Answer: You can feed your lilac with a low nitrogen, high phosphorous, high potassium fertilizer such as a 5-10-10 or 10-20-20 to encourage bloom.
A 20 year old lilac bush is probably a sight to see when it is in bloom and I suspect it does bloom or the previous owners would have removed it from the landscape years ago. Lilacs bloom in the spring. After flowering has finished the stems where the flowers were elongate and new flower buds develop over the summer months for the following spring's bloom. So pruning in the winter or early spring is not a good idea - you may accidentally prune off the flower buds. To keep your lilac looking nice, allow it to flower and as soon as the flowers are spent, prune it back a bit. New growth will develop and new flower buds will form on this new growth. They will open next spring and you can repeat the pruning process. As long as you get new growth, you will get flowers. The goal is to have new growth all over the plant, not just at the top, and pruning annually will help you accomplish your goal. Best wishes with your new landscape!
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