Most flowering shrubs need regular pruning, to keep them vibrant. Lilacs are no exception. Giving your lilac plants an annual pruning and clean-up will keep them blooming and thriving and blooming. Luckily, it won't take much effort or time, but it really is worth doing, if you want your lilac bushes to keep providing you with lush, fragrant flowers for decades.
Why Should You Prune?
Lilac bushes will flower without any effort from you. However, if lilacs are left to grow and spread on their own, without maintenance pruning, they will eventually only flower on the top branches. You will get shrubs reaching 15 to 20 feet in height and all the blooms will be way above your head. You won't get to see them or inhale their delicious fragrance.
Lilac plants that have a balance of new shoots and older stems, will be the best bloomers. 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 with regular pruning.
In general, by the time a stem reaches more than 2 inches in diameter, it should be pruned out. If you are diligent with yearly renewal pruning of your lilacs, your shrubs will be about 8 feet tall, with flowers throughout the branches. More on that below.
When to Prune
New lilac bushes don't require maintenance pruning until they are about 6 feet tall. Many of the newer lilac varieties grow quickly and may need pruning starting in their second or third year.
The time to prune mature lilac plants is just after the flowers have faded. Lilacs set their 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 time and energy to develop.
How to Prune
The pruning rule of thumb is to cut no more than 1/3 of a shrub's stems each year. That will help 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 to 12 stems of various ages, but all should be between 1 to 2 inches 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 inches in diameter
Regular pruning of old stems should keep your lilacs from becoming too tall and cumbersome. What you do not want to do is cut off just the tops of branches. Cutting back just the tops of long stems leaves the plant with an odd, unnatural shape. You are better off completely 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 and a denser shrub.
Older lilacs can have stems as thick as small trees and will flower only on the topmost branches. Fortunately, it is pretty easy to rejuvenate an old lilac in about 3 years time. There are two 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 to 8 inches from the ground, in the 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.
The only time deadheading seems to help lilacs bloom better is during the first few years of growth. New lilac plants should begin blooming within 2 to 5 years. While the plants are young, deadheading the spent flowers put the plant's energy into setting more buds. Once the plant has matured, it won'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 fewer flowers. So don't panic and give up on pruning. As long as your lilac is healthy, the flowers will follow.