What is the best growing environment for rhododendrons?
Acidic soil with a pH of 4-6 is needed for good rhododendron health. If your soil is too alkaline, you can raise the pH by adding agricultural sulfur or ferrous sulfate. Rhododendrons also require well-drained, humus-rich, loose soil for their numerous fine roots, many of which are close to the surface. So add generous proportions of compost, leaf mold, peat moss, and/or well-rotted manure to the soil.
Planting is best done in fall or spring, with fall preferred in all but the coldest climates in which Rhododendrons will grow. Before planting, soak dry root balls thoroughly. Breaking apart the root ball isn't necessary; however, loosening some of the outer roots should be done to get the roots out of the existing root ball and into the new soil. Dig a shallow hole twice the diameter of the root ball. Line the hole with a 50-50 mix of sandy soil and peat moss or leaf mold, and pack firmly. Place the plant into the center of the hole. The top of the root ball should be at or slightly above the existing soil line. (Rhododendrons and azaleas are easily damaged or killed by planting them too deeply). Fill in the soil around the root ball and firm. Water two or three times to moisten deeply after planting. Mulch with pine needles, partly decomposed oak leaves, or other acidic amendment.
Rhododendron's fine roots take a long time to grow out into the surrounding soil. Because of this, a newly planted rhododendron will get its water out of the original root ball. If this ball is allowed to dry out, it is difficult to get it wet again. The soil around the planting hole can be wet, yet the plant root ball itself can be bone dry. Often the only way to re-wet a dry root ball is to place a dripping hose at the base of the trunk of the plant and let it run for several hours. Unless you have a very wet climate, doing this weekly in addition to the regular watering the plant gets during the first growing season will help get the plant off to a good start.
Favorable planting sites in regions with cold winters are north- or west-facing. South- and east-facing sites are not ideal because plants there are exposed to strong sun while still frozen, causing winter burn. Plants facing east and south are best protected by tall evergreens such as pines or arborvitaes. Placed on the sunny side of the rhododendron, the evergreens will provide a shady environment. Rhododendrons can be harmed or killed too much direct sunlight and also by sustained winds, so a low-lying area, or one protected by a building or a hedge, is a good choice.
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- Courtesy of NYBG Plant Information Service