What Price Deep Green Color?
By G. G. Nearing
We have been told repeatedly that rhododendron leaves turn yellow (exhibit chlorosis) because the plants lack iron. It is true that lack of iron will cause such discoloration, but so will the want of any one of a number of substances, and so will the presence of lime. The most common difficulty resulting in chlorosis is not so much the absence of iron as it is a deficiency of organic matter in the soil and the absence of a proper mulch.
Leaves may also turn yellow as a result of the use of fertilizers, or of certain highly recommended modern sprays. In fact such yellowing is usually a sign of general ill health. But we should not leap to the conclusion that deep green color is a sign of good health, for in certain circumstances it can be a precursor of death. If the green comes naturally, it is usually good. If induced by chemicals, it may end in disaster.
Leaf color differs markedly from one hybrid variety to another, and unhappily, the varieties with most attractive shades of green in their foliage are usually not fully hardy, while the outstanding ironclads, capable of enduring the severest weather our climate has to offer, commonly show dull coloration, with a yellowish or brownish cast. These tints are often mistaken for chlorosis, whereas they are actually a part of the plant's response to climatic conditions, and are necessary to its health, even to its survival.
Mistaken use of chemicals to correct this discoloration and give the shade of deep green which all of us prefer, may result in severe injury to the plant, not only because such chemicals are usually poisonous, but because the plant is thereby deprived of some of its protection against heat and cold. A variety capable of maintaining the favored shade of green and at the same time resisting our climate, would captivate the gardening public at once. Catawbiense Album (the hybrid) approaches perhaps nearest to this ideal, but does not reach it,
To understand the problem, you must study the physiology of the rhododendron. The green of leaves is due to the presence of chlorophyll, an absolute essential to all growing vegetation except the odd few parasites and saprophytes, and the fungi. Even these live on the food produced by the chlorophyll of other plants. Chlorophyll is treasured and guarded by hardy plants, because if lost it cannot be replaced quickly, but requires a rather long and exhausting operation to manufacture. Iron is necessary for its development, but so are many other elements.
In winter part of the chlorophyll is withdrawn from the leaves and stored in the bark as a precaution, so that if the leaves should be destroyed, there will be a reserve from which new leaves can be supplied in spring. This partial withdrawal gives the leaves a dull brownish shade of green. Even in this condition the foliage functions in the warmer spells of Winter, performing photosynthesis and making sugar to nourish the stems and roots, and also to act as an anti-freeze in the sap. But the rate of food production is slow. After the first really warm spring weather, these same leaves take on a much richer hue of green. The sap has brought back to them most of the stored chlorophyll, and they are ready to function at top speed, providing nutrients for the impending burst of flowers and new foliage.
Then summer comes, and again the green is dulled with brownish tints, this time due to deliberate pigmentation. Intense sunlight can burn the substance of the leaf and destroy its chloroplasts. Therefore the plant deposits little brown granules above the chlorophyll layer, to shield it from the sun, furnishing needed shade within the leaf. Instead of seeing the green chlorophyll as you look at the semitransparent cells of the leaf, you see mostly these granules of grown pigment, with only a glimpse of the green which they are created to conceal. If by chemical manipulation you persuade the plant to re-dissolve these brown granules, you will see the deep green color to be sure, but so will the sun, and some hot day it may burn out the chlorophyll and destroy the leaf.
Rhododendrons sheltered in a woodland do not need so much protection from frost and sun, therefore keep greener foliage both winter and summer, than if exposed to full sun. But their growth in dense shade is not compact, and they do not flower well. For best results, moderate shade makes an effective compromise.
We must understand therefore that a rhododendron maintains a delicate balance in which the shade of green of the leaf protects it from our extremes of climate. Any treatment which induces too deep a shade of green, merely exposes the plant to impending ruin by frost or sun.
If the soil actually is deficient in iron, this can be supplied by throwing a little rusty scrap on the mulch, and this will help to hold the mulch in place. It will not cause a sudden change of color to deep green, and you do not want it to do that, but flakes of rust will gradually drop off and find their way into the soil, supplying all the iron that is needed. I am now using rusty tin cans crushed flat, which are particularly efficient in preventing the oak leaves from blowing away.