With correct pruning, azalea (Rhododendron spp.) shrubs will put out striking flowers each spring. This evergreen shrub, which grows year-round in U.S. Department of Agriculture plant hardiness zones 4 through 9, can be pruned in spring, summer or winter, but because next year's buds form shortly after this year's flowers are spent, pruning in spring ensures you won't cut away next year's buds. How you prune azaleas, depends on what you want to accomplish.
Proper pruning starts with the right tools and making sure they're in good condition. Most pruning done on azalea shrubs can be handled with sharp hand pruners for small twigs, but for branches larger than 1/ 2 inch in circumference use long-handled loppers. You'll be able to get a better grip allowing you to make clean, straight cuts. To prevent spreading disease with your pruning tools, soak them in denatured alcohol, available at home improvement stores, for at least 10 minutes before starting to prune. And if the shrub you're working on show signs of disease, spray the pruners thoroughly with a household disinfectant after each cut to prevent spreading disease.
A properly sited azalea fits the space it's growing in and needs only light pruning to shape it up, increase fullness or promote more flowering. You can perform light pruning in mid-summer with hand pruners after new growth is no longer soft and breaks when you bend it. Prune new growth back almost to where you desire new buds. New branches and buds will form just below the pruned site, resulting in a fuller shrub with more flowers next year. This may take multiple prunings over several weeks to get your desired result, because not all new growth will be ready at the same time.
Thinning for Light
Azalea branches that are near the interior of the shrub tend to be shaded and don't get enough light. This leads to branch die out. You can correct this problem by thinning away some of the outer branches. Start by pruning dead branches back to the junction between the dead branch and the next living branch, or back to the main trunk. You can test if a branch is living by scraping away the bark with your thumbnail. If it's green underneath, it's alive. After you remove all the dead branches, prune the remaining branches to the desired shape, usually with the top center branches a few inches longer than side branches. Thinning is best performed in the spring immediately after flowers have bloomed and begun to die. Thinning can be done with loppers or hand pruners.
When an azalea has become tall and spindly, or is just overgrown, it's time to cut it back and start over. Before the shrub puts out new growth in the spring, use a pair of loppers to prune the entire plant back to within 1 foot of the ground. Throughout the summer, the shrub will regrow foliage and form next year's bud so keep the soil around it moist, but not soggy, for about one month. In the spring, just before buds begin to break, prune away suckers from around the base, leaving the most vigorous stems for the shrub to grow from. A properly pruned azalea will have two or three main branches growing at the base.