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Fertilizing

In fertile soils rhododendrons and azaleas can be grown well without receiving further fertilization.  However, if plants are mulched with something like fresh sawdust or wood chips, there will be a nitrogen demand caused by the decomposition of these materials, and unless nitrogen is added, the plants are likely to show yellowish foliage and poor growth.  In this case a fertilizer such as ammonium sulfate should be added.  It is safer to use mulches other than those containing fresh sawdust or wood chips, then you don't have to be concerned with exactly how much extra fertilizer to add, as excess fertilizer can harm your plants by "burning" the roots.

R. 'Graf Zeppelin'
R. 'Graf Zeppelin'
Photo by Richard Gustafson
Copyright 2000, ARS

For rhododendrons planted 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 normally require less fertilizer than plants such as grasses and vigorous shrubs.  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.

Phosphorus in fertilizer tends to favor early 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.  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 the 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 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 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.

  Index of Topics:
  Botanical classifications  |  Use in landscape  |  Plant selection  |  Climate  |
  Protection  |  Soil  |  Planting  |  Subsequent care  |  Fertilizing  |  Pruning  |
  Insect & disease control  |  Propagation  |  Transplanting  |


American Rhododendron Society
Executive Director: P.O. Box 525,  Niagara Falls, NY 14304
Ph: 416-424-1942   Fax: 905-262-1999   E-Mail: lauragrant@arsoffice.org
1998-2014, ARS, All rights reserved.