According to the United States National Arboretum, azaleas bloom anywhere between early April through June. Peak periods of blooming vary by two weeks or more, but typically take place around the end of April.
Azaleas given too much nitrogen in early summer may not bloom. Skipping fertilizer or limiting fertilizer applications to the fall or spring is a standard practice to ensure flower bud formation.
Azaleas prefer partial shade. However, deep shade often results in fewer flower buds. Planting azaleas under a tree where the plant can receive diffused light under tall, widely spaced trees, such as oaks and tulip poplars, encourages strong blooms.