Lilacs: Spring's Favorite Perfume
Care and pruning
Left to their own devices, lilac bushes can become a bit straggly. See whether yours gets at least six hours of full sun per day. If not, move it or plant a new one (which can take up to three years to flower).
Don't be afraid to prune. How you prune this year affects next year, as lilacs set their buds on last year's wood. The best time is right after the last flower turns crispy brown. Remove each flower stalk and unruly branches that are too tall or floppy. Then stop.
If you prune in spring before the lilac blooms, you will get few or no flowers.
Blooming may suffer if you apply fertilizer with too much nitrogen, which promotes foliage growth at the expense of flowers. Avoid the area around lilacs when you fertilize your lawn. If you want to fertilize lilacs, try a bloom-booster in which the second of the three numbers (the nitrogen-phosphorus-potassium ratio) on the package is approximately twice as high as the first number.
If your lilac has lots of old wood, encourage new growth with rejuvenation pruning. For vigorous new limbs, cut back one-third of the old wood to the ground each year for three years.