Rhododendrons thrive with very few diseases or pests. The plants occur in many parts of the world, growing on the forest floors beneath a canopy of old-growth trees, which affords them wind protection, moist soil and dappled sunlight. They require rich, acidic soil with ample organic matter, which the soil of the forest can easily provide. When grown in the home landscape, the trick to success with the rhododendron is to re-create its natural habitat.
Plant rhododendrons in well-draining soil with a soil pH below 6.0. If the soil pH is above 6.0, render it more acidic by adding sulfur, iron sulfate or ammonium sulfate before planting. Soils high in lime or calcium need to be avoided because the rhododendron will be unable to thrive.
Choose a location that offers shade or dappled sunlight. Dig a hole that is twice the size of the rhododendron's root system. Mix ample organic matter such as peat moss, aged manure or bark chips into the soil. Trickle water over the rootball of the rhododendron so it is thoroughly soaked. Place the rhododendren into the hole and tamp the soil down. The plant does not tolerate air pockets in the soil because this will allow the delicate root system to dry out.
Apply three to four inches of mulch around the rhododendron to keep the soil moist. The plant has very sensitive roots that require moisture to survive. The plant does not tolerate flooded roots, but moist soil is ideal. The rhododendron has a very shallow root system that easily dries out. A dry root system can kill a newly planted rhododendron. Water the plant by allowing the water to slowly trickle near the base of the shrub so the root system receives ample water.
If the soil is nutrient-poor, fertilize the rhododendron each spring using a well-balanced fertilizer for acid-loving shrubs. Follow the directions on the label for application. Rhododendrons planted in fertile soils do not require fertilizer, according to the American Rhododendron Society. If the shrub has been mulched using bark chips or resides in a cold climate, apply nitrogen in late June.
Remove dead flower heads as they appear to encourage new flower production. In early spring, remove any dead or damaged branches. When pruning to maintain size, prune the rhododendron after flowering so new flower buds for the next season will form, according to Oregon State University.