Rhododendron Totally Explained

Everything about Rhododendron totally explained

ť This article is about the plant. For the place, see Rhododendron, Oregon. For the vessel, see MV Rhododendron.
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.
The Rhododendron is a genus characterized by 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 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 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 Taiwan. 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 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.

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 (for example 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 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. 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've 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.

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 rhododendrons

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).

Symbolism

Rhododendron arboreum (Lali Gurans) is the national flower of Nepal. Rhododendron niveum is the state tree of Sikkim in India. Rhododendron catawbiense, the predominant Rhododendron in the Appalachian Mountains, is the state flower of West Virginia, and is in the Flag of West Virginia. Rhododendron macrophyllum, the predominant rhododendron on the Pacific Coast and in the Cascade Mountains is the state flower of Washington.

Gallery

Image:Rhododendron decorum ssp diaprepes -1.jpg|Rhododendron decorum subsp. diaprepes Image:rhododendron2.jpg Image:Wild Rhododendrons in Kashmir.jpg|Wild Rhododendrons in Kashmir by Edward Molyneux; painted before 1908 Image:Rhododendron 1 Fcb981.JPG Image:Wild Rhododendrons in Whitwick Leicestershire 1155.JPG | Flowering Rhododendrons in Whitwick Leicestershire

Footnotes

Further Information

Get more info on 'Rhododendron'.

External Link Exchanges

Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:

<a href="http://rhododendron.totallyexplained.com">Rhododendron Totally Explained</a>

Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned.