Rhododendrons develop new leaf and flower buds directly below the old flower trusses, so if you trim back the branches you’ll lose next year’s bloom. You might want to make pruning a 2-3 year project, removing only some of the branch tips this year and some next year, in order to preserve at least half of the blooms each season. They can be kept in shape by removing about one-third of the length of each branch.