When and how to trim lilac bushes?
Most recommend pruning lilacs right after they bloom. Most flowering shrubs bloom more vigorously on young wood compared to older branches. Cut 1/4 to 1/3 of the oldest branches to the ground each year. Lilacs often have many suckers/shoots coming up the middle. Remove any that aren't at least pencil width and those that crowd each other. Allowing room for air and light to move easily through the bush results in a healthier plant.