Answer: The most common reasons for a lilac not to bloom are shade (surrounding trees have grown larger over the years and now shade the bush where it used to be in full sun all day) and pruning at the wrong time. The plants bloom on wood grown the previous year, so any pruning must be done in the spring immediately after the shrub blooms. Pruning in summer or fall or early spring will remove flower buds. Finally, take care you are not oversupplying the plant with nitrogen, this can cause excess foliage and fewer flowers.
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