Source: JARS V48:No.2:p85:y1994
Top Causes of Death in Rhododendrons
Jan D. Kelley
Our hopes soar with the coming of spring as we anticipate another excellent growing season for our rhododendrons with their exquisite flowers. As we ponder the fantastic new hybrids in the pages of the several catalogs that we receive our vision of being successful gardeners bursts forth. However, as you reflect upon last year's plant losses a ray of doubt creeps into your consciousness, and the nagging question emerges: "Why did that plant die?"
For the past 15 years I have enjoyed raising rhododendrons. During that period of time I believe that I have killed rhododendrons in every conceivable way. In the remainder of this brief article I would like to identify some of the various ways that rhododendrons succumb in our yards and gardens. My experience indicates that most rhododendrons die from about seven causes.
To begin with, excessive water kills about 75 percent of all rhododendrons purchased. Rhododendrons are fibrous, shallow rooted plants that need good drainage to perform well. Historically, gardeners have been told to dig a hole twice as wide and twice as deep as the root ball. After the hole is completed put the plant in the hole and back-fill it with a mixture of peat, soil and other amendments. Many rhododendrons die from this guidance. The result of digging the hole and planting the rhododendron in it is nothing more than putting the plant in a bathtub that holds excessive water. The continual presence of water around the root ball prevents the roots from taking in vital oxygen as well as serving as an excellent incubation chamber for fungus diseases. It seems that most of the native soils around the country have an excessive amount of clay in them. The presence of clay in the soil prevents good drainage, which is vital to the growth of the rhododendrons. Anyone who has ever been to the several locations around the world where rhododendrons originate knows that rhododendrons grow in shallow beds of highly organic matter. The drainage is typically excellent.
Another cause of rhododendron death is lack of water. Rhododendrons do not have taproots like trees: their roots grow very near the surface. Therefore, they need frequent watering. The acquisition of new plants in the spring requires regular watering. During the first couple of years watering the plants at least twice a week is a must. As the time goes by and the plants increase in size and root development, watering less frequently works well. After about five or six years it is possible to water weekly or even bi-weekly. Frequently sunburned leaves are the result of the lack of water. For many varieties that have burned in the sun in the past, the cause was lack of water not too much sun. Burned tips on this year's new growth is typically indicative of lack of water as the plant withdraws water from the tips of the new foliage first.
Another cause of rhododendron death is the excessive application of fertilizer. This is particularly true of applying fertilizer directly at the base of the trunk of the plant. A good rule of thumb is to fertilize more frequently with smaller amounts, rather than one large dose. This is especially true for small plants or newly transplanted plants.
A fourth reason for rhododendron death is planting too deep. As indicated earlier, rhododendrons are shallow rooted plants. Their roots grow just below the soil line. If they are placed too deep in the ground, the soil that covers the roots serves to smother them. I have found that planting too deep will basically stop the plant from growing. Eventually this leads to the death of the plant.
Another reason that rhododendrons die is from cold winter temperatures. Most rhododendron sources indicate the lowest temperature range in which rhododendrons can be successfully grown. This hardiness rating is a guide not an absolute! In general, the lowest temperature during the past five years is a good guide for making selections based on hardiness. Years ago there were very few plants that were hardy in -25°F for the extreme climates. Now we have over 100 varieties that will survive those winter temperatures. Gardeners in the East should select hardy varieties in the beginning. With time and experience less hardy varieties can be successfully tried. A rhododendron rated hardy to 5°F, no matter how beautiful it is, planted in Green Bay, Wisconsin, will not survive.
As more and more home gardeners in the Southern and Midwestern states begin to grow rhododendrons, increased attention must be paid to the hot summer sun. Most varieties exposed to unprotected all-day sun are doomed. However, there are available rhododendron varieties that can stand direct sun. In general rhododendrons in extreme climates benefit from filtered light and partial shade. Planting in a southern exposure without any protection from the sun nearly guarantees plant death.
Finally, if you create the right conditions most rhododendrons will be subject to fungus diseases. Typically we combine several fungus diseases into a general category of "die-back." The results of the disease are seen during the late spring when the plant is just beginning to grow and all of a sudden it drops dead. It is also seen during the summer when a branch turns brown and dies. Frequently the ailing plant will be lost. These phytophthora-type diseases are generally the result of conditions created by the gardener, as it is believed that the disease spores are present in the soil all over the country. Some of the ways that we promote these organisms is by planting the rhododendron too deep, thus providing a water culture for the development of the disease organisms. Puddles of water that remain more than an hour after watering also harbor disease. Watering in the late afternoon or evening encourages disease development. Finally, failure to use fungicides during the late spring and summer encourages the development of fungus.
In conclusion, you are not alone if you have lost plants to any of the above mentioned causes of rhododendron death. Most of the causes can be overcome with the intelligent selection of plants that are suited to your geographical area. Finally, think about where and how you planted your rhododendrons and what you did to promote their death.