is harvested from the wild for local use as a medicine and source of materials. It is sometimes grown as an ornamental in gardens.
Rhododendron maximum has a very wide distribution, a 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)[].
The plant is increasing its range in eastern N. America, where it can form dense thickets that exclude other plants and reduce the diversity in the forest. It can also prevent regeneration of canopy trees and so has the potential to become a weed[]..

Known Hazards

The leaves are poisonous[]. Ingestion can cause convulsions and coma[].
Although no other 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

, ,

Range

Eastern N. America - New York to Maine, south to Alabama and Georgia

Habitat

Damp places along streams, edges of bogs and ponds, woods and mountain slopes to 900 metres[, ], often forming impenetrable thickets[].

Properties

Weed Potential	Yes
Conservation Status	Least Concern
Medicinal Rating
Other Uses Rating
Habit	Evergreen Shrub
Height	3.50 m
Pollinators	Bees, Insects
Cultivation Status	Wild

Cultivation Details

Rhododendron maximum is a very cold-hardy plant, able to tolerate temperatures down to around -30°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[].
Succeeds in a most humus-rich lime-free soils except those of a dry arid nature or those that are 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[]. A pH between 4.5 and 5.5 is ideal[].
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.

The poulticed leaves are used to relieve arthritic pain, headaches etc[, ]. A decoction of the leaves is occasionally employed internally in domestic practice in the treatment of rheumatism[, ].
The leaves are taken internally in controlled dosage for the treatment of heart ailments[].
Caution is advised, see the notes above on toxicity.

Agroforestry Uses:

The large, leathery leaves can be up to 35cm long. They are retained for up to 8 years, and once shed, are slow to decompose, providing a good material for mulching acid-loving plants[, ].

Other Uses

Wood - fine-grained, hard, heavy, strong, rather brittle[, , , ]. It weighs 39lb per cubic foot[]. Used for tool handles and as a boxwood (Buxus spp.) substitute in engraving[, , , ].

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[].