Is it harmful to prune lilacs at this time of year?

Q: My lilacs are starting to look pretty dreadful – spotted with powdery fungus, overgrown and generally unattractive in the fall border.  Do I just have to wait until the leaves fall off, or can I treat and prune them now?

A:  The 3D ‘s (damaged, diseased and dead wood) may be pruned anytime, including that on early flowering shrubs such as lilacs. This is the only sort of pruning you should do on your lilacs and other shrubs in the fall.  Pruning live healthy wood now for size or shape will encourage more suckering and excessive growth next season and can lead to desiccation.

On the other hand, the best time to prune to remove suckers, sprout, crossing branches and storm damage on most trees and shrubs is in late winter (February and early March), since pruning while a shrub or tree is dormant promotes faster healing (energy is not spent elsewhere growing leaves or flowers and  the shrub or tree is less susceptible to attack from insects that might be attracted to the fresh wounds).  When you do prune your lilacs, leave plenty of swollen branch tips to guarantee spring flowering.

In the spring, right after flowers are spent, prune for shape, to lower the height of your lilacs, and to remove new suckers that have sprouted.  Another tip for early spring:  in addition to pruning for increased air circulation,control that by spraying your lilac with horticultural oil when the buds swell just before bloom time. Spraying it any other time of year will not help the infected leaves.