Rhododendrons have shallow roots. Robert suggests watering in a band around each plant, starting at the trunk and running out 80% of the way to the dripline (the point where the widest branches reach). Water until you've wet the soil 6 inches deep. Cover the roots with several inches of mulch to keep the soil cool and moist. How can you tell when a plant needs water? "If it's late in the day in summer and the leaves are drooping and then the next morning they're straightened out, the plant is fine," he says. "But if leaves are still wilted the next morning, water."