Rhododenutrition
Dave Goheen, Camas, Washington
Sometimes we may be inclined to forget, or perhaps a better word is overlook, the fact that all rhododendrons are living organisms - collections of myriads of living cells, put together in unique ways, delightful to our eyes and restful to our souls. As for all living beings, each unit has its own special requirement for energy, water, organic food and minerals. Each cell, in effect, creates its own little domain where it will strive to obtain the various elements necessary for it to thrive, be fruitful and multiply.
In almost all of our gardens, water is either plentiful or can be supplied readily. Energy is taken care of by an unfailing source in the sun. With seemingly miraculous ease, cells in the plant's leaves gather this energy and, by chemical reactions only now beginning to be understood, readily convert water and gaseous carbon dioxide from the atmosphere into the basic organic food, the sugar glucose, from which all other organic food elements are derived. Thus, it would appear that rhododendrons and other plants have it made. Everything seems to be available just for the taking.
Alas, this idyllic situation has a catch. The catch is the fact that, in addition to the endless supply of free things from the atmosphere, each cell requires varying amounts of mineral elements and these can only be obtained from the soil through the action of specialized cells in roots and rootlets. These mineral elements are generally required in relatively minor amounts but, even though the requirements are not great, they are exceedingly important. Rhododendrons, like almost all plants are static creatures, destined to stay rooted in one spot throughout their life-span.
Thus, if the particular area in which a plant develops is deficient in one or more of a number of mineral substances, varying degrees of malnutrition can become evident. To make matters even more trying, the ability of each plant to take up and utilize the minerals is greatly dependent on the acidity or lack of acidity (pH) of the soil medium. Another complicating feature is that most plants will thrive better if the soil is filled with large, complex organic molecules which result from decay of vegetative matter. The large molecules hold on to minerals keeping them from being leached away by rainfall and making them more available to growing, searching rootlets.
So, we see that in contrast to the free and easy absorption of the abundant materials from the atmosphere, the underground portions of the plants are in a constant, almost frenzied, search for elements necessary not only for growth but for the plants' very survival.
We have long known about requirements of the three most necessary plant nutrients, nitrogen, potassium and phosphorous. All of these are required for cell growth, genetic material formation and energy transformations. Magnesium and iron are needed for chlorophyll production to allow leaves to convert water, carbon dioxide and solar radiation to food. In late years, we have learned that a number of so-called trace nutrients are necessary for the production of what are called enzymes - the complex compounds that allow each cell to function as a little chemical factory. Thus, if a plant is unfortunate enough to find itself in a relatively barren spot of ground, we can supply the lacking elements by application of modern compounded fertilizers. Plant nutrition has come a long way.
There is still much to be learned, however. For example, since rhododendrons developed in well watered and well leached mountain regions, they have developed mechanisms that function well only on the acid side of neutrality, that is at pH values less than 7. So we have always called them lime haters. Growers, associating lime (and high pH) with calcium, have studiously avoided adding calcium to rhododendron environments. It has recently been found that calcium is not a rhododendron enemy at all but is important in their metabolism. What is very important is that, if calcium is present as lime or limestone, the pH will be on the alkaline side and this condition will block the plant's ability to absorb and utilize important nutrients such as iron and phosphorous. If a rhododendron encounters such a soil situation, it will literally starve to death.
We now have learned that the necessary calcium ions can be easily and safely provided by application of calcium in the form of gypsum. With this material, the pH will always be below 7 and no interference with iron and other essentials will take place. Cy Ward has pioneered the gypsum treatment in this area. I would venture to say that an application of agricultural gypsum to a yellowish, unhealthy appearing plant will do more to improve its appearance and vigor than any single other treatment. Not only will the yellowish color be deepened to a much healthier green, but the brownish leaf spots so commonly observed on such varieties as Loderi King George and others will almost miraculously disappear! Try it, you'll like it and so will your plants!
So it goes. The nutritional requirements of our favorite plants are gradually being resolved. One final item that recently caught my attention in the Quarterly Bulletin of the Society for Growing Australian Plants is worth mentioning. In this article, the possibility of controlling many plant insect pests through fertilization techniques was suggested. This ingenious idea is to add large amount of potassium to the soil medium. It turns out that plants are generally very tolerant to excessive potassium take-up, but the pests that feed on them are not! Now, how about the control of our ubiquitous adversaries, the hordes of root weevils, not to mention white flies and who knows how many others, by generous dollops of muriate of potash? I am planning to try this on weevil susceptible varieties the next growing season.
This surely will be the ultimate in "rhododenutrition''. Not only will we be able to have healthy, deep green plants, but unsightly notching from weevils and curling from aphid attacks might become things of the past. Think of it! The Utopian ideal of all rhododendron growers may be just beyond the next bags of gypsum and muriate of potash - large, healthy, deep-green, unmarred plants merely by the application of a few handfuls of inexpensive nutritional elements! I'm a dreamer and, hopefully, other growers may be too. Care to join me in experiments in "rhododenutrition"?