Late winter is the best time to prune a rhododendron, according to Gardening Know How. Planting any time during the dormant season, which is late fall to early spring, will also work.
Pruning a rhododendron can help rejuvenate the plant and clean up the shape of it. If pruned in the late spring, it will inhibit bud growth and flowering, so that is the worst time to prune according to Gardening Know How. Take only 15 to 20 inches of a branch at a time for yearly pruning, or if doing a pruning on an established plant, take all but 12 to 15 inches from the ground.