Landscapers love lilac shrubs for their dense growth and brightly colored blossoms. Like most shrubs, lilacs benefit from annual pruning. Pruning helps keep the plant growing lush and green and should be carried out in the winter, according to North Carolina State University. In some climates, bees may be active during this time and can pose a hazard when you're trying to work among your lilacs.
Wait until the evening before approaching the lilac bush. During this time, both bees and wasps are less active and many of them will have returned to their nests, according to the University of Nebraska.
Identify a dozen of the plant's largest stems. Cut off all other stems to keep the lilac bush's branches open and ventilated, and to ensure none of the branches are so close that they can rub against each other and cause self-inflicted injury to the shrub.
Cut off 25 to 30 percent of the length of the remaining stems, as measured from the tip of the stem. This encourages new growth and keeps the plant from getting too woody. Make the cut just after the leaf node on the stem. The node is the bump where new branches and leaves sprout.
Prune the plant for height. North Carolina State University recommends trimming the plant 12 inches shorter than the height you want it to be the following year.