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Ericaceae FAMILY Least Concern

Rhododendron maximum

Rhododendron maximum

Edibility
0/5
Medicinal
2/5

Safety & Hazards

The leaves are poisonous[1]. Ingestion can cause convulsions and coma[1]. 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[2].

Cultivation Details

HabitEvergreen Shrub
Growth—

Rhododendron maximum is a very cold-hardy plant, able to tolerate temperatures down to around -30°c when fully dormant[3, 4]. 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[4]. Succeeds in a most humus-rich lime-free soils except those of a dry arid nature or those that are heavy or clayey[5]. Prefers a peaty or well-drained sandy loam[5]. Succeeds in sun or shade, the warmer the climate the more shade a plant requires[4]. A pH between 4.5 and 5.5 is ideal[5]. Succeeds in a woodland though, because of its surface-rooting habit[4], it does not compete well with surface-rooting trees[5]. 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[4]. 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[4]. Plants in this genus are notably susceptible to honey fungus[4].

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[4]. 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[6]. Cuttings of half-ripe wood, August in a frame. Difficult[6]. Cite as: Temperate Plants Database, Ken Fern. temperate.theferns.info. 2026-02-21. <temperate.theferns.info/plant/Rhododendron+maximum>

Pollinators

Bees, Insects

References

  1. [1]A Field Guide to Medicinal Plants. Eastern and Central N. America. — Foster. S. & Duke. J. A. · Houghton Mifflin Co. · 1990 · ISBN 0395467225 ↩︎A concise book dealing with almost 500 species. A line drawing of each plant is included plus colour photographs of about 100 species. Very good as a field guide, it only gives brief details about the plants medicinal properties.
  2. [2]Grayanotoxin Poisoning: ‘Mad Honey Disease’ and Beyond — Jansen S.A. et al · 2012 · https://doi.org/10.1007/s12012-012-9162-2 ↩︎An in-depth review of the literature on the toxic effects of a toxin widely found in the family Ericaceae
  3. [3]Shrubs. — Phillips. R. & Rix. M. · Pan Books · 1989 · ISBN 0-330-30258-2 ↩︎Excellent photographs and a terse description of 1,900 species and cultivars.
  4. [4]The New RHS Dictionary of Gardening. 1992. — Huxley. A. · MacMillan Press · 1992 · ISBN 0-333-47494-5 ↩︎Excellent and very comprehensive, though it contains a number of silly mistakes. Readable yet also very detailed.
  5. [5]RHS Dictionary of Plants plus Supplement. 1956 — F. Chittendon. · Oxford University Press · 1951 · ISBN - ↩︎Comprehensive listing of species and how to grow them. Somewhat outdated, it has been replaced in 1992 by a new dictionary (see [ 200 ]).
  6. [6]Propagation of Trees, Shrubs and Conifers. — Sheat. W. G. · MacMillan and Co · 1948 · ISBN - ↩︎A bit dated but a good book on propagation techniques with specific details for a wide range of plants.
  7. [7]Fire Effects Information System — http://www.feis-crs.org/feis/ ↩︎An on-line information site with comprehensive information on over 1,100 species of plant. Mainly developed to supply information on the effects of fire on plants and animals, it also contains a wealth of other information on the plants
  8. [8]IUCN Red List of Threatened Species — http://www.iucnredlist.org/ ↩︎A list of plants under threat and facing possible extinction, usually with brief details of the threats and information on habitat.
  9. [9]Gray's Manual of Botany.Eighth Edition — Fernald. M. L. · American Book Co.; New York · 1950 · ISBN 0442222505 ↩︎A bit dated but a good and concise flora of the eastern part of N. America.
  10. [10]An Illustrated Flora of the Northern United States and Canada — Britton. N. L. Brown. A. · Dover Publications. New York. · 1970 · ISBN 0-486-22642-5 ↩︎Reprint of a 1913 Flora, but still a very useful book.
  11. [11]A Dictionary of Plants Used by Man. — Usher. G. · Constable · 1974 · ISBN 0094579202 ↩︎Forget the sexist title, this is one of the best books on the subject. Lists a very extensive range of useful plants from around the world with very brief details of the uses. Not for the casual reader.
  12. [12]Manual of the Trees of N. America. — Sargent. C. S. · Dover Publications Inc. New York. · 1965 · http://www.biodiversitylibrary.org/ · ISBN 0-486-20278-X ↩︎Two volumes, a comprehensive listing of N. American trees though a bit out of date now. Good details on habitats, some details on plant uses. Not really for the casual reader. It can be downloaded from the internet.
  13. [13]Native American Ethnobotany — Moerman. D. · Timber Press. Oregon. · 1998 · ISBN 0-88192-453-9 ↩︎Very comprehensive but terse guide to the native uses of plants. Excellent bibliography, fully referenced to each plant, giving a pathway to further information. Not for the casual reader.
  14. [14]Dictionary of Economic Plants. — Uphof. J. C. Th. · Weinheim · 1959 · ISBN - ↩︎An excellent and very comprehensive guide but it only gives very short descriptions of the uses without any details of how to utilize the plants. Not for the casual reader.
  15. [15]The Complete Trees of N. America. Field Guide and Natural History. — Elias. T. · Van Nostrand Reinhold Co. · 1980 · ISBN 0442238622 ↩︎A very good concise guide. Gives habitats, good descriptions, maps showing distribution and a few of the uses. It also includes the many shrubs that occasionally reach tree proportions.