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Journal ARS Article

 Vol. 46: No. 3: Year 1992

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Tips for Beginners: Rhododendron Nutrition Notions

Ted Van Veen
Portland, Oregon

Quite likely the most frequent inquiry about rhododendrons is, "When should I fertilize?"  It's a question many of us feel uncomfortable about answering with complete assurance.  We are concerned because the question implies frequency of application, formulation recommendation or even the possibility of no need for fertilizer.  Then, too, we realize a satisfactory response will require weaving together many variables including soil complexities, temperature ranges, light conditions, rainfall, water quality, age of plants and prior nutrition practices.  To more precise, each species or hybrid could demand a specific individual formula.  Thinking about this overall picture can be a bit frightening.

Were we to query ten different experts about the subject of fertilizing rhododendrons, we would receive ten diverse answers ranging from "never" to a complicated year-round program.  One authority states, "Fatten your rhododendrons with ample food and they will break into smiles of blooms each season."  Another grower facetiously responds, "Limit your fertilizer applications to a few hummingbird droppings once a year.  Too many rhododendrons are suffering from acute indigestion brought on by the force-feeding of assorted plant foods."  And, yet one other rhododendron buff wisely states, "Fertilizing is better left undone than overdone."

By this time you have surmised that established, healthy plantings probably need little or no fertilizer, and this is basically correct.  Research studies have brought out that, in general, ericaceous plants have low nutrient requirements compared to most ornamentals.  However, some basic elements are needed for survival.  An ambitious rhododendron lover will pick up the challenge for a perfectly balanced program and delight in a happier, healthier plant.

Liberal doses of nitrogen may have been poured on our beloved rhododendrons and still the foliage assumes the appearance much like 'President Roosevelt' or look as if we should correct the label to read "variety aureafolium."  Our impulse is to give a good dose of salts.  If the veins are green, it could mean a magnesium deficiency, and a drench of Epsom salts (magnesium sulphate) might very well be the solution.

Rhododendrons require phosphorus, as well as nitrogen, and adequate sunlight to produce flower buds.  Not always understood is the length of time required for phosphorus to reach the root system and be taken up by the plant.  As long as six months may be necessary for this process.

Potassium deficiency can be difficult to diagnose.  A plant must have sufficient iron to utilize available potassium.  For this reason the symptoms of potassium and iron deficiency are almost identical in the initial stages.  Potassium deficiency begins with leaf yellowing, which eventually spreads between the veins.  Leaf tips and margins show scorch and necrosis.

Soil Testing
The more avid gardener should test his soil for deficiency or surplus of basic elements.  Sometimes the soil of a particular location is known for excessive boron, for example, or for low phosphorus content.  A further problem could be the water supply which might contain too much salt or some of the heavy metals.  The zealous grower should concern himself with this potential problem.

Most authoritative books and articles state that rhododendrons are acid lovers and that the pH of the soil should be between 4.0 and 4.5.  This paper is not intended to be an all-conclusive scientific treatise, but our experience in the nursery indicated rhododendrons to be acid tolerant only.  A pH of 5.5 to 6.5 has been quite successful in our operation.

A wide selection of the native species originating in Asia grow in mountains of dolomite limestone where the pH reading approximates 6.0.  The addition of dolomite, which is a combination of magnesium carbonate and calcium carbonate, to our plantings darkens foliage color and increases flower buds.  Gypsum, the common name for calcium sulphate, is another type of fertilizer some gardeners seem to have used successfully to improve the quality of their rhododendrons.

Trace Elements
In addition to the major elements, most gardeners are familiar with iron and magnesium deficiency, but all do not realize a rhododendron requires some, but often only a minute amount of boron, manganese, zinc, molybdenum, copper and possibly aluminum.  Most of these elements are usually in the soil, but if not available, they could be the cause of poor rhododendron performance of some kind.  Many of the more expensive fertilizers incorporate these trace elements.

A frequently asked question pertains to the use of manures.  Yes, well rotten manures may be used with caution.  These products release nutrients gradually and supply humus to the soil.  However, some manures have been treated with agricultural lime or harmful chemicals to destroy flies and odors, and they sometimes introduce weed seeds.

There is little doubt many rhododendrons are sickened by excessive pampering.  Most of our rhododendron troubles are man-made.  Have a bit of sympathy for them, but practice careful neglect with established rhododendrons which are performing as you think they should.  Keep your precious plants pleasantly moist in reasonably good soil and they will depend on you for little else.  An alert rhododendron enthusiast can easily detect signals from the plant when it is not well.

Be consoled, you indulgent owners of overfed rhododendrons, a pair of sharp pruning shears can reduce their size and shape with dispatch


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