Although many rhododendrons do not require much pruning, some basic shaping and care can help to encourage better flowering in subsequent years and seasons. In some cases, an older rhododendron that has grown out of control may benefit from a judiciously timed hard pruning. Different techniques can help to manage bush thickness and encourage flowering.
Timing for rhododendron pruning is important. Prune your rhododendron in the winter, after the first killing frost. If you prune at the wrong time, you can cause your rhododendron to expend growth and flowering resources that could result in better, healthier growth. The exception, however, is for dead branches and twigs. Remove dead branches and twigs as soon as you identify the dead area. Deadwood uses up plant resources without contributing to the growth of your rhododendron.
Light pruning, although often best done in winter, can also be done throughout the year to shape and direct bush growth. However, avoid pruning during the first spring flush of growth. Light pruning is best done with a pair of hand shears. When shortening branches or thinning out the tree, clip branches just above the branch collar and shorten branches by cutting just above leaf pairs or buds.
Hard pruning may seem like a very harsh pruning method, but it can be a good way to help bring an old, established rhododendron back that has grown to the point that light pruning is not a practical way to control the bush. Hard pruning should only be done in the winter. With hard pruning, you cut an older tree back to within several inches of the ground. Bushes 12 to 15 feet tall can be trimmed back this severely and survive. By hard pruning an out-of-control bush, you can train and prune the new growth in a more manageable way.
Deadheading of rhododendron bushes is a good way to encourage flowering in following years. To deadhead a rhododendron, go around and remove the dead flowers by snapping them off. This removes the seeds. Removing the seeds directs the growth energy that would have gone to the seed to flowering the subsequent year.
Disbudding is a technique to encourage side branching on rhododendrons. To disbud a rhododendron bush, pinch off the terminal buds on new growth. This will awaken dormant buds on the sides of the branches and cause side branches to appear. Disbudding is a good way to help keep a rhododendron from growing so large that it becomes unmanageable.