Prune lilacs right after they finish blooming
Remove dead, damaged, or diseased branches first
Cut off spent flower clusters
Remove up to one-third of the oldest stems at ground level
Thin crowded stems to improve air circulation
Cut weak or crossing branches back to a main stem
Keep the shrub’s natural shape
Avoid heavy pruning in late summer, fall, or winter
Use clean, sharp pruning shears or loppers
Disinfect tools between cuts if disease is present