Rhododendron

Rhododendron
Rhododendron ponticum
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Magnoliopsida Order: Ericales Family: Ericaceae Genus: Rhododendron L.
Subgenera
Azaleastrum Candidastrum Hymenanthes Mumeazalea Pentanthera (Azaleas) Rhododendron Therorhodion Tsutsusi (Azaleas) Vireya

Rhododendron is the common and genus name for a large and diverse group of woody shrubs and small (rarely large) trees in the flowering plant family Ericaceae. It is a large genus with over 1000 species, and subdivided into various subgenera (commonly 5, 8, or 9, depending on the taxonomic scheme). The trees may be deciduous or evergreen and the spirally arranged leaves can vary significantly in size according to species, from 1 to 2 centimeters (0.4-0.8 inches) up to 100 centimeters (39 inches). Members of two of the traditional subgenera, Pentanthera (deciduous), and Titsushi (evergreen), are known as azaleas.

and most have showy flower displays. Indeed, the name rhododendron comes from the Greek rhodos, meaning "rose," and dendron, meaning "tree."

Overview and description

Ericaceae, the family to which the genus Rhododendron belongs, is known as the "heath family," and consists of herbs, shrubs, and trees with leaves that are alternate, simple, and estipulate, and mostly lime-hating or calcifuge plants that thrive in acid soils. In addition to rhododendrons, well-known examples include cranberry, blueberry, heath, heather, and huckleberry.

The Rhododendron genus is characterized by shrubs and small to (rarely) large trees, the smallest species growing to 10 to 100 centimeters (3.9-39 inches) tall, and the largest, R. giganteum, reported to over 30 meters (98 feet) tall (Francis 2006). The leaves are spirally arranged; leaf size can range from 1 to 2 centimeters (0.4-0.8 inches) to over 50 centimeters (20 inches), exceptionally 100 centimeters (39.4 inches) 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 of rhododendrons 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.

Some rhododendrons are known by the common name of azalea. These are flowering shrubs that are characterized by the lack of scales on the underside of the generally thin, soft, and pointed leaves, and typically having terminal blooms (one flower per stem), flowers with five or six stamens, and with long straight hairs parallel to the leaf surface and along the midrid on the ventral surface of the leaf. Originally, azaleas were classed as a different genus of plant, but now they are recognized as part of the rhododendrons. In the common classification of rhododendrons into eight or nine subgenera, azaleas comprise two of the subgenera: Pentanthera (deciduous) and Titsushi (evergreen).

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. Relatively fewer species occur in North America and Europe. Rhododendrons have not been found in South America or Africa.

Classification

The genus Rhododendron was created by Linnaeus in 1753, but he placed the azaleas in another genus. However, it was later recognized that a separation between azaleas and the members of Rhododendron could not be justified and they were placed together. The Rhododendron genus was subdivided into 8 sections in 1834 by George Don and these eight subcategories have been commonly recognized until today, with the azaleas comprising two of the sections, subgenus Pentanthera (typified by Rhododendron nudiflorum) and subgenus Tsutsusi (typified by R. Tsutsusi) (Henning 2007).

However, other subcategories also have been recognized. It is not uncommon for 9 subgenera to be recognized, with section Vireya within the large subgenera Rhododendron being raised to the subgenus level (Callard 2008). Furthermore, in 2004, based on genetic and chemical data, Goetsch et al. (2005) called for a new realignment in which the genus Rhododendron was subdivided into five subgenera (Rhododendron, Hymenanthes, Azaleastrum, Choniastrum and Therorhodion).

Rhododendron wardii var. puralbum

Overall, the species today are organized by subgenus, section, subsection and series. The traditional recognition of eight subgenera divides the genus 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: evergreen azaleas, 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 , 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.

A garden with tall Rhododendrons in Lynnwood, Washington

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:

• Rhododendron 'President Roosevelt'

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. 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.^{[citation needed]} 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 . 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.

Use in landscaping

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 . 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.

Commercial growing

Rhododendrons are grown commercially in many areas for sale, and are occasionally collected in the wild, a practice now rare in most areas. Larger commercial growers often ship long distances; in the United States most of them are located on the west coast (Oregon, Washington and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might, such as resistance to root rot when over-watered, ability to be forced into budding early, ease of rooting or other propagation, and saleability.

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. Mulching and careful watering are important, especially before the plant is established.

Insects and 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

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  Rhododendron decorum subsp. diaprepes

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  Wild Rhododendrons in Kashmir by Edward Molyneux; painted before 1908

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  Flowering Rhododendrons in Whitwick Leicestershire

References

ISBN links support NWE through referral fees

Chris Callard 2008 recognizes nine subgenera Subgenus Vireya http://www.vireya.net/classification.htm he Classification of Vireya Rhododendrons

The Molecular Systematics of Rhododendron (Ericaceae): A Phylogeny Based Upon RPB2 Gene Sequences Authors: Goetsch, Loretta; Eckert, Andrew J.; Hall, Benjamin D. Source: Systematic Botany, Volume 30, Number 3, July-September 2005 , pp. 616-626(11)

• 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.

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http://www.rhodyman.net/rhodyhis.html#anchor282700 http://www.rhodyman.net/rhodyhis.html#anchor282700 Rhododendron History Henning, ? 2007 Hennings Home Page

External links

• Rhododendrons from Turkey, Anatolia
• Flora of China: Rhododendron
• Description of damage caused by Rhododendrons in the UK
• 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.
• History of Rhododendrons
• Historical Survey of Rhododendron Collecting - With Emphasis on its Close Associations with Horticulture

Rhododendron Societies

• American Rhododendron Society
• UK Royal Horticultural Society
• Rhododendron Species Foundation and Botanical Garden
• Société Finlandaise du Rhododendron
• Australian Rhododendron Society
• German Rhododendron Society

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