Answer: There are two ways to bring back a big overgrown lilac. The quick and dirty method is drastic but it works. You simply cut it off at the ground in the very early spring using heavy loppers or a chain saw. It is important to do it in late winter to early spring in order to allow the plant the maximum time to recover during the coming growing season. This prevents bloom for a year, but it is simple.
Or, you can prune selectively over a three year period. If you opt for this method, prune right after it blooms. In year one, take out one third of the trunks or main stems by cutting them off at the ground level. In year two, remove half of the remaining original old stems again cutting as close to the ground as you can. In year three, remove the rest of the orginal old stems. Meanwhile, each year, also remove any new shoots that are smaller than a pencil in diameter. After three years, you will have renewed the entire plant.
Maintenance pruning is done every spring right after the lilac blooms and consists of removing a few of the oldest stems by cutting them off at the ground, and also removing any shoots coming from the roots that are smaller than a pencil in diameter. This encourages an ongoing supply of vigorous new growth.
Lilacs bloom on old wood, meaning wood that grew the previous season. That is why pruning is usually done right after the bloom season and not at other times of year. Keep in mind that it is difficult to keep a lilac bush small. If you need an overall smaller plant, it may be better to replace it with something that matures to the size you have space for. I hope this answers your question.
Good luck with your lilac!
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