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The best time to prune lilac is late spring -- immediately after they finish blooming. If you prune them later in the growing season or during winter, you'll remove flower buds and decrease the amount of spring bloom.
When to prune depends on several factors, notably the species being grown and the reason you are pruning. Pruning can actually stimulate growth. Pruning back a weak branch in late winter or early spring will often cause the new growth that replaces it to grow much faster. To slow growth down, prune in early summer. These are the two basic principles of pruning, but there are numerous exceptions.