Prune lilacs right after they finish blooming
Remove spent flower clusters by cutting just above the first pair of leaves
Cut out dead, damaged, or diseased stems at the base
Remove a few of the oldest stems at ground level each year
Thin crowded branches to improve air circulation
Cut back weak or crossing shoots
Keep healthy main stems and avoid heavy annual cutting
If needed, rejuvenate overgrown bushes by removing one-third of the oldest stems each year for three years
Use clean, sharp pruning tools
Avoid pruning in late summer, fall, or winter if you want next year’s blooms