We recently moved to the old farmstead and I discovered an area where lilacs are growing, sparsely. I don't think it has ever been prunned and the flowers are few and far between. I read that you should prune immediately after flowering. But then I read for a plant that has never been pruned and out of control you should cut 1/3 out late winter (March to April) and continue for the next three years. I am confused. If I cut back in late winter/early spring won't I be cutting off the buds for that year? So I am wondering when exactly I should trim the plants?
You are right that if you prune before it blooms, you will be cutting off some of the buds for that season and it will not bloom as much. But if you are only taking 1/3, the other 2/3 that is untouched should bloom.
If you want as much blooms as you can, prune right after it blooms. For rejuvenating old plants, they do say to cut out up to 1/3 of the oldest stems, by cutting them off at ground level.
Check out my post here about how to keep it blooming well after that: