Lilacs are old-fashioned favorites for good reason. They are hardy, long-lived shrubs that produce beautiful flowers in mid-spring after the early bulbs but before early summer's diversity of bloom. Trimming these lovely bushes requires a little common sense and general pruning knowledge. They require little pruning to produce enough blooms to fill the house with outsized bouquets and sweet aromas. An understanding of these carefree shrubs and a few sharp tools should keep your lilacs growing and producing for many years to come.
Pick lilacs freely--it's part of the maintenance program. Cutting lots of blooms gives branching and growth for next year's blooms an early start. Use very sharp, very clean garden shears to cut lilac blooms. Cut above a node, the bumpy places on branches where new branches will form, to force two new branches to form to replace the one that bloomed this year.
Trim untidy bushes within two weeks of the point where blooms begin to fade. Take no more than 20 to 30 percent of a branch to keep growth compact. If no reduction is needed, remove the dead blooms to improve appearance and to encourage the shrub to put its energy into producing new branches rather than seed. Resist the urge to shape your shrub in the fall; wait until after those oddly-shaped or protruding branches have flowered next spring and then correct them.
Remove old branches in late winter before buds begin to form. Old branches with dark, rough bark shade new growth, causing loss of leaves and bloom in the lower parts of the shrub. They also are attractive to pests such as the lilac borer. Remove about three of these old branches with a pruning saw, cutting them to a height of 6 inches or less. Perform this mid-winter clean up every 3 to 5 years.
Eliminate "suckers"--new shoots that grow straight up in the spring. They steal energy and congest the center of the shrub, locking out air and light. Both are important in the control of late-season mildew, a harmless but unattractive problem. Cut suckers at ground level with garden shears or a sharp knife whenever they appear, beginning in early spring.
Renew overgrown, unproductive shrubs by cutting several of the largest branches back about one third of their length in late winter each year. After a few years, the bush will be reduced and fully blooming once again.