Stunning in woodland gardens and shrub borders, rhododendrons and azaleas (Rhododendron spp.) are similar shrubs with confusing nomenclature. Even the experts have trouble distinguishing some plants that may have characteristics of both types of shrubs. Even if you're struggling with the differences, the growing requirements are the same. Azaleas and rhododendrons need partial shade and an acidic, well-drained soil with lots of organic matter. The hardiness of azaleas and rhododendrons varies, depending upon the type. Most azaleas and rhododendrons are hardy in U.S. Department of Agriculture hardiness zones 5 through 8 with some extending into zone 9.
Both azaleas and rhododendrons belong to the genus Rhododendron. The differentiation between the terms rhododendrons and azaleas is a difference of common usage, not taxonomy. When in doubt, it is correct to call either shrub a rhododendron.
The traditional way to tell the difference between rhododendrons and azaleas is to count the stamen while the flowers are in bloom. The original taxonomists classified those with 10 stamens as rhododendrons and those with five as azaleas. Although there is some variation because of modern criteria changes, this method still proves true most of the time. Azaleas flowers are generally smaller and bloom earlier than rhododendrons. Both azaleas and rhododendrons bloom in spring or early summer, and "Encore Azaleas" bloom in early spring and again in summer.
Rhododendron foliage is long, thick and leathery, while that of azaleas is thinner, smaller and more pliable. The foliage of both azaleas and rhododendrons can be smooth and glossy or covered with bronze hairs. You can create an interesting display by planting a variety of species and cultivars with different textures. Some deciduous azaleas have attractive fall foliage colors, and evergreen types add solid, year-round structure to the garden.
Evergreen or Deciduous
Azaleas may be deciduous or evergreen, and some are deciduous in cool climates and evergreen in warmer regions. Rhododendrons are always evergreen. Evergreen azaleas produce two sets of leaves each year: One in early spring and another group in summer. The loss of leaves and production of new ones is gradual and the casual observer would not notice the process if it weren't for the fallen leaves under the shrub. The second set of leaves is smaller and thicker than the first set. Evergreen azaleas are more sensitive to cold winters than rhododendrons or deciduous azaleas.
- Lady Bird Johnson Wildflower Center: What is the Difference Between Rhododendrons and Azaleas
- Virginia Cooperative Extension: Growing Azaleas and Rhododendrons
- The United States National Arboretum: Azalea Questions
- The Homeowner's Complete Tree & Shrub Handbook; Penelope O'Sullivan
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