Buds and Blooms nursery publishes a newsletter called The Blooming Journal. The following appeared in the fall/winter 1996 issue and was based upon an article in the May issue of the American Rhododendron Society. Doug Torn from Buds and Blooms gave me permission to share this with the agents in NC. You may not agree with every word but he certainly covers a lot of bases regarding rhododendron culture.
Gardening Tips for Rhododendrons
The number one reason most Rhododendrons die is excessive water. Rhododendrons are a fibrous, shallow rooted plant that requires good drainage to perform well. General planting guidelines, (plant in a hole twice as large as the rootball and back fill with a mixture of peat, soil, and other amendments), have resulted in many plant failures. The result of these guidelines places the rhododendron in nothing more than a bathtub, holding excessive water, preventing the roots from receiving oxygen and forming an incubator for fungus diseases. I recommend back filling with a mixture of 50% pine bark mulch and 50% topsoil, be sure to leave the top of the root ball two inches above the ground and mulch well around the plant.
On the other side of the spectrum, another cause of rhododendron death is lack of water. Rhododendrons do not have tap roots like trees. Their root system develops very near the surface, requiring frequent watering. With new plants, regular watering is critical during dry spells for the first several years. Burned tips on this years new growth is typically indicative of lack of water as the plant withdraws water from the tips of the new foliage first.
The excessive application of fertilizer directly at the base of the trunk is another cause of death for rhododendrons. A good rule of thumb is to fertilize with smaller amounts more frequently out from the base of the trunk, rather than one large dose at the trunk base. This is particularly important with small and newly transplanted plants.
Another reason that rhododendrons die is planting too deep. As shallow rooted plants, their roots grow just below the soil line. The soil that covers the roots can serve to smother them. Planting too deep will stop the plant from growing and will eventually lead to its death.
Cold winter temperatures will cause plants which are not hardy enough for the region to die. The hardiness rating is a guide not an absolute. In general, the lowest recorded temperature for the past five years is a good guide for making selections based upon hardiness. Years ago very few varieties were hardy to - 25 F, today over 100 varieties will survive those winter temperatures. Beginning gardeners should select hardy varieties, and with time and experience less hardy varieties can be successfully grown.
As more southern and midwestern gardeners begin to grow rhododendrons, increased attention must be made to the hot summer sun. Most varieties exposed to unprotected all day sun are doomed. In general, rhododendrons will benefit from filtered light and partial shade. Planting in southern exposure without protection from the sun nearly guarantees plant death, and western exposure is also tough on them. Northern and eastern exposures are best suited for rhododendron growth in the south.
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 are seen in late spring when the plant is just beginning to grow and suddenly it drops dead. It is also seen during the summer when a branch turns brown and dies. Frequently the ailing plant is lost. These phytophthora-type diseases are generally the result of conditions created by the gardener. As it is believed, the disease spores are present in the soil over the country. One way we promote these organisms is by planting the rhododendron too deep, thus providing a water culture for the development of the disease organism. Puddles of water which remain more than an hour after watering can harbor disease. Watering in the late afternoon and evening will also encourage disease development. The use of fungicides during spring and summer will help inhibit the development of these diseases.
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 suited to your region, and careful consideration to the planting and care of your rhododendron.
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North Carolina State University