There are some 9,000 species of colorful rhododendrons worldwide. Native rhododendrons are found in most of the Northern Hemisphere except deserts. They grow in broad-leaved and coniferous woodlands, rain forests and alpine regions. The most diverse species range from the Himalayas to the mountains of the Indochina Peninsula, Japan and Korea. Tropical rhododendrons are found in Southeast Asia to northern Australia, some of which are epiphytes, meaning they grow on other plants.
Most species of rhododendrons grow as shrubs. The smallest species grow from ½ to 40 inches tall; the largest, R. giganteum, is a tree growing to 90 feet high. The leathery leaves, rounded or long and thin, grow from ¼ inch to more than 3 feet long; they can be evergreen or deciduous. The tops of the leaves are glossy; the undersides are often fuzzy. This fuzz, called indumentum, can have colors ranging from cinnamon to silver or white.
R. campanulata ssp. Aeruginosum, a wild species growing in Northern India and Nepal, has leaves with a bluish metallic shine and with indumentum that turns from white to a fawn color when the leaves are older.
From spring to early summer, rhododendrons produce spectacular displays of bell-shaped flowers that may be orange, red, pink, purple, white or yellow. The blossoms range from 1 to 1 ½ inch long.
Rhododendrons like loose, crumbly soil with a pH between 4.5 to 5.5 and that is rich with decaying organic matter. Rhododendrons grow naturally in the mountains, where water drains off steep slopes, so their soil should be moist but well-drained. They prefer slopes to hollow, flat areas; hot sun from the south is not good for their roots, which need to be kept as cool as possible.
North American Species
Twenty-seven species of rhododendron are native to North America. The rhododendron is the state flower of Washington state and West Virginia.
In 1762, Karl von Linnaeus, the Swedish naturalist, described Rhododendron canadense that is today found in eastern Quebec to Newfoundland and Nova Scotia and south to northern New Jersey and Pennsylvania.
In 1792, Archibald Menzies, a Scottish naturalist, discovered the Pacific rhododendron (R. macrophyllum) that is found in abundance from Northern California to British Columbia.
In 1855, Robert Fortune, a Scottish botanist, brought R. ovatum and R. fortunei from China to the United States, where they have spread along the East Coast.
Early UK Introductions
In 1656, Flemish botanist Charles l'Ecluse classified the Alpine Rose (R. hirsutum), the first rhododendron to be introduced to the UK. In 1736, Peter Collinson, a Quaker, and John Bartram, a Pennsylvania farmer, introduced R. canescens, R. nudiflorum and R. viscosum to England. Some regions of the UK are overrun by R. ponticum, native to Armenia, which was found in southern Spain around 1750 and introduced to England in 1763.
There were 12 species of rhododendron in cultivation in the UK by 1800.
In 1811, Thomas Hardwicke, a British sea captain, introduced the first Southeast Asian species to the UK--R. arboretum, a tree species has blood-red flowers.
All parts of rhododendrons, especially the leaves, contain grayanotoxin, a poison that can cause severe stomachache, liver damage and pneumonia. Keep grazing animals away from rhododendrons.