Keeping lilacs strong, healthy and blooming season after season, requires some maintenance. Now is the time to prune lilacs. Most lilacs bloom on new wood, so timing is essential. The flower buds form during the summer, so you want to avoid pruning in the fall or late winter, if you want the plant to bloom the following spring. To keep your lilac growing vigorously and to avoid having to do major renovation, remove any dead, broken or crossing branches. Make sure you remove spent blooms and seed pods. This will help the shrub produce more flowers next season. To assure the shrub is always growing on the best wood, remove about a third of the canes each year. This includes suckers. You can leave two or three suckers to grow into mature canes.
This is also a good time to look for evidence of scale or borer damage. Severe insect damage should be pruned out. Persistent insect problems can be treated with systemic.
If indeed, your lilac has seen better days, consider cutting every cane of to 4”- 6” above the ground. This drastic approach will result in many vigorous canes next season, forming a two-three foot shrub.
Finally, work in 2” of sheep, peat and compost; add a balanced fertilizer, such as 11-15-11 and mulch to about 3”. Your lilac will be prepared for the rest of the summer and ready to bloom next spring.