Prune lilac bushes immediately after they finish flowering
Remove spent flower clusters
Cut out dead, damaged, or diseased stems
Remove weak, crossing, or crowded branches
Thin out up to one-third of the oldest stems at the base
Cut the oldest stems to ground level
Keep the strongest young stems to maintain shape
Trim back overly long shoots to a healthy outward-facing bud
Avoid heavy pruning in late summer, fall, or winter
Do not remove more than one-third of the bush in a single year
Use clean, sharp pruning shears or loppers
Disinfect tools before and after pruning
Shape the bush naturally rather than shearing it flat
Repeat light pruning yearly to maintain vigor