Lilacs respond best when pruned right after the flowers fade and turn brown. The pruning should occur before June. Pruning later likely means fewer flowers the following season.
Lilac bushes begin setting buds for the following growing season shortly after the flowers die. Pruning during summer or fall months removes the buds growing on those branches, meaning fewer flowers the following spring. When the old flowers die, it is best to prune no more than one-third of the existing stems and focus on dead or damaged stems, thin suckers and older stems thicker than 2 inches in diameter. This early-season pruning encourages new stems to grow by allowing the bush to put more energy toward the new growth.