In fertile soils rhododendrons and azaleas can be grown without receiving further fertilization.  In less fertile soils, a complete fertilizer designed for acid-loving plants may be applied in late winter or early spring.  Be careful to use only the amounts recommended for rhododendrons and azaleas, which do not need as much fertilizer as other plants.  Excessive fertilization can result in damaged roots and leaves, and some rhododendron varieties can be killed with fertilizer.

In cold climates, nitrogen fertilizer should not be applied after late June as it may promote new lush growth susceptible to winter damage.  Recent research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved.

If plants are mulched with materials like fresh sawdust or wood chips, there will be a nitrogen demand caused by the decomposition of these materials, and unless nitrogen fertilizer is added, the plants are likely to show yellowish foliage and poor growth.  In this case, an organic nitrogen fertilizer, such as cottonseed meal, canola meal, fish meal or blood meal, can be added.  Mineral nitrogen fertilizers are associated with increased problems, such as chlorosis where leaves produce insufficient chlorophyll.  Mulches other than fresh sawdust or wood chips are recommended, then you don't have to be concerned with exactly how much fertilizer to add.

Phosphorus is required in the production of flower buds.  If your soil is deficient in phosphorus and since phosphorus does not readily move through the soil, phosphorus should be incorporated into the soil at planting time if needed.  Do not use phosphorus fertilizers unless a soil test indicates a deficiency.  The popular perception that rhododendron and azalea flowering is enhanced by phosphorus is incorrect.

Magnesium in the form of Epsom salts is sometimes recommended for rhododendrons.  Magnesium is an essential element and lack of it will cause yellowish areas between green leaf veins on older leaves.  If the leaves are a solid green the addition of Epsom salts would not be useful.

Lack of iron causes much the same symptoms as lack of magnesium, but with the younger leaves showing yellowing between the green-colored veins.  Iron deficiency is frequently caused by too high a soil pH, often the result of mortar or mortar building debris in the soil near the roots.  A soil test should be performed to see whether high pH is a problem and if it is the soil should be acidified.  For a quick but temporary solution, ferrous sulfate can be added to the soil or chelated iron can be sprayed on the foliage, but the soil pH should be corrected for long term good growth.

Calcium is also essential to good rhododendron growth.  Calcium can be obtained either from gypsum or from agricultural lime.  Gypsum will not raise soil pH, while lime will, therefore, lime is not generally recommended in areas with naturally alkaline soil or water.

Rhododendrons and azaleas require minute amounts of boron, manganese, zinc, molybdenum, and copper.  Most of these elements are usually in the soil, but if not available, they could be the cause of poor performance.

More information about rhododendron and azalea fertilization can be found in the following resources:

  Rhododendron Nutrition by Ted Van Veen
  Feeding Rhododendrons Organically by Terry Richmond
  Rhododendron and Azalea Fertilizing by Steve Henning
  Phosphorus and Nitrogen Nutrition of Rhododendrons by George F. Ryan
  Vermicomposting by Soni Cochran