I have a lilac shrub that is probably 12 feet tall . . . all the limbs are quite substantial in size. I'd like to bring it down to about 8 feet tall, but am not sure how to go about pruning (topping) old limbs without killing or deforming it.
How to Prune Old Lilac
I understand after trimming a lilac, it will take a year or two before it will rebloom. We had the same problem two years ago. A huge, ugly lilac bush. As there were so many bare branches inside the bush, I didn't know if trimming it back would do any good. I decided to "cure or kill" and had my husband trim it to about 6" off the ground one spring. It came back with new shoots and is now full and bushy. We even moved it that fall after trimming it, so the roots were cut too. It will take a few years for it to mature and bloom again, but wow... what a difference! Since it came back trimming down to the ground, maybe if you trim back to the center, it will produce new green branches from the main branches. I would do it in the spring though. Good luck.