Nutritional Requirements of Rhododendrons
Dr. F. W. Herrick, Shelton, Wash.
Part of a talk given Mar. 21, 1967 before the Shelton Chapter.
Rhododendrons should be grown in porous, well aerated and well drained soils containing a good quantity of organic material. The soil should be acidic (pH 4.5-6) and maintained in a humid condition. Organic mulches may be added on the surface to help retain soil humidity. Many types of mulches may be used as peat moss, sawdust, leaves or conifer needles, or well-decomposed manure. Such mulches will contribute to soil fertility and good plant growth as they become part of the soil. Under average conditions, rhododendrons require only a limited amount of fertilizer and, particularly, very little nitrogen. Phosphorus is essential to flower bud development.
These three elements - nitrogen, phosphorous and potassium are essential to the annual growth cycle of all plants. Rhododendrons utilize nitrogen principally in the form of ammonia or its salts, and possibly as urea and as amino acid or other nitrogen compounds derived from such protein sources as liquid fish fertilizers. Phosphorus is utilized as "phosphate" salts or as dilute phosphoric acid. Potassium is utilized as one of several soluble salts such as potassium chloride or sulfate. Several commercial fertilizers that are especially blended for rhododendrons contain N-P-K in percentages expressed as 4-8-8 or 4-12-12, and are satisfactory for general use. Small plants (6-inch diameter) are able to tolerate as much as one teaspoon of such fertilizers. A plant three feet in diameter can utilize one cup, this being uniformly spread on the ground in the area under the foliage, which corresponds to about the same area of root growth. By blending concentrated components like urea, ammonium phosphate and potassium chloride it is possible to prepare concentrated fertilizers that will be less expensive than commercial blends, but must be used very sparingly. Liquid fertilizers are also available. Young rhododendrons benefit from dilute fish fertilizer (one tablespoon per gallon of water) applied as a water spray, once or twice a month.
Other Required Elements, Micronutrients
Sulfur, though required in appreciable amounts, is normally present in most fertilizers or soils in adequate amounts. Powdered sulfur or sulfate salts (ammonium sulfate) are often used to provide increased soil acidity. Several "metal" elements are required for proper plant growth and development. Although traces of iron and magnesium are required by rhododendrons generally, it is also believed that even smaller traces of zinc, manganese, cobalt and copper provide needed requirements. All of these "micronutrients" can be provided in the form of water-soluble complexes that are more easily assimilated by the plant. Many products are available either in fertilizer blends or as separate products to supply these plant needs. Special micronutrient treatment by water spraying very dilute solutions together with insecticides or other materials is recommended where plants are obviously "chlorotic."
Organic Matter, Fungi
Organic matter in the soil and especially "humic acids" are now believed to be contributors to plant nutrition via soil conditioning, metal complexing and other reactions. This organic material also functions to support soil fungi which, in the case of rhododendrons, are a necessary part of the nutritional or food cycle. The so-called "mycorrhizal" fungi grow around and in the rhododendron root system and provide part of the food utilized by the host plant. The benefits of good organic mulches are thus eventually noted in better soil condition and growth response in the plant.
Application of Fertilizers, Timing
Rhododendrons should be fertilized early in the year (February, March, April) so that phosphorous is available prior to the beginning of new plant growth, which will contain the nascent flower buds, if other conditions are favorable to bud formation. A second light fertilizer application may be made after blooming to insure adequate growth. However, no fertilizer should be used after about July, since the plant must complete its growth cycle and be hardened well in advance of fall frosts. Some rhododendrons are quite sensitive to excess fertilizer which will cause leaf burn if too much nitrogen is present. Mulches may be applied throughout the winter, spring and summer, but should be minimized during the fall to aid in hardening the plant for the next winter.