Answer: The most common reasons for lilac to fail to bloom are lack of sun and/or pruning at the wrong time of year. Lilacs do best with full sun all day long, or a minimum of six hours of sun including the hour of noon. If the spot where your lilac is planted is shaded by nearby trees, for example, then it is possible they may have grown to shade the lilac more than they used to. Since it did manage to bloom in year four, though, I suspect most of the problem is related to pruning.
Lilacs bloom on wood that grew the year before, so pruning in fall or early spring will remove the flowers for the coming season. (You saw the results of fall pruning when it bloomed only on the old wood left at the bottom after you pruned it back hard. If you had not pruned it, it probably would have bloomed nicely all over the shrub.) The time to prune a lilac is in early summer right after it blooms.
Lilacs are best pruned by thinning out a few of the oldest stems each year. Do this immediately after it blooms. Cut them off at the base. Remove only about one fifth of the plant this way each year. Also, at the same time, remove any shoots from the ground that are thinner than a pencil. This promotes vigorous new growth while allowing for good air flow and light penetration into the plant.
You would not want to overfertilize a lilac; some gardeners will top dress with compost or a general purpose granular or slow release fertilizer per the label instructions each spring. If your lilac is adjacent to a lawn that is being fertilized, or if your local soil is naturally good and rich, you may not need to fertilize it separately.
I hope this helps.
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