Posted: 01/06/2014 at 08:23
Yes, you can rejuvenate lilac by cutting back hard and it will grow back. If it is multi-stemmed, the best way is to remove 1/3rd of the stems (oldest first) right back to the ground each year for 3 years. As new growth won't flower for 2-3 years, doing that means you will still get some flowers each year. If it has a single main trunk, you can cut that back to about 6-8 inches from the ground. New stems will appear, but you won't see any flowers for 2-3 years. The best time to do it is in early spring, so a bit late to do it this year but you could risk it, especially as you are considering removing it anyway. When the new stems appear, prune out the weak spindly ones next Spring. One word of warning though - hard pruning can make lilac throw up lots of suckers in the general area and these can be difficult to deal with.