Rejuvenation in Pruning Lilacs

Pruning Time

• Lilacs produce their flower buds about one month after they finish flowering in spring or early summer. When pruning, it's best to cut back the branches during this period so you don't remove next year's flowers. It's also possible to prune in late winter or early spring before new growth appears, but you will cut off some of the current season's buds and reduce flowering. Lilacs may still flower poorly for one to two years after severe pruning as the plant recovers, but once recovered, it will likely reward you with even more blossoms.

Diseased Lilacs

• Cane borers, scale insects or root damage can result in poor flowering and trunk dieback. Pruning out the older, most-affected trunks allows the plant to focus energy on the younger, more resilient trunks. The oldest trunks are the thickest. Cutting these out at their base with a pruning saw helps the younger trunks put on a renewed flush of healthy new growth. Remove about one-third of the oldest trunks in the first year. In the second year, remove half the remaining old trunks and remove the remainder of the old trunks in the third year. Pruning over a three-year period gives the lilacs time to recover with minimal stress.

Overgrown Lilacs

• Healthy lilacs that otherwise are too tall benefit from more extreme rejuvenation pruning. Although it's possible to cut the entire plant down to within 8 inches of the ground during late winter dormancy, not all lilacs recover and those that do will require two or more years before they resume bloom. In the first two years prune out the oldest trunks, removing half the first year and the other half the second year. Cut back the remaining young trunks the third year, cutting them back to 3 to 4 feet tall. Beginning in the fourth year, trim back the trunks annually to the desired height so the lilac doesn't become overgrown again.

Pruning Tips

• Deadheading further ensures lush spring blooms each year. By removing the old flowers as soon as they wilt, the lilac can't set seed so the plant's energy is diverted to producing more flower buds. Deadheading also improves the appearance of the lilac since it isn't covered in dead flowers or brown seed pods for the rest of the year. Lilacs will also continue to grow new stems from the base even after you have completed rejuvenation pruning. Cutting out the oldest stems, or about one-third of the existing stems, every three to five years prevents your lilac from becoming too dense or overgrown again.