Do not add fertilizer to the backfill at the time of planting because you take the chance of burning new foliage and roots by high fertilizer salt content around the roots. Once established, azaleas are quite tolerant of doughty soils. However, watering newly planted azaleas is essential. If planted in well-drained soil, frequent watering will be required. Azaleas planted in wet or waterlogged soils are prone to many root rot diseases.
Azaleas grow best with a moderate level of fertility.
For safety sake, split fertilizer applications into three applications with small amounts applied in March, May, and July. Be extremely cautious when using fertilizers with high levels of nitrate nitrogen. Azaleas cannot metabolize this form of nitrogen and plant death can result in some situations. Some folks are repulsed at the thought of pruning azaleas. However, many azalea varieties benefit from pruning.