Answer: In general, lilacs need only average soil to bloom. They also need full sun and an adequate amount of water. You might try giving it some compost and/or well rotted manure and/or a complete fertilizer according to the label instructions in early spring and again in late spring. The most common reasons for lilac not to bloom are pruning at the wrong time and lack of sun. They should be pruned in late spring only, because pruning later than that can remove the next year's blooming wood. Finally, some lilacs such as the Persian Lilac or "Miss Kim", simply do not bloom until they have reached a certain maturity, and this can take a number of years.
Q&A Library Searching Tips