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