When to Fertilize Azaleas
There are thousands of varieties of azaleas (Rhododendron spp.), the most hardy of which can be grown as far north as United States Department of Agriculture plant hardiness zone 3. Other azalea varieties grow well in climates that do not experience frost, to zone 10. With such a wide climate range, there is an azalea for every region. With few fertilization requirements, azaleas are a low-maintenance flowering bush. To keep azaleas healthy and blooming, keep them well mulched and make sure the soil is slightly acidic.
Improving Soil pH for an Azalea
The key to keeping azaleas healthy is having the right soil acidity. Soil that is too acidic or too alkaline will not produce the best blooms. Azaleas like a soil pH of 4.6 to 6.0, which is slightly acidic. Garden centers sell soil-testing kits which can determine the acidity of soil around an existing azalea or an area where an azalea is to be planted.
For soil that is too alkaline, above 6.0, add 3 ounces of elemental sulfur per azalea plant to lower the soil pH one point. This can be done once a year until the ideal pH is reached. For existing azalea bushes, both recently planted and well-established plants, work the sulfur into the top few inches of soil around the plant in an area about 6 inches to 12 inches away from the stem. Be careful not to damage plant roots.
It is rare for soil to be too acidic, but if it is below 4.5, add 12 ounces of lime per azalea plant to raise pH by one point, working it in the same way sulfur is added once a year.
Planting New Azaleas
Test soil acidity before planting new azaleas and add sulfur or lime as needed at the amounts listed previously. Work the amendments into the top few inches of soil before planting. Ideally, amendments are worked into the soil a few weeks before planting, but can be added up to the day of planting if necessary.
While azaleas are not heavy feeders -- they do not require a lot of nutrients in the soil to perform well -- they may display signs of a nutrient deficiency. If leaves start turning yellow between the veins, feed them a fertilizer specially formulated for azaleas, such as a 30-10-10 mixture at a rate of 1 tablespoon per gallon of water per plant, but follow label directions. Fertilize established plants one time when they are dormant, between late fall and early spring. Do not fertilize newly planted azaleas, as this may stress the plant by forcing it to put out too much new growth on roots and stems too quickly.
For azaleas that are performing well and do not show signs of nutrient deficiency, it is not necessary to fertilize. Keep azaleas mulched with 4 inches of organic matter such as pine needles, leaves or wood chips to provide a steady supply of decomposing matter to feed the bush. Do not mulch with inorganic mulch like rocks or rubber mulch. This can cause the soil to retain too much water and does not provide the necessary nutrients.