Apr 02, If lilac bushes are already too large or becoming unsightly, however, pruning the entire bush or tree to about 6 or 8 inches ( cm.) off the ground may be necessary. Keep in mind that you may have to wait for flowers, as it takes about three years for them to develop once the entire shrub has been cut. When to Trim Lilac Bushes. Jul 08, When to Prune Lilac Bushes.
Many of the newer lilac varieties grow quickly and might need pruning starting in their second or third year. In general, by the time a stem reaches more than 2 inches in diameter, it should be pruned. If you are diligent with annual pruning of your lilac, the shrub will grow to about 8 feet tall with flowers throughout the branches.