In general, rhododendrons require little pruning. In fact, the American Rhododendron Society recommends trimming the bush to control its size, if you want to pick the flowers for an arrangement or to remove spent blooms through the season. But if your rhododendron has become leggy with age, prune it in winter to rejuvenate it. This plant's stems are covered with dormant buds that spring to life when the wood is cut. This feature facilitates a quick recovery and gives you an invigorated shrub that could pass for new.
Locate the shrub's primary stems, which are the trunks that sprout from the base of the plant and the wood you'll be pruning.
Look for the latent buds on your rhododendron's primary stems. They look like small pink dots. You'll see them in groups and as single spots. Make your pruning cuts above clusters whenever possible. Position your shears ¼ inch above the buds and cut the wood at a 45-degree angle.
Cut the primary stems back to about 2 feet above the soil line. Prune each of the trunks slightly shorter or taller than the others so they all have different lengths. This preserves the rhododendron's natural look. You might need lopping shears or a saw depending on how thick the wood has become at that level.
Some rhododendrons also survive hard pruning. Find out whether yours can take it by pruning one of the primary stems 6 inches from the ground. Cut the others the same way only if new growth sprouts from the hard-pruned trunk.