Q&A: Pruning Lilacs
Q: We recently bought a house with a garden that was planted in the 1930s. The lilacs are taller than the power lines and have trunks as big as my arm. Is there any hope of rejuvenating these monsters or should I cut them down and buy new plants?
A: Lucky you! No, don't cut them down and start over. Try this first. As soon as they have finished flowering, cut down one-third of the bush with your pruning saw; cut the oldest and fattest trunks as close to the ground as you can comfortably get. You'll get some new wood this year. Then next year after it blooms, do the same thing. You'll get more new wood after that. The third year, cut out the remaining old part of the bush, and you'll have an entirely new shrub, growing on old, strong roots. It should reward you mightily. Plus, all the flowers will be within reach for smelling and cutting. You'll love the results!
— National Gardening Association