Rhododendron

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Rhododendron

Rhododendrons, Azaleas
Genus:Rhododendron
Family:Ericaceae
Type:Shrubs and trees
Transplant:Easily transplanted
Disease issues:Many, some serious
Pollination:Insects

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.

Description

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.

Species

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:

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.

Species

Uses

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].

Harvesting

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

Powdery Mildew

Leaf Spots

Leaf Scorch

Leaf Blotch

Leaf Galls

Petal Blights

Bud Blasts

Blights

Stem Cankers

Diebacks

Damping Off

Root Rots

Rusts

Wilts

Crown Rots

Dieback

Nematodes

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

Chlorosis: Caused by insufficient Iron available in the soil, amlost 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)

Aphids

Scales

Hoppers

Whiteflies

Mealybugs

Bugs

Lacebugs

Thrips

Maggots

Beetles

Weevils

Caterpillars

Sawflies

Wasps

Mites

References

  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. 1 2 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
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