Unfortunately, as lilacs mature, the lower portions of the shrub become shaded and usually lose their leaves. As a result, large, overgrown specimens are often leggy and unattractive. Their thick, heavy stems are very attractive to lilac borers. Start young plants on a regular pruning program in the fourth or fifth year of growth as outlined below. Old, neglected lilacs can be renewed or rejuvenated through pruning. Home gardeners can choose between two different pruning methods.
Three-year Plan
The best way to prune lilacs it to initiate a three-year pruning cycle. Begin the procedure the first year by removing one third of the largest, thickest stems at ground level in late winter. For young plants, this may mean you are only remove 2 or 3 stems.
The following year, again in late winter, prune out another third of the thickest stems. Repeat the process again in the third year.
For plants, this method has the advantage of not removing all stems with their stored carbohydrates all at the same time. And it leaves the plant with an ample amount of foliage each year so it can photosynthesis normally, maintaining health and vigor. Since lilac wood needs to be 3 or more years of age before it blooms, this pruning method also allows gardeners to enjoy flowers every spring.
When properly pruned using the 3-year plan an old, overgrown lilac can be transformed into a vigorous attractive shrub within a few years.
Renovation Pruning
A more drastic way of renewing an overgrown lilac is to cut the entire plant back to within 6-8 inches of the ground in late winter, March or early April. This severe pruning will induce a large number of shoots to develop during the following growing season.
In late winter of the next year, select among last year's new shoots and retain several strong, healthy ones to form the shrub framework. Remove all other shoots at ground level. To encourage the development of branching in these young stems more quickly, cut them back at staggered heights just above an outward facing bud.
This method has the disadvantage of removing all flower buds and delaying flower development for about 3 years. But it does allow for the pruning work to be done all at one time. Once rejuvenated, start the 3-year pruning program to keep your lilac beautiful and healthy.