Updated: June 10,
Unless plants pruning lilac shrubs unsightly or broken branches, you can usually not bother with pruning until your plants are ten years old or more. If your plants are six to ten feet high, maybe the same across, with older wood (thick branches, distinct bark like a tree), or perhaps branches that cross and rub on each other, you may think about pruning.
Apr 02, How to Prune Lilac Bushes.
While the plants are young, deadheading the spent flowers helps to direct the plant's energy into setting more buds.
When pruning lilacs, cutting back the tops of overgrown stems is oftentimes not enough. It is generally better to cut the entire stem. Trimming lilacs is best accomplished using clippers.
Remove spent blooms all the way to the stems to prevent seeding and encourage more blooms later on. Cut back about a third of the branches.