Q&A: Pruning is the key to keeping lilacs from growing too tall

In the Garden

Q: My lilac has grown tall and most of the flowers are up at the top where I can’t enjoy them. Is there any way to keep this from happening if I start over with a new one?

A: Beloved as they are, lilacs can be somewhat of a disappointment. That’s because they often take a long time to begin blooming after planting, and once they do finally begin flowering, before you know it, they grow tall with most of the blooms occurring out of reach in the top third of the shrub.

When you buy a new lilac, the only pruning needed is to cut back overly long stems to nodes farther back on the branch to promote bushy growth. Once flowering begins, however, annual pruning is needed to control for height and to promote heavier blooming.

Soon after the flowers fade, remove the spent flower by cutting back to a pair of leaves or a side branch slightly farther down on the stem. Removing the spent flowers before they are able to set seed will prevent the tree from wasting energy ripening seed, which will encourage increased flower-bud production while helping to keep the tree from growing too tall.

If your lilac grows taller than desired, right after the blooms fade cut back a few of the tallest stems to vigorous side branches a third or more of the way down the branch. Remember, however, that blossoms are produced on previous seasons’ growth, so don’t cut too many branches down hard or you’ll pay for the reduction in size with reduced flowering the following spring.

If you’re in the market for a lilac, keep in mind that there are new dwarf varieties available, including the repeat bloomer ‘Boomerang’ that tend to stay much lower in height and, with yearly pruning, can easily be kept shorter than 6 feet tall.

Q: Several of our deciduous trees are heavily infested with tent caterpillars. Do I have to spray with something to keep them from harming my trees?

A: Tent caterpillars make an appearance in our area from time to time, and although they often show up in huge numbers, there’s no need to panic. A healthy tree can withstand the loss of a third of its foliage without suffering lasting damage, and even if a tree is defoliated, it will usually recover as long as the defoliation does not occur two years in a row.

The good news is that many beneficial critters help keep the populations of these intruders in check. Birds feast on tent caterpillars, and baldfaced hornets and paper wasps consider them a gourmet treat. Surprisingly, one of the most beneficial creatures when it comes to tent-caterpillar control is a fly. Tachinid flies look like houseflies but they attach tubular, one-sixteenth-inch-long eggs on the caterpillars’ bodies. The maggots that hatch from the eggs bore into the caterpillars, putting the “el Kabotski” on hundreds of them.

Despite the efforts of these natural predators, sometimes equally high populations of the caterpillars return the following spring. In that case, a spray might be needed to knock numbers down enough to prevent defoliation from occurring two years in a row. Fortunately, there is a commercially available biopesticide, Bacillus thuringiensis (Bt) that is a safe, naturally occurring bacteria that is effective at controlling caterpillars. It does not harm birds nor beneficial insects; however it is lethal to butterfly caterpillars, so use it only when and where necessary. Bt works best when the insects are newly hatched. The caterpillar must eat leaves that have been sprayed, but sun breaks down the bacteria, so if possible, apply Bt on a cloudy morning on a day it is not expected to rain. Then let nature’s helpers finish the job. By the third year, it’s highly likely the troublesome tent caterpillars will cease to be a problem.