When is the best time to trim lilacs and how much can be trimmed off?
Certified GKH Gardening Expert
I would not prune it back by more than 1/3 in any one year. Cutting it back further than that could harm the plant. This article should help with that: https://www.gardeningknowhow.com/ornamental/shrubs/lilac/pruning-lilac-bushes-when-to-trim-lilac-bushes.htm