The Syringa genus contains a host of shrubs with heart-shaped leaves and panicles of pastel-colored blooms. Most famous for the lilac-colored species, the name lilac has become synonymous with this popular garden bush. Lilacs have been known to grow for hundreds of years, and may become poor bloom producers or take on an awkward shape if not pruned regularly. Gardeners may use the process of renewal pruning to get the mature shrub back in shape and blooming beautifully.
Prune lilacs in the spring just after flowering has ceased. Lilacs flower on old wood, meaning they will form the buds for blooms prior to the spring growing season. Waiting too long to prune could result in removal of the bloom buds and thus flowers. In addition, the shrub will spend lots of energy attempting to make seeds, which saps flower-making potential for the following season.
Use garden shears to cut back any diseased or dead canes and any suckers to ground level.
Clean shears between each cut with a solution of equal parts bleach and water. This prevents the spread of disease to the rest of the shrub during pruning.
Cut back one-third of the oldest canes to ground level the first year of pruning. New growth will then be free to emerge, but the bulk of the shrub remains to provide some flowers and foliage for the next season.
Prune the next third of oldest canes to the ground in the following season.
Cut down the remaining third of oldest canes in the third year to complete the renewal pruning process. The shrub will begin to fill in during the first two years of pruning, and the shape will begin to improve as will the number and health of blooms.