How to Prune Lilacs to Prevent Legginess

Remove up to one-third of the oldest stems each year, cutting them out in midwinter when the lilac is still dormant. Cut through these stems at their base, using a small pruning saw to make a clean cut.

Cut off the old flower heads after they begin to wilt. Make the cut within 1/4 inch of a leaf or leaf bud beneath the flower structure, using bypass pruning shears.

Trim back overly long or overgrown stems to the desired height immediately after flowering. Remove up to one-third of the stem's length. Make the cut just above an outward-facing leaf or leaf bud, using shears for thin stems or a pruning saw for thicker stems.

Examine the base of the lilac for suckers, or quick-growing upright stems that emerge from the soil surrounding the main trunk or stems. Cut out the suckers 1 inch below the soil surface. Leave two healthy suckers in place for bush lilacs so they can replace the older stems during the next winter pruning. Remove all the suckers from lilacs grown as trees.

Prune out dead or damaged wood at any time during the growing season. Cut back the damaged branches to the nearest healthy wood, making the cut just above an outward-facing leaf or bud.