Azaleas are called "the royalty of the garden" and can be healthy, long-life plants when properly cared for, with some azaleas in Japan growing to be a hundred years old. One of the most confusing aspects of taking care of azaleas for most gardeners is fertilizing–whether you should fertilize, when to do it and what type of fertilizer to use.
Examine your azaleas closely to see how healthy they appear. In general, azaleas require little or no fertilizer, and excess nutrients may promote large populations of pests, like lace bugs and azalea whiteflies. Consider instead having humus in the soil and maintaining an organic mulch around your azaleas. However, if the foliage loses its deep green color, turns yellow, or there is stunted growth, test your soil to make sure the pH isn’t too high. Soil test results should show the amount of nitrogen, phosphorus, potassium and other important elements, accompanied by specific fertilizer recommendations to balance the soil. Check with your county agricultural agent to arrange testing at little or no cost.
If a soil test reveals that nutrients are out of balance, use a slow-release fertilizer that is acid forming, and apply a very light scattering around the edge of the root zone. Make sure the plant is dry before applying fertilizer, or it can burn the leaves, and don’t apply more than a few tablespoons at one time. Spread the fertilizer around the root system of the plant, which can extend beyond the drip line or outer-most branches. Start by broadcasting 4 to 6 inches away from the center of the plant, using a broom or rake to knock any fertilizer off the leaves when you’re finished. Be sure to water well after fertilizing–too much fertilizer and not enough moisture can also burn the plants. Fertilizer granules dissolve slowly and release food evenly and gradually, but liquid fertilizer can be used more often, sprinkled directly onto leaves and around roots, giving a quick nutrient boost. Time fertilizing for spring or fall, but in cold climates, don’t apply nitrogen fertilizer after late June, or it may promote growth susceptible to winter damage. Wait to fertilize after the plants have flowered; otherwise new growth can cover the flowers and blooms won’t be as spectacular.