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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.
prune roses just as or before new growth emerges from the canes (cut off the wood that died over the winter. this will promote strong, healthy shoots that will produce lots of blooms. it will also give plants a more open habit (?) which helps them resist diseases like black spot