Rhododendrons prefer growing on a slope, where water quickly drains away. If your yard is level, plant in a raised bed. Robert tills the soil and then uses crossties cut in half to make a 4-foot-square raised bed for each plant. When filled with soil, the bed raises the roots above ground level so they won't sit in water after a heavy rain.
The Right Soil
This is critical. Rhododendrons rot and die if planted in heavy clay. They need loose, acid (pH lower than 6.5) soil that contains lots of organic matter. Robert uses a soil mix consisting of 2 parts finely ground bark and 1 part sand. Before planting in spring, he slips a new plant out of its pot, soaks the root-ball, and then uses his fingers to gently remove the soil from the bottom of the root-ball while preserving the roots, leaving the root-ball only 6 to 8 inches high. Next, he spreads the roots in the hole and backfills with soil mix to the top of the root-ball. "I don't want the roots to go down," he explains. "I want them to go out."