Lilacs require full sun for maximum bloom, except for areas of the hottest sun, where it needs partial sun. They prefer a slightly alkaline soil (pH 7.5). When they are planted in strongly acidic soil, lime should be cultivated into the soil beneath the drip line to adjust the pH. Most Lilacs require a pronounced winter cold period. When your lilac finishes blooming, prune the spent flowers back to the point just above where the new buds are forming. Dead or damaged stems should be cut off, and a few of the oldest stems should be cut back to the ground. Heavy pruning will result in the loss much of next years bloom, because next years flowers are formed on the plant this year. Don't get carried away when you are pruning.