Answer: Lilacs bloom on wood grown the previous year, so any pruning must be done in the spring immediately after the shrub blooms. Pruning in summer or fall or early spring will remove flower buds. If you don't mind not having flowers next spring, you can prune now and remove up to one-third of the plant to renovate it. If flowers are important, wait until flowering finishes next spring, then prune your shrub back. Pruning is as much of an art as it is a science. One-third of the plant is the most you should remove in any one pruning session. If your lilac is quite overgrown, plan on reducing its size over a 2-3 year period and you'll reduce the stress.
Best wishes with your landscape!
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