Answer: If a lilac is very young and small, you don't need to prune it. But as it grows it will need to be pruned, both to keep it looking neat and compact, and to promote flowering. Lilacs produce flower buds on new growth during the summer months for the following spring's floral display. If you never prune your lilac all of the new growth will be from the ends of the branches and all the flowers will end up developing only on the uppermost part of the plant. If you want flowers all over your lilac, you'll need to prune to promote new growth throughout. Prune immediately after flowers have faded to give your lilac all summer to develop next year's flower buds.
Best wishes with your lilac!
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