Rhododendrons are evergreen shrubs that are best known for their showy clusters of flowers, which range in color from white to a deep red. Azaleas, which are also classified in the rhododendron genus, are deciduous and much smaller than the plants that are commonly called rhododendrons. Care of these desirable plants requires special attention to the pH level of the soil, according to information published by Ohio State University.
Most rhododendrons grow best in a relatively humid, mild climate, according to information published by the University of Missouri. They thrive in the rainy weather of the Pacific Northwest and other similar areas. Many cultivars are hardy to U.S. Department of Agriculture (USDA) zone 4, and some can even survive the cold winters in zone 3, but it's best to check the cultivar information before planting one in your climate.
Choose a planting location that slopes a bit to the north or east, if possible, which will protect from southerly drying winds and allow for good drainage, according to the University of Missouri. Buildings and trees can also be used as wind blocks. Protection from the wind is vital for avoiding leaf scorch or splitting of the wood and bark.
A planting site with filtered sunlight (such as through a deciduous tree) is best, but morning sun followed by afternoon shade also works well for rhododendrons. Too much sunlight will cause the flowers to bleach, even if the plant appears to be growing vigorously.
Soil and Planting
Rhododendrons thrive in rich, organic soil that is well draining. Most importantly, they require acidic soil in order to thrive. Acidic soil has a pH level of 6.0 or below. Soil testing kits, which are available at most garden centers, can tell you the pH level of the soil. To increase the acidity of your soil, amend it with aluminium sulphate. Then, plant your rhododendron in a hole that is as deep and wide as the root ball. The top of the root ball should sit around 2 inches above the surface, according to the University of Missouri. Backfill the hole with the removed soil, and water thoroughly. Finally, add a 2-inch layer of wood chips that extends to the edges of the plant's canopy.
Water and Food
Rhododendrons are not heavy feeders, but they can benefit from a light application of acidic fertilizer in the spring. They also require extra iron. It is best to choose a fertilizer made specifically for rhododendrons and azaleas. Follow the application instructions on the label according to the size and age of your plant. Water enough so the soil remains moist, but never soggy, as this can cause fungi to develop in the soil. Let the soil dry out a bit in the fall, watering again at Thanksgiving and then not again until the next spring.