Growing lilacs, those fragrant harbingers of spring, represent a gift to future generations. Lilacs can grow, often quite neglected, for hundreds of years. It's not unusual to see abandoned farmhouses near which old, untended lilacs still flourish. Yet following a routine of regular pruning, feeding and pest control ensures that your shrub, whether common lilac (Syringa vulgaris) or one of the many cultivars and hybrids, will not just survive but retain a pleasing shape and bloom vigorously every year.
Get your young lilac shrubs off to the best start by establishing them where they'll receive at least six hours of sun each day. They also need well-tilled, neutral soil. Send a soil sample to your local extension service or buy your own soil-test kit. The pH level of neutral soil registers as 7.0; lilacs will tolerate a range of about 6.0 to 8.0. If your soil is too acidic (low pH), incorporate bone meal, wood ashes or garden lime in the soil. Neutralize alkaline soils (high pH) with peat moss or rotted oak leaves. This organic matter will also ensure the well-drained soil in which lilacs thrive, as will compost and screened topsoil.
Unless they receive regular pruning treatments, lilacs tend to follow a biennial schedule of spectacular bloom one year followed by so-so blossoms the next. Ensure annual splendor by pruning the branches after the flowers die in early summer. Remove the dead blossoms and the adjoining stem section as far back as the closest set of leaves. This method prevents the shrub from setting seed, allowing it to put its energies into next year's flower growth.
Another, more complicated pruning method involves correcting the "leggy" look for which lilacs are notorious. This three-year program requires cutting some of the oldest stems back to just above ground level each year in early spring, before the shrubs leaf out. Remove about one-third of these stems each year for three years, and the lilac will develop into a more vigorous, compact plant.
Feed your lilacs twice---just before blooming and just after. Use either synthetic 10-10-10 or an organic equivalent. Use the amount recommended on the package for shrubs. Using pelleted fertilizer enables you to target the nutrients around the roots of the lilac shrub.
Once lilacs become established in the garden, they rarely require extra watering. Check the soil often during the first growing season, and if the soil feels dry, water the lilac until the soil feels moist at least an inch under the surface. In ensuing years, lilacs need little in the way of consistent watering. But in times of drought, treat the shrubs to deep, weekly watering sessions.
It's pretty tough to kill a lilac. Nonetheless, limiting pests and diseases keep the plants more robust and attractive. Oyster-shell scale afflicts the stems of the shrub with brown or gray bumps. Prune the "wartiest" stems and treat the remaining ones with dormant oil spray. Lilac borers leave small holes in both the stems and lower branches. As with scale, partial removal and dormant oil spray is the best treatment.