The National Gardening Association recommends pruning lilacs immediately after flowering. Removing broken and dead branches before trimming aids in achieving a desired shape and size. Stems longer than 2 inches should be trimmed; lilacs may be cut back by as much as one-third.
Applying a layer of compost and a 2-inch thick layer of mulch in the spring helps the lilac tree to retain moisture and discourages weed growth. While many lilacs are drought-resistant, summer weeks producing less than 1 inch of rainfall require adequate watering of the lilacs to ensure lively growth. Bushes do not need regular pruning until they are between 6 and 8 feet in height.