It's hard to resist rhododendrons and azaleas. There are about 900 species found throughout the world ranging in size from 80-foot trees to low-growing plants that creep along the ground. Most have truly spectacular blossoms.
They grow at sea level and in high altitudes, in rain forests, mild maritime climates and even in alpine tundra. The one thing most have in common is their love for a good acidic and organic soil — a tendency that causes problems here in Utah where the soils are alkaline.
Jerry Goodspeed, Utah State University Extension horticulturist, says rhododendrons and azaleas take a lot of extra effort and dedication. "They are not plants for the casual gardener. The biggest trick is creating a micro-environment around them, fooling them into thinking they are actually in South Carolina or Western Washington."
The first step is to amend the soil, Goodspeed says. Add plenty of organic matter at planting time. Peat moss is probably the product of choice, with coco peat, leaf mold and compost also good additions. These materials should be mixed into the soil before planting and later used as a thick mulch layer yearly to keep the soil rich in organic material.
The next concern is allowing for proper drainage, which rhododendrons and azaleas require, he says. They are susceptible to root rot and iron chlorosis, which is aggravated by excess water. Never leave standing water around their roots.
Rhododendrons and azaleas are shallow rooted plants, Goodspeed explains. Be certain they remain that way by not planting the root ball too deep. Because of their shallow root system, keep them moist but not wet.
"Grow these beautiful plants in a protected, partially shaded location," he says. "Hot summer sun and dry winds will burn their leaves. An eastern exposure is probably the best, with cool light in the morning, and protection from heat in the afternoon. However, they will not bloom as prolifically if planted in complete shade."
Fertilize every year in the early spring with an acidic fertilizer, Goodspeed says. This actually does little to improve the acidity of the soil, but every little bit helps. Some nurseries even sell special rhododendron and azalea fertilizer.
The flowers are produced in the late spring or early summer, he explains. After blooming, pull off the spent blossoms. This encourages vegetative growth instead of allowing the plant to put energy into seed production. When pulling off the spent blossoms, be careful not to damage the new growth that grows right below the flowers.
"I am often asked if floral azaleas, grown and sold as potted plants, can be planted outdoors in the landscape," Goodspeed says. "These plants, grown in a greenhouse for two years and forced to flower, are beautiful when in bloom, but usually make lousy landscape or houseplants.
"After they are through blooming and you have enjoyed them in the home, quickly put them out of their misery by tossing them in the garbage. If this is too difficult, they can indeed be planted in the landscape, but they will eventually die."
For more information, contact your local USU County Extension office.
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Issued in furtherance of Cooperative Extension work, Acts of May 9 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Robert L. Gilliland, Vice-President and Director, Cooperative Extension Service, Utah State University, Logan, Utah. (EP/09-98/DF)