If you didn't prune your lilacs right after they bloomed, you still have a little time left, but don't delay: Pruning them too late can take off next year's blooms. Here's what you need to know about pruning lilacs.
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Lilacs flower on old wood, not on the new growth in spring. That's why you can't prune them once summer begins, at which point next year's buds are developing. Prune now for best results next year. You should prune off about a third of the branches and clip any shoots around the stem, right at ground level. Lilacs put out prodigious quantities of new shoots, but you want only a handful of main stems coming out of the ground.
Reach deep inside the lilac to do your pruning; the one-third you remove needs to come from the interior to allow light into the plant. Keep the plant's overall height in mind as you prune; unless you want all the blooms over your head, you'll want to trim the plant back to no more than six feet tall.
If a lilac has become completely overgrown and unwieldy, you can cut the entire plant back to within inches of the ground and wait a few years for it to bloom again. Regular maintenance, however, should keep you from having to take such a radical step.
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