Trimming rhododendrons can be done at nearly any time of year, although several times are preferable to preserve the large beautiful blooms we prize. Follow the growth cycle of rhododendrons to reshape them and renew their vigor without damaging next year's flowers. Even beginning pruners can use several strategies to keep their rhodies healthy and productive.
Use loppers or even hand-clippers to do minor reshaping of rhododendrons in spring, early or fall. Remember that new growth provides the source of new flower buds. To remove small overgrown branches or dead wood, cut back each branch to its juncture with another branch or branches. This is informally referred to as going back to the V, where newer branches have sprouted from older ones. If a large dead branch must be removed (1 inch in diameter or more), cover the cut edge with wound paint to prevent disease.
Time major reshaping to avoid damaging next year's flowers. This means removing (deadheading) flowers as soon as they have bloomed; usually in late spring or early summer, depending on the variety. Some harassed gardeners joke that you have roughly 20 minutes between flower die-off and new bud formation, but that serves only as a reminder to do this as promptly as possible. Thorough deadheading and pruning gives your plant the summer and fall to complete new growth and form next year's flower buds.
Use this same time period or work into early summer to do major reshaping of old and overextended plants. This usually requires a pruning saw. The system used is commonly called doing the 3s. Select the largest third of the branches and cut them back to within 6 inches of the soil. Next year, you will do the same with the next largest third of the branches; the last third gets done the third year. While this seems like a draconian measure, it can restore great vitality to shrubs that are putting most of their energy into maintaining large woody branches, rather than the leaves and flowers you desire. Your renewed plants will be smaller, bushier and healthier overall, even if they were planted decades ago.
Avoid pruning rhododendron--or any large established shrub--severely during the height of summer. Heat, humidity and the possible absence of adequate water add to the stress of pruning and make plants disease-prone.
Follow your regular feeding and watering schedule even if you are doing heavy pruning. Avoid, however, feeding late in the season, which can stimulate new growth that cannot survive winter-damage.