Japanese Azalea - Rhododendron japonicum

Japanese Azalea Rhododendron - Rhododendron japonicum (Gray) Sur.
Live plants photographed at The Morton Arboretum, Lisle, Illinois, USA
All parts of Rhododendrons are toxic to animals if ingested.
Blooms mid-May (this plant was photographed May 26, near Chicago).

Japanese Azalea - Rhododendron japonicum
Japanese Azalea has gorgeous yellow flowers

A decidious shrub growing to 2m. Hardy to zone 5. Flowers April to May. The flowers are hermaphrodite (have both male and female organs) and are pollinated by insects. The plant prefers light sandy, well-drained but moist soil. It can grow in semi-shade (light woodland) or full sun.

Japanese Azalea - Rhododendron japonicum
Japanese azalea grows well in light shade or full sun

Rhododendron (from the Greek: rhodos, "rose", and dendron, "tree") is a genus of flowering plants in the Heath Family (Ericaceae). It is a large genus with over 1000 species. Most have very colorful, showy flowers. It includes the plants known to gardeners as azaleas.

Rhododendron grow as shrubs or small trees. The smallest species grows to 10-100 cm tall, and the largest, R. giganteum, has been reported at 90 feet. 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.

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. Most large commercial growers in the United States are located on the west coast.

Rhododendrons are 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 such as R. arboreum and R. falconeri eventually growing to 10-15 m or more tall.

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 need a planting mix similar to that required for 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.

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. [1]

All parts of Rhododendrons are toxic to animals if ingested. Symptoms include vomiting, diarrhea, hypersalivation, weakness, coma, hypotension, CNS depression, cardiovascular collapse and death. [2]

References:

NATIONAL AUDUBON SOCIETY, National Audubon Society Field Guide to Rhododendron --E: Eastern Region, Chanticleer Press Ed (Knopf, 1980).
ASPCA, Animal poison control Toxic plant list
David Streeter and Richard Lewington, The Natural History of the Rhododendron: An Intricate Visual Exploration, 1st American ed (1993).

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