There is no secret fertilizer for lilacs. If your bush lack vigor use a complete product at the medium label rate. Don't prune the bush in winter. If you need to prune do so after the normal spring blooming season. Keep the soil moist but not soggy wet. Make sure the plant is in plenty of sun. These all are important in helping a lilac to bloom well. In your area lilacs are marginal due to your long hot summers and relatively short mild winters.
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