Most flowering shrubs need regular pruning to keep them vibrant, and the common lilac (Syringa vulgaris) is no exception. Lilac bushes will typically flower without any effort from you. However, if left to grow and spread on their own, common lilacs will eventually only flower on the tops of the uppermost branches. Thus, you should be pruning lilacs annually to maintain their form and create a balance of new flowering shoots and older stems. Your lilac also might occasionally require a more extensive "rejuvenation" pruning to revive a bush with very thick stems and minimal flowering.
When to Prune Lilac Bushes
Many of the newer lilac varieties grow quickly and might need pruning starting in their second or third year. In general, by the time a stem reaches more than 2 inches in diameter, it should be pruned. If you are diligent with annual pruning of your lilac, the shrub will grow to about 8 feet tall with flowers throughout the branches.
The time to prune mature lilac plants is just after the flowers have faded in the spring. Lilacs set next season's flower buds almost immediately, so late pruning will mean sacrificing the next year's flowers. Pruning early also gives new shoots more time and energy to develop, ensuring plenty of blooms for the following spring.
Equipment / Tools
- Bypass pruner
- Pruning saw (optional)
- Stepladder (optional)
- Mature lilac bushes
Regular Maintenance Pruning
The general shrub pruning rule is to cut no more than a third of the stems each year. That will help the plant remain constantly vital, with new stems developing as old stems bloom. Your goal is to have a lilac bush that has somewhere between eight to 12 stems of various ages, all of them between 1 to 2 inches in diameter.
Prune Unsightly Features
Begin by pruning dead or diseased stems, pencil-thin suckers, and twiggy growth. Cut these back all the way to ground level. Pruning shears or loppers will generally handle these stems.
Remove Any Stems Thicker Than Two Inches in Diameter
This regular removal of entire old stems will prevent your lilac from becoming too tall. Avoid cutting off just the tops of long stems because this can leave the plant with an odd, unnatural shape. With very large stems, a pruning saw might be necessary. Thick lilac stems can be very tough.
Prune Remaining New Stems
Finally, if you want your lilac to fill in more and become shrubbier, prune the remaining new stems to an outward-facing bud. This means pruning just beyond buds that face away from the center of the plant. This technique will cause more branching and create a denser shrub.
Older lilacs can have stems as thick as small trees and will flower only on the topmost branches. Fortunately, rejuvenation pruning can revive an old lilac in about three 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 third rule. Prune a third of the oldest branches all the way to the ground each year for three consecutive years. 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. After three consecutive years of pruning your overgrown lilac in this way, new shoots should comprise the bulk of the plant. The plant will begin to bloom all over, and you can do regular maintenance pruning from that point 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 plant to about 6 to 8 inches above the ground in the early spring. Fertilize the plant with compost or a balanced fertilizer to prompt new growth. New shoots will develop throughout the growing season; let them grow through the summer. The following spring, begin pruning out the spindly growth, and maintain the healthiest shoots while considering the shape of the plant. Encourage branching by cutting back the remaining shoots to just above a bud. Carry on with regular maintenance pruning after this.
Working With Japanese Tree Lilacs
The Japanese tree lilac (Syringa reticulata) is a plant that can grow as high as 25 feet and has become increasingly popular in urban environments. It has a vase-shaped crown with spreading branches, and it produces showy white flowers in June.
Consistent with its tree-like shape, these plants should be pruned in the same manner that most small trees are handled. Prune to maintain an open interior and several main branches that form a vase shape. Japanese tree lilacs generally require little, if any, pruning until they are about 3 years old.
After this, any necessary pruning should be done immediately after the flowering period is over. Remove dead or diseased branches, as well as any branches that interfere with the overall vase shape or clutter the interior of the tree. If the tree grows too tall, you can cut back individual branches to around 1 foot below the desired height to prompt dense growth at the top.
Tips for Pruning Lilacs
Dwarf lilacs, such as ‘Palibin’ Meyer lilac (Syringa meyeri ‘Palibin’) and ‘Miss Kim’ Manchurian lilac (Syringa pubescens ssp. patula ‘Miss Kim’), look similar to the common lilac. But they rarely require maintenance pruning, though you can prune when necessary for shape. And like other lilac varieties, they also can benefit from deadheading.
Deadheading is the practice of removing dead blooms from a plant by hand. With some plants, this helps to stimulate continuing blooms. But with lilacs, the only time deadheading seems to help them bloom better is during the first few years of growth.
New lilac plants should begin blooming within two to five years. While the plants are young, deadheading the spent flowers helps to direct the plant's energy into setting more buds. However, once the plant has matured, it won't need this encouragement, and you'll likely have so many flowers that the task would be too time-consuming.
As with any plant, some years your lilac will bloom magnificently and some years not so much. Blooms are often dependent on the weather. A pleasant summer during which healthy new growth develops will reward you with abundant blooms the following year. A summer with extreme weather will yield fewer flowers. So don't panic if your lilac isn't as vibrant one year. As long as the plant is healthy and you keep up with maintenance pruning, the flowers will follow.