Answer: Themost common reasons for a lilac to fail to bloom are insufficient sunlight (perhaps neighboring trees have grown larger and now shade it) or improperly timed pruning (Lilac should only be pruned in spring right after it blooms because it blooms on old wood grown the previous year.) When just one out of a group of plants fails to bloom, you need to do some detective work. Perhaps there was a root injury due to voles, or a borer has made holes in the trunk, or there has been a change in drainage patterns near that shrub? Or maybe it was pruned during the summer or fall or early spring for some reason.
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