How and when should lilacs be pruned?
Common lilacs (Syringa vulgaris and its cultivars), also known as French lilacs, should be pruned every year to produce a pleasing shape and a healthy plant. Remember that lilacs bloom on old wood (last year's growth) so it's important to prune immediately after flowering to give ample time for the new growth and flower buds to develop.
First, when the lilac blooms have died they should be removed (dead-headed). This will encourage stronger growth and encourage the production of next year's buds. Following this, the bush can be shaped if necessary by "tipping", i. e. cutting back any branches that stick out or are too tall, to an outward pointing bud. How much you cut back will depend on the final shape that you are hoping to achieve.
Cut any small suckers at ground level or where they come out of the main trunk. A few suckers can be left if the main part of the shrub is old wood. Do not give the shrub a flat top but aim for a rounded form. As with any pruning job, dead or dying branches should be removed.
If the lilac is overgrown, too large or too dense, a more vigorous regeneration pruning may be carried out. In early spring, cut about 1/3 of the branches down to the ground. Do this every year for three years. If your lilac plant has been grafted on to a different root stock then take care to cut above the graft (visible as a swollen band on the stem). If the shrub is very overgrown, and unsightly, then you can cut all the branches down to ground level, leaving just a few new shoots. As the shrub grows, prune it for shape and it will rebloom in a couple of years.
There are a few dwarf lilacs, e. g. Meyer lilac (Syringa myeri and its cultivars) and Manchurian lilac (Syringa pubescens and its cultivars) which do not need regular pruning.
You will find more information on caring for your lilac in the NYBG guide "Trouble Free Lilacs".
Courtesy of the NYBG Plant Information Service