Lilacs are hardy shrubs, meaning they need little care to survive and thrive. The biggest concern for lilacs during the winter is dehydration caused by dry winter winds and frozen ground. Choosing a lilac hardy in your zone and correctly siting and planting it is the best way to ensure it survives the winter without damage.
Sprinkle 1 to 3 cups of lime, also called calcium carbonate, around the lilac at the drip line in mid to late fall or before the ground freezes in your area. The drip line is the imaginary line around a tree or shrub where the tips of the branches reach. This is where most of the small feeder roots that absorb nutrients are located. Small lilacs, under 4 feet, only need 1 cup of lime. Larger lilacs between 4 and 6 feet need 2 cups, and lilacs over 6 feet tall need 3 cups.
Give your lilac a good watering (2 to 3 gallons) before the ground freezes to ensure the shrub is hydrated before winter starts. If you have had a rainy fall skip this step. You can water your lilac during winter warm-ups as long as day and nighttime temps will stay above freezing. Be sure not to over-water; a half-gallon of water is plenty.
Use 3 to 4 inches of organic mulch, like shredded leaves or bark, around the base of your lilac to prevent winter heave, the exposure of roots due to the ground freezing and thawing. Lilacs over the age of seven generally don't have a problem with winter heave due to their deep roots.
Wrap your lilac in burlap, leaving the top open, in zones 5 and below to prevent damage to buds and tender growth. One to two layers is enough to prevent damage. Wrap from the bottom up, overlapping the layers. Gently lift the branches up slightlyt towards the main trunk to prevent snow from accumulating on the burlap and braking branches.
Secure the burlap with twine.