The key is to determine when and where the plant sets its buds for.
Prune summer-flowering shrubs in late winter or early spring. The pruned shrubs will bloom in summer on the current season's growth. Some deciduous shrubs don't produce attractive flowers.
These shrubs may possess colorful bark, fruit, or foliage. Prune these shrubs in late winter or early spring before growth begins. Do not prune deciduous shrubs in late summer. Pruning shrubs in August or early September. You want to prune before buds are set to keep from disturbing the following year’s blooms. As a rule, flowering plants that bloom in early spring on old wood (or growth from the previous season), like azaleas, should be pruned a week or two after flowering.
The new growth that follows is where buds will form. Apr 20, DO Prune Flowering Shrubs at the Right Time of Year Spring-flowering shrubs, such as lilac and forsythia, develop buds that will display the following year during the Estimated Reading Time: 5 mins.