The lilac (scientific name Syringa) is one of the most beloved landscape plants, valued for its over-the-top, staggeringly fragrant flowers that emerge in the early spring. Most varieties grow to between three and five feet high. The larger, most common cultivars, which include Syringa vulgaris, Syringa oblata and Syringa x hyacinthiflora, need regular pruning to maintain an attractive shape.
Wait till mid-winter to seriously prune your lilac shrubs. Then cut out one-fourth to one-third of the largest stems. Leave a maximum of six to 12 main stems on the shrub.
Trim out any stems that are rubbing against other stems.
In the spring or early summer, after the lilacs have finished flowering, you'll probably find new shoots, called "suckers," coming up around their bases. Cut all but one or two of the suckers off at least an inch below the soil.
If your lilacs are growing too tall, wait till they're done flowering and then cut their tops down by as much as a foot.
No matter what the season, cut out diseased or damaged wood as soon as you see it.