Answer: Lilac bushes tend to send out suckers from the root system as their way of colonizing an area. If allowed to grow without being pruned, the new suckers will develop into a thicket of lilacs and make a nice, thick hedge. The suckers are attached to the roots of your old lilac and will grow and flower just like the original. You can prune your old lilac down but cutting it back to only 4' in height will be far to stressful and it could die, taking the suckers with it. Instead, try pruning it back by only one third this year, and pruning again next spring (by one third). Eventually the suckers will grow up and you'll prune the old lilac down to where everything is the same height. Then, if you want to be rid of the original plant you can cut it down to ground level after the suckers are growing well (about the third year). Best wishes with your lilac!
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