Answer: First, let me apologize for the delay. I remember answering your question, but the response was obviously lost in cyberspace.
Previously you said the nursery recommended applying one pound of fertilizer per foot of tree height. This is not a good practice! You'll be overdosing your lilac and too much phosphorus and potassium in the soil can result in deformed and off-color foliage. The fertilizer package recommendations are correct; one tablespoon per foot of the diameter of the canopy is the amount you should apply. The fertilizer package should also recommend the correct frequency of application.
Best wishes with your lilac!
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