Rhododendrons, Azaleas
Type:Shrubs and trees
Transplant:Easily transplanted
Disease issues:Many, some serious

Rhododendron (from the Greek: rhodos, "rose", and dendron, "tree") is a genus of flowering plants in the family Ericaceae. It is a large genus with over 800 species. and most have showy flower displays. It includes the plants known to gardeners as azalea, which was once treated as a separate genus.

Species of Rhododendron are widely distributed, occurring throughout most of the Northern Hemisphere except for dry areas, and extending into the Southern Hemisphere in southeastern Asia and northern Australasia. The highest species diversity is found in the Sino-Himalayan mountains from central Nepal and Sikkim east to Yunnan and Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japan and Taiwaan. In addition, there are a significant number of tropical rhododendron species from southeast Asia to northern Australia, with 55 known species in Borneo and 164 in New Guinea[1]. Relatively fewer species occur in North America and Europe. Rhododendrons have not been found in South America or Africa.



The species are shrubs and small to (rarely) large trees, the smallest species growing to 10-20 cm tall, and the largest, R. arboreum, reported to 50 m tall. The leaves are spirally arranged; leaf size can range from 1-2 cm to over 50 cm, exceptionally 100 cm in R. sinogrande. They may be either evergreen or deciduous. In some species the underside of the leaves are covered with scales (lepidote) or hairs (indumentum). ; some of the best known species are noted for their many clusters of large trumpet-shaped flowers. There are however alpine species with small flowers or with small leaves.

Growing conditions


Most species require rich, moist, but well drained soils, sun to shade. Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5-5.5. Rhododendrons have fiberous roots and prefer well-drained soils high in organic material. In areas with poorly-drained or alkaline soils, rhododendrons are often grown in raised beds using mediums such as composted pine bark.[2]. Mulching and careful watering are important, especially before the plant is established.



Rhododendrons are extensively hybridized in cultivation, and natural hybrids often occur in areas where species ranges overlap. There are over 28,000 cultivars of Rhododendron in the International Rhododendron Registry held by the Royal Horticultural Society. Most have been bred for their flowers, but a few are of garden interest because of ornamental leaves and some for ornamental stems.

Some species (e.g. Rhododendron ponticum in the United Kingdom) are invasive as introduced plants, spreading in woodland areas replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

The species are organised by subgenus, section, subsection and series. These are currently divided into four large and four small subgenera:

  • Subgenus Rhododendron L.: small leaf or lepidotes (with scales on the underside of their leaves); several hundred species, type: Rhododendron ferrugineum. The tropical rhododendrons (sect. Vireya, about 300 species) are usually included as a section in this subgenus, but sometimes split off as a ninth subgenus.
  • Subgenus Hymenanthes (Blume) K.Koch: large leaf or elepidotes (without scales on the underside of their leaves); about 140 species, type: Rhododendron degronianum.
  • Subgenus Pentanthera G.Don: deciduous azaleas; about 25 species, type Rhododendron luteum.
  • Subgenus Tsutsusi: about 15 species; type Rhododendron indicum.
  • Subgenus Azaleastrum Planch.: five species; type Rhododendron ovatum.
  • Subgenus Candidastrum (Sleumer) Philipson & Philipson: one species; Rhododendron albiflorum.
  • Subgenus Mumeazalea: one species, Rhododendron semibarbatum.
  • Subgenus Therorhodion: one species, Rhododendron camtschaticum.

Recent genetic investigations have caused an ongoing realignment of species and groups within the genus, and also have caused the old genus Ledum to be reclassified within subgenus Rhododendron. Further realignment within the subgenera is currently proposed [3], including the merging of subgenus Hymenanthes into subgenus Pentanthera. Most species prefer acidic soil conditions; some tropical Vireyas, and other species, grow as epiphytes.




Both species and hybrid rhododendrons (including azaleas) are used extensively as ornamental plants in landscaping in many parts of the world, and many species and cultivars are grown commercially for the nursery trade. Rhododendrons are often valued in landscaping for their structure, size, flowers, and the fact that many of them are evergreen [4]. Azaleas are frequently used around foundations and occasionally as hedges, and many larger-leafed rhododendrons lend themselves well to more informal plantings and woodland gardens, or as specimen plants. In some areas, larger rhododendrons can be pruned to encourage more tree-like form, with some species such as R. arboreum and R. falconeri eventually growing to 10-15 m or more tall [4].







Some species are poisonous to grazing animals. Some Rhododendrons have a toxin called grayanotoxin in their pollen and nectar. People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers. Xenophon described the odd behavior of Greek soldiers after having consumed honey in a village surrounded by rhododendrons. Later, it was recognized that honey resulting from these plants have a slightly hallucinogenic and laxative effect. The suspect rhododendrons are Rhododendron ponticum and Rhododendron luteum (formerly Azalea pontica), both found in northern Asia Minor. Eleven similar cases have been documented in Istanbul, Turkey during the 1980s [5].

Pests and diseases


There are a number of insects that either target rhododendrons or opportunistically will attack them. Rhododendron borers and various weevils are major pests of rhododendrons, and many caterpillars will attack rhododendrons. Major diseases include Phytophthora root rot, stem and twig fungal diebacks; Ohio State University Extension provides information on maintaining health of rhododendrons.

Rhododendron species are used as food plants by the larvae of some Lepidoptera species.

Crown Gall

  • Agrobacterium tumeifasciens

Powdery Mildew

  • Erisiphe
  • Microsphaera alni

Leaf Spots

  • Cercospora rhododendri
  • Coleotrichum
  • Corynospora cassiicola
  • Cylindrocladium scoparium
  • Coryneum rhododendri
  • Coryneum triseptatum
  • Cryptostictis mariae
  • Diplodinia rhododendri
  • Gloeosporeum rhododendri
  • Gloeosporium ferrugineum
  • Hendersonis concentrica
  • Laestadia rhodorae
  • Lophodermium melemeucum
  • Lophodermium rhododendri
  • Melasmia rhododendri
  • Monochaetia
  • Mycosphaerella
  • Pestalotia macrotricha
  • Phyllosticta maxima
  • Physalospora rhododendri
  • Venturia rhododendri

Leaf Scorch

  • Septoria azaleae

Leaf Blotch

  • Botrytis cinerea

Leaf Galls

  • Exobasidium vaccinii

Petal Blights

  • Ovulinia azaleae:

Bud Blasts

  • Pycnostysanus azaleae:


  • Briosia azaleae
  • Monilinia azaleae
  • Pestalotia macrotricha

Stem Cankers

  • Cylindrocladium


  • Botryosphaeria dothidea
  • Phomopsis

Damping Off

  • Pellicularia filamentosa

Root Rots

  • Armillaria mellea


  • Chrysomyxa ledi
  • Chrysomyxa piperiana
  • Chrisomyxa roanensis
  • Pucciniastrum vaccinii


  • Phytophthora cinnamomi
  • Phytophthora ramorum

Crown Rots

  • Phytophthora cryptogea


  • Phytophthora cactorum
  • Phytophthora citricola


  • Aphelenchoides fragariae
  • Ditylenchus
  • Trichodorus christiei
  • Tylenchorhynus claytoni
  • Tylenchus

Loss of vigor: Highly susceptible to Juglone poisoning (from Black Walnuts)

Chlorosis: Caused by insufficient Iron available in the soil, almost always a result of high pH.

Boron Toxicity:Too much Boron in soil solution

Dry Bud or Bud Drop (Caused by drought during bud formation)

















  • Cox, P. A. & Kenneth, N. E. The Encyclopedia of Rhododendron Species. 1997. Glendoick Publishing. ISBN 0-9530533-0-X.
  • Davidian, H. H. The Rhododendron Species. In four volumes from 1982-1995. Timber Press. ISBN 0-917304-71-3, ISBN 0-88192-109-2, ISBN 0-88192-168-8, ISBN 0-88192-311-7.
  • Ann Fowler Rhoads and Timothy A. Block (2000). The Plants of Pennsylvania: An Illustrated Manual. Anna Anisko, illustrator. Morris Arboretum, University of Pennsylvania Press. pp. 384–386. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)
  • Staff of the L. H. Bailey Hortorium (1976). Hortus Third: A Concise Dictionary of Plants Cultivated in the United States and Canada. Cornell University Press. pp. 949–965. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)
  • Pirone, Pascal P. (1978). Diseases & Pests of Ornamental Plants (Fifth Edition ed.). John Wiley & Sons, New York. pp. 448–457. {{cite book}}: |edition= has extra text (help); Cite has empty unknown parameter: |coauthors= (help)
  • Cranshaw, Whitney (2004). Garden Insects of North America: The Ultimate Guide to Backyard Bugs. Princeton University Press. p. 617. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)
  • Pippa Greenwood, Andrew Halstead, A.R. Chase, Daniel Gilrein (2000). American Horticultural Society Pests & Diseases: The Complete Guide to Preventing, Identifying, and Treating Plant Problems (First Edition ed.). Dorling Kindersley (DK) Publishing, inc. p. 196. {{cite book}}: |edition= has extra text (help); Cite has empty unknown parameter: |coauthors= (help)CS1 maint: multiple names: authors list (link)
  1. Argent, G. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1-902896-61-0
  2. Soil information for planting rhododendrons
  3. Goetsch, L. A., Eckert, A. J. & Hall, B. D. (2005). The molecular systematics of Rhododendron (Ericaceae): A Phylogeny based upon RPB2 gene sequences. Sys. Bot. 30(3): 616-626.
  4. a b Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan
  5. Nurhayat Sütlüpmar, Afife Mat and Yurdagül Satganoglu Poisoning by toxic honey in Turkey. Archives of Toxicology. Volume 67, Number 2, pages 148-150, February, 1993