Rhododendrons and azaleas (Rhododendron spp.) are a diverse group of plants with slight variances in growing range and cultural preferences, depending on type. While rhododendrons prefer some shade, azaleas may take some sun. Your best bet for a healthy, long-lived plant is to refer to its individual needs, but some general rules about planting location do exist.
Rhododendrons and azaleas are a very large group of plants, comprising over 800 species, maybe as many as 1,000. Although most of them are evergreen, with large, leathery green leaves, the subgroup Pentathera refers to deciduous azaleas. A few rhodies are also deciduous. While it is always correct to refer to both rhodies and azaleas, deciduous and evergreen, under the umbrella term “rhododendrons,” most gardeners do distinguish between the two. Their hardiness zones range depending on species: they are often winter hardy down to U.S. Department of Agriculture plant hardiness zone 4, and can withstand southern extremes up to zone 9.
Rhododendrons prefer to grow in partial shade. Despite their reputation as a shade-loving plant, however, they do best in dappled or partial shade. They will grow in full shade, but only if their branches are pruned high. The best bet is to grow them where shade is dappled, letting in a little light all day long, or where they get morning sun -- until 1 p.m. -- but are protected thereafter. The PJM rhododendrons (Rhododendron (PJM Group)), for instance, are hardy in USDA zones 4 through 8 and will tolerate full shade, but prefer sun-dappled sites. If you don’t have larger plants to shade them in the heat of the afternoon, consider providing a fence for shade.
Rhodoendrons specifically do appreciate some sunlight, but only if dappled or in the morning. Although they may tolerate sites in full sun, they often will not perform their best. Azaleas, on the other hand, do better with sun. Some species may even thrive in full sun, although their flower production may suffer and blooms may be dry and papery. The pinxterbloom azalea (Rhododendron periclymenoides), winter hardy in USDA zones 4 through 9, will tolerate sun better than many other species of rhododendron or azalea.
Although rhododendrons can tolerate some sun, they are especially prone to adverse effects like leaf scorch -- browning or curling of leaf margins -- when they are exposed to too much light during dry winter periods. To avoid this, consider planting rhodies on the north-facing side of a house or in the shade of an understory. Make sure that wherever you put them also has protection from drying winds, as this can cause moisture reduction and leaf scorch as well. Always supply good drainage, as rhododendrons and azaleas hate wet feet.