I have a friend with a beautiful garden, designed and installed by professional landscapers. She loved it, but her one complaint was that her lilacs had bloomed the first year and then never really put on much of a show after that. When I visited her lovely garden my one question, that revealed the problem, was “When did you prune your lilacs?” It turns out she hadn’t pruned her lilacs, but the guys taking care of her garden (a grass cutting service that purported to also be a gardening service) always pruned them in late summer. This was the problem.
Lilacs set their blooms for the following season shortly after the current year’s bloom is finished. In my part of the world, lilacs usually bloom in mid-May, the flowers die off in June, and then the shoots for next year’s blooms develop in July. If you prune your lilacs after those shoots have started to form you won’t get any blossoms the following year–you’ve cut them off.
Last year, after having this conversation with me, my friend told her lawn service not to touch her lilacs. Recently I asked her if they’d bloomed “they’re beautiful! You should see them!” she said. Yup, nothing wrong with the plants, just the person wielding the secateurs.
Pruning is required:
To keep the lilacs blooming my friend will need to cut off this year’s old blooms now. If she doesn’t, the plant will put all of its energy into turning those blooms into seeds, and she won’t get a very good flower show again next year. Her question to me this year: “Where do I cut them?”
The answer is: right below the bloom, above the first set of leaves or new growth.
If you want to prune out more of the plant (for shape, or to keep its size in check) you would trim it like any other woody shrub (a good clean cut, right above a growth node). But if all you’re trying to do is ensure a spectacular crop of blooms next year, take 10 minutes this week and cut out all of this year’s finished blooms. If you have a large lilac you may need a ladder–with mine I can get away with just bending the taller branches toward me enough so that I can just reach.