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Water is the primary need for rhododendrons and azaleas in August. After more than six weeks of summer dry weather, these plants show stress. The welcome rain of the second week in August wasn't enough to saturate their roots. Many of them are native to regions such as the foggy lower slopes of the Himalayas or the eastern U.S. Smoky Mountains, where summer rain falls frequently and copiously. Rhododendrons have fine roots that grow very close to the soil surface. They can dry out fast under adverse conditions.
Use soaker hoses or in-ground drip systems if possible for watering. Rhododendrons and azaleas require deep, slow soaking that penetrates through the entire root ball. A wave of the hose for a minute or two won't do it, even if the surface of the soil looks satisfactorily damp. Take a trowel and check gently around the root ball to see how far the water has penetrated. Gardeners are constantly advised to "water deeply and slowly" but this isn't a simple skill. Check your results and use those slow-delivery systems.
Several readers have commented this month on needing help with ideas to get rhododendrons to "green up." Lack of water can be a contributor to leaf yellowing, because the nutrients that enter roots must be in water to move into the plant. A dry, stressed plant is a plant with nutrition problems as well as its other difficulties.
Another reason for observing and assisting with extra water this month is that the rhododendrons and azaleas are setting flower buds for next spring's bloom, and they require water to form healthy buds. Rhododendrons neglected in July and August will disappoint observers looking for bloom next April, May, and June. Taper off the watering in September to allow the plants to move into dormancy.
Some symptoms of foliage stress can be relieved in some plants by using one of the chelated iron products formulated for broadleaf evergreens. Never apply this to a dry plant! In general, all fertilizers and minerals should be applied when water is available, either through rain or supplementary water. August isn't the month to do a general fertilizing of rhododendrons and azaleas, however. They are usually fertilized once early in spring, and once immediately after bloom, and in all cases by the end of June.
In August the plants are slowing their growth in preparation for dormancy and should not be shocked into excess soft growth by fertilization. (This is true for all trees and shrubs, not just for rhododendrons.) If the rhododendron has been neglected and not fertilized during this growing season, it's better to fertilize it in the mid-winter than to cause too much soft growth now. Some growers start their spring fertilization in late January, repeat once during bloom, and give a final application by the end of June when bloom has stopped.
Mulch, if not previously applied to the plants, can help to retain moisture and keep the surface roots cooler. Don't dump mulch at this time of year over a dry root ball --- water it well first. A layer of mulch that's appropriate for these plants will be no more than 2-3 inches deep, and very porous to allow oxygen to be available to the roots. Bark chips, coarse compost (not hot, unfinished compost), and chipped yard trimmings all make excellent rhododendron mulches. Don't pile it against the trunk or place it too deeply over the roots. Roots need air!
Simple, basic care can keep rhododendrons and azaleas giving results as year-round garden assets. Remember, they are garden features during the 11 months of the gardening year when they are not in bloom.
Hortsense: Managing plant problems with Integrated Pest Management
