Lilac trees grow vigorously in the northern regions of the United States and tend to perform poorly in the hot, arid areas of the South. As they grow, lilacs become leggy and unattractive. Proper pruning can prevent this and give you a healthy, flowering plant. To ensure that lilac trees bloom every spring, they must be pruned at the right time.
Prune right after spring flowering occurs. Cutting the lilac tree much later than this will remove the formation of summer blossoms, which bloom the next year.
Sterilize your pruning shears. Rub the blades with rubbing alcohol and a rag to minimize the spread of disease between plants.
Cut one-third of the oldest, woodiest branches to the ground. This will encourage new branching and eliminate the thickest stems, which are attractive to borers. Cut down any disease-infected branches.
Rejuvenate the bush if it becomes leggy and overgrown. Cut all of the branches down to 6 to 8 inches above the ground. The lilac tree will grow back healthy and produce blooms again in three years.
Destroy all plant materials that remain from pruning. This will decrease the spread of diseases the lilac may possess.