How & When to Prune an Old Lilac Tree
The Japanese tree lilac (Syringa reticulata) grows to a height of 20 feet. Bearing long, wide panicles of cream-coloured blooms, this lilac can be grown as a tree or as a large shrub.
When the lilac becomes old or has not been pruned properly, the years of lilac borer infestation, powdery mildew and old growth can stifle blooms and cause weak new growth. Rejuvenate your lilac tree by following simple pruning procedures to regain your lilac's former beauty.
When to Prune
Lilacs must be pruned immediately after flowering in the spring if you expect to see blooms in the following season. Lilacs flower on old wood, or the previous season's growth. This means that after blooming ceases in the late spring, the tree or shrub immediately begins storing energy to produce buds that will overwinter and create blooms the following spring. If you prune in the late fall, winter or early spring, you will remove the buds and thus the blossoms for that year. The only exception is in cases when severe renewal pruning is needed during late winter or early spring, which removes all the canes to allow for brand-new growth.
Severe renewal pruning is best in cases where there has been confirmed lilac borer disease or powdery mildew. Also, an old tree or large bush that has not been properly cared will grow large, unhealthy canes around the outside of the bush that shade the centre from sunlight and cause dwindled growth and blooms.
To renew these types of lilacs, cut back all the canes to within 6 to 8 inches from the ground. Use sharp gardening shears, and always clean your tools with a solutions of equal parts water and bleach to ensure disease doesn't spread between plants.
In the spring, when new shoots emerge, chose up to seven canes to keep, and prune back the others to ensure a good form to your renewed lilac. Keep in mind that you are forfeiting blooms for that season in favour of promoting long-term health of the lilac.
Over the years, a lilac tree that has not been pruned to grow as a tree will form a large shrub shape. To correct this and promote more growth and better flowering, renewal prune one-third of the oldest canes close to ground level just after flowering ceases in the spring.
Follow this same procedure every year for the next three years, until all the oldest canes have been cut back. Not only will you ensure continuous blooms over the three years, you will gradually change the shape of the bush as well as create healthier growth for years to come.