Answer: Lilacs produce blooms on new growth which develops in the previous summer so pruning immediately after flowers are spent will encourage new flowering growth. Each year you should remove new shoots from the ground that are smaller than a pencil plus a portion of the oldest trunks or stems by cutting them at ground level. Typically, you might routinely take about 10 to 20% of the older stems each year. On a long neglected shrub, in addition to removing the smallest suckers, also remove one third of the oldest stems in year one, half the remaining oldest stems in year two, and the rest of them in year three. This three year plan renews the entire plant. Alternatively you can cut the entire plant to the ground in early spring, but this will sacrifice a year of blooms. Make clean cuts and they will heal over quickly - no need to put wound dressing on your pruning cuts. Lilacs do fine with a general purpose granular fertilizer such as 10-10-10 and/or well rotted manure and/or compost applied in late fall after the leaves drop or in very early spring and again in late spring after bloom. They also do best in full sun and with adequate moisture.
Best wishes with your lilac!
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