Answer: The most common reason lilacs fail to bloom is lack of sunlight. They do best in full sun. In part shade they may bloom sparsely. If your lilacs are in full sun, perhaps the flower buds were nipped by a late frost. Take a close look at the shrub to see if there is evidence of any flower buds. Finally, you may have purchased a variety that takes a long time to mature and produce blooms. If this is the case, they will flower - eventually.
Lilacs produce next spring's flower buds on this year's summer growth so timing your pruning is essential. Prune immediately after the flowers fade. In the case of non-blooming lilacs, prune in late June. This will give your shrubs the rest of the summer to produce new stems, leaves, and flower buds for next year's floral display.
Best wishes with your lilacs!
Q&A Library Searching Tips