When to Fertilize Azaleas & Rhodies
Azaleas and rhododendrons are not heavy feeders, but they benefit from a yearly application of fertilizer. A formula for acid-loving plants, cottonseed meal or a general 6-10-4 formula applied on the soil surface is recommended. Fertilizers supplying nitrogen in the ammonium form are preferred.
Fertilizing When Planting
Azaleas and rhododendrons do not normally need fertilizer at planting time. If your soil test indicates a need for phosphorus, you may mix a superphosphate or triple-phosphate fertilizer into the soil before planting.
Fertilize While Dormant
You may fertilize in late fall after a hard freeze or early in the spring while plants are still dormant. A light side-dressing may be applied near each plant, with caution to avoid damaging tender roots near the surface of the soil.
Fertilize in Spring
You may also fertilize in spring after new growth appears. Do not fertilize after July 1. Fertilizing too late in the summer will stimulate fall growth, which will be too tender for winter cold.