Azaleas (Rhododendron spp.) vary in their winter hardiness and heat tolerance, but generally grow in U.S. Department of Agriculture plant hardiness zones 5 through 8. Many varieties are native to North America, while others come from Asia. It is acceptable and advisable to prune azaleas in late spring, especially if the plant has become overgrown.
When to Prune
Azaleas set flower buds for the next season just a few weeks after they bloom in spring. So, most sources generally recommended you prune them shortly after the spring bloom cycle is complete to avoid accidentally removing next year's blossoms. You may choose to prune azaleas in late fall or late winter to early spring. If you do so, understand you will sacrifice at least a portion of the upcoming season's blooms.
Tools to Use
Don't use hedge clippers or electric shears to prune azaleas. Pruning them with that type of tool prevents them from developing their natural, graceful shape and increases the amount of annual pruning maintenance needed, unnecessarily. Use sharp, hand-held pruning shears for light pruning of branches 1/2 inch or less in diameter. For a seriously overgrown azalea, you can use loppers to cut back larger branches fairly deeply into the basal growth. Azaleas tolerate this reasonably well and generally produce new shoots quickly, filling out again by summer.
How to Prune
Use one of two methods to prune azaleas. Thinning involves using hand pruners to remove small side branches and spent flower clusters. This method can be used to maintain your shrub's shape and remove any errant, leggy branches. Heading involves more drastic cuts to larger branches, often down to the main trunks of a mature azalea. This method may be a better choice for an overgrown shrub or one that will benefit from nearly complete rejuvenation. After heading, it may take an extra season for your azalea to reach full bloom production.
Keeping Azaleas Healthy
You generally don't need to fertilize azaleas. Azalea root structures are generally shallow and broad, so they benefit from an annual application of mulch to help conserve soil moisture. The United States National Arboretum recommends using pine needles, chopped pine bark, shredded leaves or leaf mold.