Anyone who has old lilacs growing on their property knows that lilac plants put out enough suckers to keep themselves alive for centuries. To keep your lilac thriving and blooming, some periodic pruning is advised.
Why Prune Lilacs?
If lilacs are left to grow and spread on their own, without maintenance pruning, you will get shrubs reaching 15 - 20' in height and all the blooms will be way above your head. Lilac plants that have a balance of new shoots and older stems, will bloom best. The newer stems won't bloom for a couple of years, but to keep the flowers coming steadily, you need to constantly renew the plants. In general, by the time a stem reaches more than 2" in diameter, it should be pruned out. If you are diligent with yearly renewal pruning of your lilacs, your shrubs will be about 8' tall, with flowers throughout the branches.
When to Prune Lilacs
New lilac bushes don't require maintenance pruning until they are about 6 - 8' tall. The time to prune mature lilac plants is just after the flowers have faded. They set the next season's flower buds almost immediately, so late pruning will mean sacrificing next year's flowers. Pruning early also gives the new shoots more energy to develop.
How to Prune Lilacs (Maintenance Pruning)
The pruning rule of thumb is to cut no more than 1/3 of a shrub's stems each year. That will keep the plant rejuvenate, with new stems developing and older stems peaking and blooming. Your goal is to have a lilac bush with somewhere between 8-12 stems of various ages, but all between 1-2" in diameter. You won't always need to remove 1/3 of the plant, so use some discretion.
Begin by pruning:
dead or diseased stems
pencil thin suckers and twiggy growth
stems thicker than 2" in diameter
stems that cross or rub against each other
Regular pruning of old stems should keep your lilacs from becoming too tall and cumbersome. Cutting back just the tops of long stems leaves the plant with an odd, unnatural shape. You are better off removing any stem that has become overgrown. If you want your lilac to fill in more and become shrubbier, after pruning out the older stems, prune the remaining new stems to an outward facing bud. This will cause more branching.
Reviving an Old, Overgrown Lilac
Older lilacs can have stems as thick as small trees and will flower only on the top most branches. Fortunately, it is pretty easy to rejuvenate an old lilac, in about 3 years’ time. There are 2 approaches you can take:
The less drastic approach to getting an overgrown lilac back into shape, is to use the 1/3 rule of thumb. Prune 1/3 of the oldest branches each year, all the way to the ground. Start by taking out the thickest stems first. Although you'll be losing some flowers for the current year, pruning overgrown lilacs is easiest early in the spring, before the branches leaf out. You can really see into the shrub and there is easier access to the base of the stems.
After pruning your overgrown lilac by 1/3 for 3 years, the new shoots should be the bulk of the plant, the plant should begin to bloom all over and you can do simple maintenance pruning from then on.
If you can't stand the look of your old lilac or you just want a quicker approach, you can take the drastic measure of cutting back the entire lilac plant to about 6-8" from the ground, again in early spring. New shoots will start to develop throughout the growing season. Let them grow during the summer. The following spring, begin pruning out the spindly growth and maintain the healthiest shoots, giving consideration to the shape and structure of the plant. Then cut back the remaining shoots to just above a bud, to encourage branching. Carry on with maintenance pruning.
Do You Need to Deadhead Lilacs?
The only time deadheading seems to help lilacs bloom better is during the first few years of growing. New lilac plants should begin blooming within 2-5 years. While the plants are young, deadheading the spent flowers puts the plant's energy into setting more buds. Once the plant has matured, it doesn't need this encouragement and hopefully you'll have so many flowers the task would be too time consuming. As with any plant, some years your lilacs will bloom magnificently and some years not so much. Bloom is often weather dependent. A pleasant summer will reward with abundant blooms the following year. A stressful summer will yield less flowers. So don't panic and give up on pruning. As long as your lilac is healthy, the flowers will follow. -Birdy