Answer: The most common reasons for lack of bloom are too little sunlight (lilacs like full sun all day long) and improperly timed pruning. Lilacs should only be pruned in spring right after their bloom season because they bloom on old wood, meaning late summer, fall, winter or early spring pruning would remove flower buds. Since lilacs develop flower buds during the summer months for the following year's display, summer water stress can retard the formation of flower buds, as well. Keep your lilacs well watered this summer and inspect the stem tips for signs of flower buds. As long as it gets full sun, adequate water and a pronounced winter chill, your lilacs should bloom next year.
Good luck with your landscape!
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