Old, neglected lilacs can be renewed or rejuvenated by pruning.

A second way to prune old lilacs is to cut back the overgrown shrubs over a three-year period.

If you are diligent with annual pruning of your lilac, the shrub will grow to about 8 feet tall with flowers throughout the branches.

Begin the procedure by removing one-third of the large, old stems at ground level in late winter. The following year (again in late winter), prune out one-half of the remaining old stems. Also, thin out some of. Jul 08, How to Prune Lilac Bushes Regular Maintenance Pruning.

Retain several well-spaced, vigorous stems and remove all the others.

The general shrub pruning rule is to cut no more than a third of the stems each year. 2  Rejuvenation Pruning. Older lilacs can have stems as thick as small trees and will flower only on.