Journal ARS Article
Vol. 46: No. 3: Year 1992
Tips for Beginners: Rhododendron Nutrition Notions
Ted Van Veen
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.
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.
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