Drastic pruning stimulates dormant buds on the remaining stumps, and usually large clusters of new shoots develop. During the winter, prune back the majority of these shoots to their point of origin, leaving two or three of the strongest, best-placed shoots on each stump to form the framework of the rejuvenated lilac.
Continue to keep the shrub well mulched and watered during this year. Any woody stumps that fail to produce new shoots should be cut out cleanly at ground level.
By now the framework of the renovated shrub will have taken shape. Prune to encourage an open, airy branching structure, and if your lilac sends up a few blooms, remember to deadhead them.
Enjoy your thoroughly revitalized, floriferous lilac. And if you want to avoid performing similar future horticultural amputations, remember to prune your shrubs lightly each year right after their flowers begin to fade.