The Spruce - How to Prune Lilac Bushes⚓︎
Lilacs, like Mock Orange, set next season’s flower buds almost immediately, so late pruning will mean sacrificing the next year’s flowers. Do not prune in the fall or you will lose next years buds.
The general rule of thumb is to prune mature lilac bushes just after the flowers have faded in the spring.
July will likely be too late to prune lilacs if you want to maintain next years buds.
In general, by the time a stem reaches more than 2 inches in diameter, it should be pruned. Thicker stems may require a saw as lilac can be very tough. Your goal for a lilac pruning is to have a bush with somewhere between 10 and 12 stems, all of them between 1 and 2 inches in diameter.
If you want your lilac to fill in more and become shrubbier, trim new stems to an outward-facing bud. This means pruning just beyond buds that face away from the center of the plant. This technique will cause more branching and create a denser shrub.