Answer: There are several reasons for lilac not to bloom, but assuming the shrub is growing well, the most common is probably incorrect pruning. Lilac blooms on "old wood" and should only be pruned right after it blooms. Pruning later in the summer will remove the next year's blooming wood and/or flower buds. Other possible causes would be insufficient light (lilac prefers full sun), and possibly overfertilizing (too much nitrogen can cause an excess of leaves and reduced blooming) or less likely but still possible, underfertilizing if your soil is truly terrible. Another reason is immaturity, but after six years I would have expected a lilac (Syringa vulgaris) to have bloomed. Finally, there are some special, more unusual varieties of lilac which tend to be smaller plants and do not bloom until they have reached a certain age, so perhaps you purchased one of these.
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