Experts and BHG readers answer.
pruning hydrangeas and lilacs
Thanks for writing. The best time to prune both of these shrubs is in early summer, right after they finish blooming. The plants have already made next year's flower buds, so if you prune them now, over winter, or next spring, you'll be cutting off the buds and stopping your plants from blooming.
---Justin, Senior Garden Editor, BHG.com