As gardeners agree, the vigorous growth of a healthy plant can be both good news and bad news. In the case of woody shrubs that, like lilacs, set blooms on new growth, energetic growth can have you getting out the ladder to pick the fragrant flowers. Eventually, blooms diminish greatly, because the plant puts so much energy into maintaining its woody limbs. One of several pruning strategies is in order, either right after the bush has bloomed or before it comes out of dormancy in early spring.
Use the rule of threes to bring an overgrown lilac slowly into control. Lilacs tend to send out new shoots every year, and they quickly become woody stems. Study your bush and select the largest third of these stems. Cut them back to the ground. Next year you will choose the largest third and cut them back. The following year, you will do the same. Sometimes this means removing only one or two large branches. Cutting them back forces new growth from the bottom of the plant, without sacrificing all the blooms, which form on the ends of branches.
Use the rule of twos to reshape your bush over two years. Remove the largest branches one year, the others the following year. Reshaping happens faster with this method, although taking out half the branches can result in an odd shape for a year and flowers way up and way down.
Plan a severe pruning if your bush is extremely overgrown and still growing vigorously but barely or no longer flowering. Cut the whole bush to within 6 inches of the ground, and start all over. Experienced gardeners would point out that branches could be cut to ground level without doing harm, but 6 inches will do. At the worst, you will lose a year or two of blooms while your bush regrows and reshapes, but this may be the only way to maintain the health of a severely overgrown lilac.
Maintain control by selecting at least one of the largest branches to remove every year. Lilacs that are well established in hospitable ground can easily reach up to 15 feet in height. Cutting a large woody stem or two each year keeps the bush within bounds--and the flowers within reach.