Rhododendrons are a large group of shrubs that are especially admired for their handsome dark evergreen leaves and clusters of bell-shaped flowers. They grow naturally all over the world, including the United States where they have been cultivated for over a century. Hybrid rhododendrons bred from native American species were introduced in 1876 at the Centennial Exposition in Philadelphia. These are the most popular rhododendrons in the northeastern United States, representing 95% of the rhododendrons sold there. New types of rhododendrons from southern Asia have recently been introduced into this country, providing homeowners an even wider choice of these stunning shrubs. Although they dislike clay or alkaline soil and hot summers, they are extremely elegant, carefree shrubs when conditions suit their requirements.
While rhododendrons and their azalea relatives are often considered to be southern shrubs, many rhododendrons are cold hardy as far north as the Ohio Valley, into southern Pennsylvania and along the Atlantic coast into New England (zone 5). Many manage well where winter temperatures seldom dip below -10°F. The climate in the Pacific Northwest is proving to be ideal for these shrubs
Size: Rhododendrons are slow growers. Dwarf types may reach only 3 feet at maturity. Standard rhododendrons often grow as tall as 15 or 20 feet, and will spread equally wide.
Foliage: Rhododendrons are called broadleaf evergreens. They have elegant leaves which are elliptical and usually a dark glossy green above and lighter and duller beneath. Unlike those of their azalea relatives, rhododendron leaves are smooth on top. Leaves of individual types of rhododendron may have leaves anywhere from 2 to 10 inches long. The foliage of most species is evergreen and is an especially welcome sight in winter. PJM hybrids boast gorgeous purple foliage in the fall.
Flowers: Flowers of standard rhododendrons are tubular, with spectacular flaring petals. They grow at the ends of branches in rounded bunches of 5 or 7 blooms. These clusters may be 6 to 10 inches across (3 to 6 inches for smaller types) and literally cover the shrubs in a gorgeous spring display. Appearing from early April (PJM hybrids) through May into early summer, various types of rhododendrons offer many colors. Flowers are often magenta or shades of purple, pink or red, or white, sometimes marked with interesting speckles and splashes of color. Breeders are working to develop some with yellow blossoms and more summer blooming types that will offer a show in late June or even July.