Prune fragrant lilacs to maximize bloom

Last week I received an e-mail from Pat Felton, graduate of the Hancock County Cooperative Extension’s Master Gardener Program, Class of 2002, now gardening in Belfast. Her message contained questions and observations that will be great fodder for future articles. Since I am on “the pruning roll,” as she put it, let’s start with this one:

“Lilacs [are] one of my family pet peeves. My father always wanted to cut the ground suckers away. I always wanted to cut the big 2-inch-plus-thick old wood and thin out to a few new suckers. You might want to explain to people the rationale behind the best approach.”

Let’s begin with the basics in annual pruning of the common lilac (Syringa vulgaris) to promote healthy shrubs with abundant flowers. Keep in mind that lilacs flower on 1-year-old wood growing off of older wood. Your objective in pruning is to encourage development of young shoots at a height that will maximize enjoyment of the flowers’ color and fragrance. This is best accomplished in four steps.

First, after the flowers fade, cut some of the oldest stems as close to the ground as possible, selecting those stems approaching 2 inches in diameter. Second, shorten any remaining stems that are growing too tall to a strong branch. Third, remove young shoots so that they are at least two inches apart. Finally, direct the plant’s energy to growth rather than seed production by snapping off the spent flower heads immediately after flowering.

Unfortunately for most lilacs, this annual maintenance approach to pruning is neglected. The result is an old lilac for which renovative pruning is the only solution for restoring both health and bloom. Renovation involves drastically reducing the shrub’s height while removing any trunks or branches more than 2 inches in diameter. These large branches are more susceptible to the lilac borer, an insect that can do serious harm.

Renovating an old lilac is best done in early summer, after the flowers have faded. Because many varieties of lilac are sensitive to removal of too much wood in one year, it is best to space the renovation of an old shrub over three years, removing one-third of the overgrown trunks or branches each year, always focusing on those that are greater than 2 inches in diameter.

The need for renovation is often a hard sell to those who have grown up with the old shrub, who associate its massive gnarly character and the sweet perfume of its spring flowers with memories of relatives or old friends. They may not recognize the problems associated with neglect, the susceptibility of the larger trunks or branches to boring insects or the fact that the shrub’s flowers are now too high for their fragrance to be enjoyed. They may not notice that the numbers of flowers have declined in recent years.

They need convincing that renovation is the only solution for correcting these problems.

Maintain renovated lilacs on the annual pruning schedule discussed above with particular attention to step three. Removing old wood during renovation will stimulate growth of new shoots that are often considered “suckers” because they emerge from buds deeply embedded in below-ground stems. Typically there are far too many such young shoots to allow all of them to grow. Selectively remove most of the shoots, leaving a few widely spaced stems to replace the old trunks that were removed.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reeser

manley@shead.org. Include name, address and telephone number.