Lilac shrubs are stunners in the landscape, producing lush panicles of blooms in white or hues of purple during the spring and very early summer. Hardy in USDA zones 3 through 7, lilacs flourish in all but the warmest and coolest climes. According to the University of Nebraska at Lincoln, lilac shrubs should be pruned each year to remove spent flower heads, compromised wood and under-performing branches.
Prune your lilacs in the late spring or early summer each year, immediately after the shrub finishes flowering and its last blooms have died back.
Remove one-third of the older blooming wood branches every year. Remove the entire branch from the crown of the plant, just above the soil line. Use loppers or a pruning saw to make a clean cut just above the crown of the plant. Pull the entire branch from the canopy and recycle or discard it. Old branch removal will trigger development of new, flowering branches.
Rejuvenate older, defoliated, long-neglected or non-flowering lilac shrubs with a regimen of severe pruning over three consecutive years, during the late winter when lilacs are dormant. Remove one-third or the largest-diameter branches in year one, cut down to the crown of the plant just above the soil. Prune away half of the remaining stems one year or older, again down to the crown. Prune away all of the older-wood, large-diameter branches in year three. Leave thin young branches under 5 years of age. Resume a regular annual light pruning regimen in year four and continue it thereafter.