In Missouri, azaleas should be fertilized in May. They should not be fertilized after July 1, as late summer fertilization can force tender fall growth which will be killed by the winter cold.
Azaleas require little in the way of fertilizer. This type of plant requires a relatively low nutrient level to thrive. A fertilizer with an analysis close to 6-10-4 can be applied at two pounds per 100 square feet. Fertilization must be done carefully, as the azalea's root system is close to the soil surface and can be easily damaged. A good natural fertilizer for azaleas is cottonseed meal.