I’ve been trying to get rid of a lilac bush that grew out of control and wasn’t in a good spot anymore. A couple of years ago I cut the whole thing down to ground level, thinking that should kill it. But no – it came back stronger than ever. Lilacs obviously enjoy a good haircut.
Ross Penhallegon is an extension horticulture agent at Oregon State University. He says lilacs will benefit from annual pruning, although the rule of thumb is to cut no more than one-third of the shrub’s stems each year. Plants that have a balance of new shoots and older stems are the best bloomers.
"As we reduce the size of the lilac bush, we would prune it down to where we want it to be, and that can be many sizes, shapes, and forms. But every time we prune or make a cut, that’s going to produce new growth whether it’s the old growth at the base of the plant, or if it’s new growth on the top of the bush," says Penhallegon. "So, both those cuts will produce new flowers the next year."
Lilacs that are left on their own will get taller, and eventually the flowers will be way above your head. We typically want to reduce the height, so take out some of the oldest growth from the base of the plant, leaving 4-6 inches where you cut. Don’t worry about hurting the lilac. It can take it.
"Pruning the older branches near the base of the plant, it’s called returning the bush to a younger, more vigorous state which then produces flowers tends to be kind of the concept that we’re after," says Penhallegon. "Yes you can prune it vigorously, and we can prune things fairly hard."
Remove the flowers immediately after they’re done blooming. Pruning late winter or early spring will mean sacrificing next year’s blooms. You can leave the flowers on, but the plant may put its energy into trying to produce seed pods rather than new growth for next year’s flowers.
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