Answer: There are three or four things that contribute to a good show of flowers on lilacs: they prefer alkaline soil, a late summer drought, correct pruning, and a pronounced winter chill to help force them into bloom. The easiest things to provide are correct soil pH and pruning - the most difficult are summer drought and winter chill. These factors are the reasons lilacs have a terrific bloom some years, and a sparse showing in other years. Have your soil tested and add lime to raise the pH to about 7.5. Prune to renew the branches and encourage the growth of new flowering stems. Flower buds form on the tips of new shoots in late summer. If you're pruning off the new growth, you may be pruning off potential flowers. Stop watering now to stress the plant a bit and encourage blossoms in the spring, and hold off on fertilizing (high nitrogen fertilizers will produce lots of green growth at the expense of blooms). If your lilacs are in a mostly sunny location and are otherwise healthy, the above should help produce blooms next spring.
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