Watering: A First Priority
Tips for Beginners
H. Edward Reiley
Reprinted from a subchapter in Mr. Reiley's recent book Success with Rhododendrons and Azaleas, Timber Press, 1992, in the chapter "Care in the Landscape"
Rhododendrons and azaleas are, by virtue of their broad leaves, constantly losing water through transpiration, and an adequate supply of water is the first priority in their maintenance. "Adequate" is indicated by maintaining a uniformly moist soil. In hot, dry areas where moisture loss is high, a steady, dependable source of water is required.
Old, established plants can survive long periods of drought in late summer when they are no longer in soft active growth. Though leaves may wilt for many consecutive days, no damage is indicated as long as the foliage is turgid again by morning. If foliage is still wilted in the morning, a deep soaking is called for. Sustained daytime wilting over lengthy periods, even though turgor returns overnight, will stress established plants. Consequences of such stress may be the appearance of disease, twig blight, or dieback, as well as symptoms of mineral deficiency in the leaves. A water stressed plant is more susceptible to insect and disease attack as it is not absorbing the soluble nutrients necessary for good health. Water stress induced problems are sometimes reduced by selecting more appropriate plants. Gable's 'Caroline' or any of the R. yakushimanum species or first generation R. yakushimanum hybrids seem seldom to suffer dieback or twig blight under severe stress.
Misting the foliage of established plants during the heat of the day can extend the period between waterings1. Mist nozzles controlled by a timer set to the appropriate cycle to prevent serious wilting require relatively small volumes of water.
Newly transplanted plants, on the other hand, require close attention to watering. They cannot be allowed to wilt for any extended period of time. When in active growth, soft, new foliage may wilt on hot afternoons, especially in full sun, but if the foliage recovers an hour or two after sundown, no serious damage will result. Misting new growth during the heat of the afternoon is very beneficial and poses no threat of creating overly wet soil. The root ball and the soil around and below newly transplanted plants need to always be kept moist. The amount of water needed to achieve this condition varies with ambient and soil temperatures, evaporation, and other factors, so check the soil beneath the mulch weekly to be sure the soil is moist but not wet.
Watering of newly transplanted container grown plants differs somewhat from that suitable for field grown plants. The soil around the roots of field grown plants dries out more slowly, and thereby plants require less frequent watering. Conversely, the coarse medium surrounding the roots of container grown plants holds little water and will dry out quickly. Container grown plants must therefore be watered every 2-3 days with just enough water to thoroughly wet the root ball. The soil around the root ball needs to be watered only when it starts to dry, not nearly as frequently as the root ball.
The death of newly transplanted plants, and especially container grown plants, can most often be traced to inadequate watering and hence drying out of the root ball. Extra attention to the water needs of newly transplanted plants is necessary for up to three years depending on how quickly the roots extend out into the soil. The intensity of care declines each year, and the frequency and quantity of water can be lessened as the root system develops.
Reduced watering of both old and new plants in late summer and early fall is necessary so that a new growth cycle is not encouraged late in the season. Tender, new growth induced by watering late in the growing season cannot harden off before freezing weather. Vigilance is still in order in the case of new transplants, and in the event that continued heat and sunlight lead to extremely dry soil conditions, water will be needed to maintain plant health and continued root development2.
Water quality needs monitoring if extensive irrigation is required. Water with a high pH will cause soil pH to move above the level suitable for best plant growth. Water should not be used that is from sources containing root rot organisms or herbicides. The water quality of flowing streams is usually sufficiently high that no serious problems arise, yet it is best to consult local growers or other knowledgeable persons to be certain.
1 Editor's Note: For more information on mist systems see article by Jean Minch, "Armchair Watering", Journal ARS, Vol.41, No. 2.
2 Editor's Note: Dry late summer and early autumn conditions are common in the Western U.S., and vigilance in watering, as Mr. Reiley suggests, is often necessary at this time (see ARS Quarterly Bulletin, Vol. 28, No. 3).