Left unpruned, many mature syringa bushes--commonly known as lilacs--easily reach a height of 15 to 25 feet, providing handsome summer foliage and unreachable, unseeable blooms. To bring lilacs to a more manageable height, regular pruning is needed. Lilacs may look like small trees, but they have in common with shrubs the ability to send forth a regular supply of new shoots that can become flowering branches. Learn to keep your lilacs trimmed, to improve their healthy growth and add to your enjoyment of their fragrant flowers.
Prune overgrown branches back to a height of 6 inches to 1 foot as soon as blooming is over, using a pruning saw or pruning shears. Lilacs set new flower buds soon after blooming, so major pruning should be done as soon as flowers fade. Choose the largest 1/3 of branches, beginning with those over 2 inches in diameter. Cutting them back to near ground level will stimulate the bush to send up new growth.
Cut back the next 1/3 of the biggest branches the following year with the pruning saw or shears. The third year, remove the final 1/3 of old, overgrown branches. During your initial three-year cycle of heavy pruning, you are likely to notice fewer blooms, as each branch you cut takes with it many smaller offshoots. By the end of three years, newer growth will have branched out and flowers should be abundant.
Cut 1/3 of the branches every subsequent year to keep your lilac under control. Your goal is to keep it at a constant height of approximately 6 to 8 feet, tall enough to perform any needed screening function but short enough to enjoy its flowers.
Remove branches that are broken, diseased or dead at any time of year, using the pruning saw or shears. It's unnecessary to wait for annual pruning to remove unhealthy growth.