Azalea: The Essential Southern Plant
Rhododendrons and azaleas are arguably one of the Southern gardener's favorite shrubs. Many people think of them as entirely different plants, but they both belong to the genus Rhododendron, which comprises more than 800 species and 10,000 named selections. Even to the untrained eye, one difference between the two groups is noticeable: rhododendrons generally have much larger leaves. From a technical standpoint, rhododendron flowers are bell-shaped and have ten or more stamens, while azalea blooms are typically funnel-shaped and have five stamens.
About Azaleas and Rhododendrons
Selecting a species that will thrive in your climate allows gardeners in every part of the South to enjoy these plants—even growing some in containers. Rhododendrons generally do better in the Upper and Middle South, though several selections thrive in the Lower South. Azaleas, however, are more accommodating. With necessary attention to soil, light, and proper selection, these plants can thrive throughout the South.
Rhododendrons and azalea species can be evergreen or deciduous. Plant sizes vary somewhat within groups, but most individual plants are roughly equal in height and width.
How to Plant Azaleas
Soil To Use
Rhododendrons and azaleas have many similar basic requirements for soil and water. They need acidic, well-drained, organically enriched soil that should neither get too dry nor remain soggy.
The recommended practice in areas rich in alkaline soil, such as Texas and Oklahoma, is to build raised beds for planting. Create beds 15 to 18 inches deep and fill them with a half-and-half mixture of finely milled bark and coarse sphagnum peat moss (be sure to mix the two thoroughly with water before serving the beds). Irrigating with alkaline water will slowly raise the pH level. To keep it in the desired range of 5.0–6.0, prepare a mixture of three parts garden sulfur to one part iron sulfate, then apply it at the rate of one pound per 100 square feet of the garden bed—this should lower the pH by one point.
Soil To Avoid
Planting in heavy clay is a no-no: root rot often ensues, indicated by yellowing, wilting foliage, and collapse of the plant. Planting in limy, alkaline soil is another mistake—lack of iron quickly results in chlorosis (yellow leaves with green veins).
How To Grow Azaleas
Plant azaleas and rhododendrons with the top of the root ball slightly above soil level. Don't cultivate around these plants, as they have shallow roots.
Because they absorb water through their foliage, wet the leaves and root zone when you water. Overhead watering with sprinklers works well, but to prevent fungal diseases, water in the morning so that leaves dry by the afternoon. Avoid drip irrigation―it doesn't wet the root system uniformly.
The sun tolerance of azaleas and rhododendrons varies by species and selection. Most types generally prefer the partial sun or filtered shade beneath tall trees. The east and north sides of the house are better locations. Too much sun bleaches or burns the leaves, and too little results in lanky plants that don't bloom. Make sure to position plants in partial sunlight to prevent compact shrubs in heavy sunlight or lack of blooms in deep shade.
Avoid varieties that love the sun if you live in a climate susceptible to cold winters with heavy snowfall. Choosing a suitable plant variety helps azaleas to flourish year after year.
The Vireya rhododendron, from the tropics of Southeast Asia, manages nicely in frost-free and nearly frostless zones. They are also good container plants (even indoors), so they can be grown in colder zones if brought inside for the winter. They need an especially fast-draining potting mix (many species are epiphytes in the wild). A combination of peat moss, ground bark, and perlite works well.
Typically, plants flower on and off throughout the year rather than in one blooming season. They bear waxy-textured blossoms in exciting shades of yellow, gold, orange, vermilion, salmon, and pink, plus cream, white, and bi-colors.
Pruning rhododendrons is simple―just follow these general guidelines. Tip-pinch young plants to make them bushy and prune older, leggy plants to restore shape by cutting back to a side branch, leaf whorl, or cluster of latent buds.
Do any extensive pruning in late winter or early spring. Pruning at this time will sacrifice some flower buds, but it diverts the plant's energy to latent growth buds, which will then be ready to push out their new growth early in the growing season. You can do some shaping while plants bloom—try using cut branches in arrangements.
To prevent seed formation, which can reduce next year's bloom, clip or break off spent flower trusses, taking care not to damage growth buds at the base of each truss.
For evergreen species, cutting the occasional wayward branch restores symmetry. To keep bushes compact, tip-pinch frequently, starting after flowering ends and continuing until mid-June.
Prune deciduous azaleas while they are dormant and leafless. Azaleas don't need as careful pruning as rhododendrons―the leaves are relatively evenly spaced along the branches, with a bud at the base of each leaf, so new growth will sprout from almost anywhere you cut (in either bare or leafy wood).
In spring, just after the blooms fade, apply mulch and fertilize with a controlled-release, acid-forming fertilizer such as cottonseed meal or commercial azalea/camellia food.
Do not mulch in the fall. Mulching will hold heat in the soil and delay the onset of dormancy, increasing the chances of winter damage. Also, don't fertilize before bloom―you'll encourage leafy growth at the wrong time.
Pests and Diseases
Insects and diseases seldom bother healthy, vigorous plants. However, rhododendrons growing in heavy clay often fall victim to Phytophthora, a deadly soil-borne fungus that causes dieback.
Azaleas growing in full sun are often plagued by sucking insects called lace bugs.
Common Azalea and Rhododendron Species
Most people know rhododendrons as big, leathery-leafed shrubs with rounded clusters ("trusses") of stunning white, pink, red, or purple blossoms. These are primarily hybrids of catawba rhododendron, R. catawbiense, native to the Appalachians. There are dwarf species just a few inches tall, giants species that reach 40 feet (or even 80 feet) in their native Southeast Asia, and a host of species and hybrids of intermediate size. Hybrids with Asian parentage may display exotic colors of yellow, apricot, and salmon. Unfortunately, plants with these colors are often less tolerant of the South's summer heat.
These are some of the selections that accept the long, hot summers of the Lower South: 'A. Bedford', 'Album Elegans', 'Anah Kruschke', 'Anna Rose Whitney', 'Belle Heller', 'Caroline', 'Cheer', 'Chionoides', 'Cynthia', 'English Roseum', 'Fastuosum Flore Pleno', 'Ginny Gee', 'Holden', 'Janet Blair', 'Jean Marie de Montague', 'Lee's Dark Purple', 'Nova Zembla', 'Purple Splendour', 'Roseum Elegans', 'Scintillation', 'Trude Webster', 'Vulcan'.
Most of the hybrids listed here are fairly cold-hardy. The following can take temperatures to at least –20°F: 'Album Elegans', 'America', 'Boule de Neige', 'Catawbiense Album', 'Catawbiense Boursault', 'English Roseum', 'Nova Zembla', 'PJM', 'President Lincoln', 'Ramapo', 'Roseum Elegans'.