Answer: Two of the most common reasons for lilac not to bloom are pruning at the wrong time and not enough sunlight. Lilacs may be pruned immediately after they flower in the spring and then should not be pruned for the rest of the season. Pruning later in the season will remove the flowering wood for the following spring. Lilacs need to be planted in full sun all day for the best blooms, although they may tolerate partial shade and still bloom well if they receive about six hours of sun a day including the noon hour.
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