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Gardening: Spring and summer care for rhododendrons and azaleas

rhododendrons & azaleas by Mary Robson, Area Horticulture Agent,Washington State University
Bright colors, soft hues, and frilly blossoms mean rhododendrons and azaleas in the maritime Pacific Northwest spring. It's hard to imagine spring landscapes without the showy, or subtle, flowers of these plants.
   How can the shrubs be cared for to keep them in good health when the bloom is over and they settle into leafy-green quiet for another year?
   Rhododendrons and azaleas originate in climates with foggy or rainy summers: the Smoky Mountains of the eastern U.S., the foothills of the Himalayan Mountains, and areas in China and Japan. Maritime Northwest summers offer the cool temperatures these plants prefer, but far too little normal summer rainfall to keep them in good health. Be sure they are installed in loose, well-drained, organically enriched soil, and supply extra summer water. Soaker hoses curled around the plants can make the difference between healthy survival and mediocre plants.
   Group rhododendrons with plants that have similar water requirements. They do not need water daily, but do need a deep soaking, all the way to the root level, at least twice a month. Since water conservation is a practical necessity for gardens during Northwest summers, concentrate on applying water efficiently and on having the water-needy plants gathered in one area of the garden. Water needed for rhododendrons and azaleas can be saved by installing low-water use plants in other areas of the garden.
   Mulch is beneficial for retaining summer water, but don't pile deep mulches over the roots. Rhododendrons are shallow-rooted, and need oxygen at the soil surface level. An inch of mulch over the roots would be sufficient. Don't scuffle under the plants with hoes or weeding tools, either. The surface roots are easily damaged. If moving or planting a new rhododendron, be sure the root ball stays at the same level it was in the container or in the previous location.
   Fertilize moderately. One fertilization after bloom, with an acid-based fertilizer formulated for these plants, will be sufficient for summer. (Most growers also fertilize the plants in early spring as they begin to grow.) Do not fertilize a dry plant; be sure to water first, then fertilize.
   Is it necessary to take off all old blooms on the plant? Rhododendrons and azaleas can set bloom for the next year even if the old blooms are allowed to go to seed on the plant, but most gardeners remove the old flowers to improve the overall appearance of the shrub. Look carefully at the plants when snapping off old flowers. The new growth emerges quite close to the old bloom stalk and it's easy to break off new growth with old seed heads.
   Pruning and shaping these shrubs is best done now, during bloom and right after bloom. These plants aren't harmed by pruning‹in fact, they benefit from being trimmed back to keep the branches from getting floppy and lanky. Prune back to a whorl of leaves or to an obvious growth point where new green growth is emerging. The plant will fill in rapidly if properly pruned.
   Don't chop off a branch in the middle where there is no green growth bud. The buds far down on the stems are small and easy to miss; they are rounded and small, slightly larger than a heavy pencil point. If the stem is pruned back to one of the growth buds, it will put out a new whorl of leaves. Rhododendrons need good cultural care after pruning, so don't neglect the summer watering.
   In mid to late summer, the plants form new flower buds for bloom the following spring. Pruning too late removes these new buds, so be sure to trim and prune rhododendrons and azaleas now. The quality of next year's bloom depends on the summer care and the supply of moisture the plant receives. To repeat, these plants do not need to be splashed with water every day, but they do need to have the roots deeply watered at least two times a month. More watering may be needed if the plant is in an exposed, hot position, particularly where it gets bright western sun in the afternoon. Plants that are located like this may show signs of scorching or stress. If they do, move them to a more protected location when the plant stops growing in fall.
   All rhododendrons and azaleas will be more satisfactory landscape plants when they receive necessary basic cultural care through the summer. And since these plants are primarily leaves 11 months of the year, think of selecting rhododendrons for the beauty of the leaf as well as the handsome flowers. One species, Rhododendron yakushimanum, with many beautiful cultivars, has beautiful leaves with soft brown felted undersides.
   Start looking beyond the bloom to the value of the entire shrub in the landscape year round.

Visit WSU's Agriculture Site on the World-Wide Web at http://www.cahe.wsu.edu.