Answer: Lilacs tend to be long ived plants, so you might trouble shoot as follows. The plants need full sun to bloom their best, often neighboring trees grow and shade what was originally a sunny spot. This is one of the most common reasons they fail to bloom well. Next is improper pruning. After cutting a bush short in an effort to rejuvenate it, there will be a forest of new stems from the ground. This year especially, remove any that are smaller than a pencil an remove smallest shoots each spring. Also, after bloom this year, you may tip prune to encourage branching. In terms of fertilizer, lilacs like a rich soil so you could use compost, well rotted manure, or an all purpose complete granular fertilizer such as 10-10-10 according to the label instructions. Apply it in early spring and possibly again in late spring, possibly one more time in mid fall after the leaves have fallen. Do not however fo overboard as overfertilizing with nitrogen can cause excess leaf growth at the expense of flowers. Finally, some old lilacs bloom profusely only in alternate years, so this may be an off year for yours.
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