A favorite spring bloomer, lilacs can be timed to bloom from early spring to early summer. Although their most familiar form is as a shrub, they can be pruned to grow as trees or bought in tree form. Once established in a sunny area, lilacs are one of the most reliable and fragrant members of the garden. In addition to low-nitrogen fertilizer and a mildly acidic soil, most lilacs require some annual pruning to stay productive. The lilac tree is no different.
Cut lilacs freely and trim fading flowers to prevent the tree from setting seed. Not only does this help control the shape of the tree, but it also conserves strength that would be expended making seed. Cut flowers with as much branch as you want---just not more than a third of its total length.
Shape your tree immediately after it blooms before early summer when it is making growth that will support blooms next spring. Remove shoots that crowd out light and air in the crown and cut back any branches that are growing too far outside the crown of the tree.
Complete major shaping or crown reduction during the tree's dormant season---in late winter or very early spring. Reduce branches by no more than a third and remove old growth. Branches trimmed during the dormant season will not flower the following spring but should flower in the following season.
Renovate old trees in late winter, too. Cut old trunks back to about 6 inches to let new shoots take over in spring. Prune the new growth back a third after flowering time each spring to encourage branching. New trees will take 3 or more years to begin flowering.
Trim lilac trees annually to keep them manageable; some varieties can grow to 30 feet tall. Remove dead branches anytime or tie a string around them and remove them when the tree is dormant.