Aside from removing spent flowers, rhododendrons generally need little pruning. However, sometimes homeowners find themselves with a rhododendron that looks gangly, out of proportion to its surroundings or just plain neglected. Cutting back an overgrown rhododendron is one way to restore it to a more youthful appearance. The job should not be done randomly, however. Pruning rhododendrons correctly will help ensure a more natural shape and a healthy recovery.
Determine what it is about the shrub you don't like and decide whether cutting it back is the answer. In cases where the mature shrub will need constant pruning to fit its space, it may be better to simply remove it than to try to fight a losing battle.
Time your work. Early spring, just after flowering, is an ideal time to prune rhododendrons. This will allow the shrub to recover quickly and produce fresh new growth to hide your cuts.
Examine the structure. The branching structure of a rhododendron can be one of its finest features. Would you prefer a multistemmed shrub or one with a tree-like shape and one main stem? Decide on the overall structure you'd like to achieve and use that goal to decide which limbs to keep or remove.
Choose tools. Depending on the diameter of the limbs to be removed, you can do the work with a small hand pruner, a lopping shears or a pruning saw. Do not use a hedge clipper to prune rhododendrons. These are meant only for formal sculpted hedges.
Cut selectively. Don't trim every branch to the same height. Instead, select an individual branch that's too long. Follow it back to an outwardly pointing interior bud or side branch and make the cut just above the bud. This will encourage new growth from that point. If you'd rather remove the branch entirely, make the cut just above the point where it attaches to the parent branch. Continue to prune in this way but do not remove more than one third of all of the rhododendron's limbs.
Follow up. Repeat these steps for approximately 3 years, removing about a third of the shrub with each pruning. Your end result should be a smaller, healthier looking rhododendron.