Rhododendron
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 Rhododendron ponticum | ||||||||||||
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*Azaleastrum
Source: RBG, Edinburgh |
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 1000 species and most have showy flower displays. It includes the plants known to gardeners as azaleas. It's the national flower of Nepal.
The Rhododendron is a genus characterized by shrubs and small to (rarely) large trees, the smallest species growing to 10-100 cm tall, and the largest, R. giganteum, reported to over 30 m tall[1]. 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 is covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. There are alpine species with small flowers and small leaves, and tropical species such as section Vireya that often grow as epiphytes.
Distribution
Rhododendron is a very widely distributed genus, 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 Himalayan mountains from Uttarakhand, Nepal and Sikkim to Yunnan and Sichuan, with other significant areas of diversity in the mountains of Indo-China, Korea, Japan and Taiwan. Rhododendron, also known as "Lali Guras" in local language is the national flower of Nepal and the state tree of the state of Uttarakhand in India. In Korea, Rhododendron schlippenbachii is particularly common (called 철쭉(choltjuk)). 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. [2] Relatively fewer species occur in North America and Europe. Rhododendrons have not been found in South America or Africa.
Classification
The species are organized 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.]].
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][4], including the merging of subgenus Hymenanthes into subgenus Pentanthera.
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 bark or 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.
- Sample species
- Rhododendron atlanticum
- Rhododendron canadense
- Rhododendron catawbiense
- Rhododendron chapmanii
- Rhododendron ferrugineum
- Rhododendron groenlandicum
- Rhododendron hirsutum
- Rhododendron lochiae
- Rhododendron luteum
- Rhododendron macrophyllum
- Rhododendron maximum
- Rhododendron moulmainense
- Rhododendron occidentale
- Rhododendron ponticum
- Rhododendron schlippenbachii
- Rhododendron spinuliferum
- Rhododendron tomentosum
A sample hybrid:
Toxicology
Some species are poisonous to grazing animals. These 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. Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant, although most horses tend to avoid it if they have access to good forage.
Planting and care
Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5-5.5; some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids. Rhododendrons have fibrous 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.[5] Mulching and careful watering are important, especially before the plant is established.
Insects and diseases
- Main article: List of rhododendron diseases
There are a number of insects that either target rhododendrons or will opportunistically 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 dieback; Ohio State University Extension provides information on maintaining health of rhododendronsRhododendrons can easily be suffocated by other plants.
Rhododendron species are used as food plants by the larvae of some members of the Order Lepidoptera (butterflies and moths) (See List of Lepidoptera that feed on rhododendrons).
Gallery
Rhododendron decorum subsp. diaprepes
Wild Rhododendrons in Kashmir by Edward Molyneux; painted before 1908
Flowering Rhododendrons in Whitwick Leicestershire
Footnotes
- ↑ Tree rhododendrons
- ↑ Argent, G. Rhododendrons of subgenus Vireya. 2006. Royal Horticultural Society. ISBN 1-902896-61-0
- ↑ 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.
- ↑ Summary of Goetsch-Eckert-Hall results
- ↑ Soil information for planting rhododendrons
References
- 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.
See also
- List of plants poisonous to equines
- Chinese herbology 50 fundamental herbs
External links
- Wikipedia article
- Flora of China: Rhododendron
- Information on rhododendrons at the Ericaceae web pages of Dr. Kron at Wake Forest University.
- Information on Vireyas
- Information on Rhododendrons by Marc Colombel, founder of the Société Bretonne du Rhododendron.
- Extensive information on rhododendron species: the history of their discovery, botanical details, toxicity, classification, cultural conditions, care for common problems, and suggestions for companion plants by Steve Henning.