One of the most glorious and fragrant heralds of spring is the much anticipated blooming of lilacs. Copious mounds of purple, white and lavender grace us with their beauty and heady aromas for a short two to three weeks annually. Fortunately, the remaining shrub is quite attractive and easy to grow and maintain. With a little thoughtful pruning after your lilacs are three to four years old, you can keep them healthy and attractive, as well as promote disease resistance, throughout the remainder of the year. You'll also want to manage the growth of this plant that can easily reach a height of up to 30 feet tall. Prune your lilac each year as soon as it's through blooming in the spring. This must be done before June or July, when the plants set their buds for next year's flowering. For most maintenance pruning, keep in mind that you should never remove more than one-third of the plant's healthy stems at any one time.
Cut off all the spent and wilted blooms just behind the flowers. Trimming these stems back any further may remove many of next year's developing bloom buds. Use clean, sharp pruning shears and try to make your cuts at 45-degree angles whenever possible.
Remove suckers, unruly shoots and runners throughout the entire plant. These tend to grow wildly in random directions and usually don't produce blooms. They also give the lilac shrub a spreading appearance, when a taller, vase-like shape is much more desirable. Removing this type of growth will improve air circulation to the center of the plant and stimulate your lilac to bloom even more profusely next year.
Shear out any stems or branches that are growing straight downward, as they are untidy and totally unproductive. Cut out any branches or stems that cross over one another to enhance good air circulation. This also removes unnecessary clutter from your lilac's foliage.
Remove old, hard, woody branches every year. These will probably be fairly large, and you may need to use a very sharp saw for this part of the job. This will allow plenty of healthy new shoots to take the place of older wood, thereby promoting fullness and increased blooming capacity.
Rejuvenate older, overgrown lilacs over a three-year period, pruning in late winter. Prune out one-third of the largest stems at ground level the first year. Leave the stems that appear to be the healthiest and have the most attractive growth habit.
Remove half of the older stems that remain during late winter next year. Take them out at ground level, again retaining those that appear healthiest and most attractive. Thin a little new growth from the remaining stems to encourage new sprouts to emerge.
Cut out all remaining older wood at ground level late in winter of the following year. Thin the smaller shoots to encourage new growth. This pruning method will not only restore an old lilac bush to its former beauty but also will allow the plant to continue flowering for you each spring throughout the process.