Cutting back a lilac
I have a several old lilac bushes in my backyard. They are quite old with some very large old growth along with some new. They only have leaves on the top half of the bushes and do not put on many flowers in the spring. How & when can I trim out the old growth so that they will continue to grow or should I take them out and plant new bushes?
The following bulletin discusses two ways to accomplish your purpose.
Cutting the old trunks to the ground may seem severe and the result unsightly but healthy lilacs pruned this way will produce enough new growth to be presentable in a single growing season. Early spring is the best time to prune this way.
The second method (removing up to 1/3 of the old growth over a period three years) takes longer but allows some flowering and leaf cover through that time.