Northeast Nursery suggests that pruning times for ornamental shrubs like rhododendrons depend on the age of the woody part to be pruned; old wood parts of the plant should be pruned immediately after flowering in summer, while new wood should be trimmed in late winter or early spring.
Generally, pruning is effective during winter months when the plant is dormant. With old flowering wood, however, new buds begin to grow after blossoms fade; pruning prior to the formation of these buds keeps the shrub healthy and still gives the plant enough time to form new buds for the following spring. Northeast Nursery suggests that annual pruning is vital to keeping the plant healthy. By removing dead and damaged parts of the shrub, it allows the plant to focus on new healthy growth.