Lilacs (Syringa spp.) burst into bloom in early spring, delighting home gardeners with their large clusters of blooms and pleasant fragrance. The showy flowers and sweet scent make them irresistible to little hands and noses. Lilacs are non-toxic and easy to grow, making them good projects for your young gardeners. The plants bloom on old wood. Deadhead them as the flowers fade.
There's a difference between removing spent blooms from a plant and cutting back the stems to prune it. The former is called deadheading, and it causes a plant to redirect its energy from producing seeds to storing food and developing next year's flowers. Deadhead your lilac blooms as they fade, so that they don't go to seed. Clip off the flower panicles at the point where they attach to the stem. You will not harm the plant if you neglect deadheading, but the spent flowers will be unsightly, and the next year's blooms will be fewer and smaller than if you had deadheaded the old blooms on time.
Lilacs need regular pruning to maintain a full, bushy form and remain in good health. The bushes tend to get leggy if they aren't pruned. Judicious annual removal of some older wood encourages the plant to produce new growth and larger blooms. It's fine to remove last year's neglected flowers before new blooms emerge in the spring, but hold off on pruning either until you deadhead this year's blooms in late spring or summer, or until late winter, depending on your lilac's age.
When to Prune
If your lilac is less than 5 years old, just trim the plant each year as the season's blooms fade and you deadhead them. Your goal is simply to maintain a pleasing shape. If your plant is older than than 5 years, maintain it by pruning away about a third of the thickest stems all the way down to the ground each winter. If you have inherited a severely neglected lilac, you may need to rejuvenate it by cutting the entire bush to about 8 inches from the ground. The plant won't bloom for about three years. Once it starts blooming, maintain it by deadheading on time and pruning it every winter.
Deadheading and pruning are important aspects of lilac care, but the plants have other needs if they are to bloom to their best advantage. Plenty of sun exposure is important. Lilacs need at least 6 hours of sun per day for optimum flowering. The plants prefer rich, moist soil that drains well. Soil that is too wet will cause root rot. Water your lilacs so that they receive about an inch of water per week, and keep grass from growing too close to the stem. The Old Farmer's Almanac recommends placing a 16- to 24-inch circle of ground cloth around the plant to prevent grass growth. Cover the cloth with rocks or heavy bark to keep it in place.
Climate and Species
Lilacs vary slightly in hardiness depending on the species, but most are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 7. Common lilacs (Syringa vulgaris) are hardy in USDA zones 3 through 7, although Syringa vulgaris "Lavender Lady" was bred to survive the heat of USDA zone 8. Meyer, Manchurian and Japanese lilacs (Syringa meyeri, Syringa pubescens patula and Syringa reticulata) are hardy in USDA zones 3 through 7.