Spring and early summer pruning questions and answers
A. Lilacs produce buds for next year shortly after flowering. So if you wait too long, say until August or September, you will be removing buds for next spring, thereby reducing next year’s display. So timing is important. My advice is to prune as soon as your lilac has finished flowering. That will give it plenty of time to develop new flower buds. Q. When should I prune my spireas? A. That depends on the spirea. If it’s a spring flowering type (e.g. bridal wreath, Spirea prunifolia, S. nipponica), prune shortly after flowering. These types can become woody, leggy and less productive ...
Source: newsoptimist.ca