The Basics- Rhododendrons and Azaleas

Rhododendrons and azaleas are arguably the South’s favorite shrubs. Many people think of them as entirely different plants, but they both belong to the genus Rhododendron. One difference between the two groups is obvious: rhododendrons generally have much larger leaves. Also, rhododendron flowers are bell shaped and have ten or more stamens, while azalea blooms are typically funnel shaped and have five stamens.

Rhododendrons generally do better in the Upper and Middle South, though a number of selections thrive in the Lower South.  Azaleas, however, are more accommodating; with the necessary attention to soil, light, and proper selection, they can be grown throughout the South.

Rhododendrons and azaleas have much the same basic requirements for soil and water. They need acid, well-drained, organically enriched soil that should neither get too dry nor remain soggy. 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). 

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 both the leaves and root zone when you water. Overhead watering with sprinklers works well, but to prevent fungal diseases do this in morning so that leaves dry by afternoon. Avoid drip irrigation―it doesn’t wet the root system uniformly.

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 fall; this will hold heat in the soil and delay the onset of dormancy, increasing the chances of winter damage. And don’t fertilize before bloom―you’ll encourage leafy growth at the wrong time.

The sun tolerance of azaleas and rhododendrons varies by species and selection. In general, most types prefer the partial sun or filtered shade beneath tall trees. The east and north sides of the house are better locations than the west and south. Too much sun bleaches or burns the leaves; too little results in lanky plants that don’t bloom.

Pruning rhododendrons is simple―just follow these general guidelines. Tip-pinch young plants to make them bushy; 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 the plant’s energies will be diverted 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 are in bloom; use 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 base of each truss.

Evergreen azaleas are dense, usually shapely plants; heading back 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. You don’t have to prune azaleas as carefully as you do rhododendrons―the leaves are fairly evenly spaced along the branches, with a bud at base of each leaf, so new growth will sprout from almost anywhere you cut (in either bare or leafy wood).

Info Courtesy of Southern Living http://www.southernliving.com/home-garden/gardens/azalea-plants