Answer: Lilac's have a reputation for taking several years to become established. Once they do, they grow and flower prolifically. It sounds as though you have one of those reluctant lilacs. The best approach is to do nothing other than to water regularly during the spring and summer months and to prune it back by one-third after it has finished flowering next spring. Pruning it back will promote healthy new growth which should also result in the development of flower buds for the following year's bloom. Lilac's prefer full sunshine but will adapt to partial sun. They do not flower as well if they receive afternoon shade.
Best wishes with your landscape.
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