When you prune your lilac depends on why you're pruning it: trimming the height, cleaning up the bush or rejuvenating an old shrub. If you prune a lilac bush in the wrong way at the wrong time of year, you'll have few blossoms to enjoy.
To keep your lilac bushes at a desired height, trim them when they've finished flowering in late spring to early summer. Use hedge clippers to cut the tops of the lilacs to 1 foot shorter than the desired height, then trim around the bushes. Trimming lilacs before they flower reduces the number of flowers.
Vigorously prune lilacs in late winter to maintain a healthy shrub with adequate flower coverage. Remove one-fourth to one-third of the stems each year in late winter, cutting the stems off at the ground. Because the wood of lilacs blooms only after it is 3 years old, this method promises flowers each spring. Remove the largest and oldest stems and ensure that stems do not touch each other.
While these are the two major pruning times and pruning techniques for lilacs, you may notice dead, damaged or diseased wood at any time of year---after a storm or high winds, for example. Cut off the dead or damaged wood as soon as you notice it to protect the lilac's health.