Most flowering shrubs need regular pruning to keep them vibrant. The common lilac (Syringa vulgaris) is no exception. Giving your lilac shrubs an annual pruning and clean-up will keep them full of lush, fragrant spring flowers from top to bottom for decades. How you prune your lilacs will depend on your goals, and whether the bush has been well maintained or is in need of a major rejuvenation.
Why Common Lilacs Need to Be Pruned
Lilac bushes will flower without any effort from you. However, if left to grow and spread on their own without regular maintenance pruning, common lilacs will eventually only flower on the tops of the uppermost branches. You will get shrubs reaching 15 to 20 feet in height and all the blooms will be far above your head; as a result, you won't really see them or inhale their delicious fragrance.
Lilac plants that have a balance of new shoots and older stems are the best bloomers. The newer stems won't bloom for a couple of years, but to keep the flowers coming steadily, you'll need to constantly renew the plants with regular pruning.
When to Prune Lilac Bushes
New lilac bushes don't require maintenance pruning until they are about 6 feet tall. Many of the newer lilac varieties grow quickly and these may 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 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.
The time to prune mature lilac plants is just after the flowers have faded. Lilacs set 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 and ensures plenty of blooms for the following spring.
- Working Time: 1 to 2 hours for each moderate-sized shrub
- Total Time: Plan on one pruning session each year
- Materials Cost: None, unless you need to buy a bypass pruner ($5 to $25) or loppers ($25 to $50)
What You'll Need
- Bypass pruner
- Pruning saw (if needed)
- Stepladder (if needed)
Although many of the principles are the same, there are slight differences between the annual pruning that lilacs routinely require and the occasional "rejuvenation" pruning that helps revive a lilac shrub that has developed very thick stems and now flowers only in the top portion.
Regular Maintenance Pruning
With all pruning of lilac shrubs, the general pruning rule of thumb is to cut away no more than 1/3 of a shrub's stems each year. That will help the plant remain constantly vital, with new stems developing as older stems peak and bloom. Your goal is to have a lilac bush that has somewhere between 8 to 12 stems of various ages, all of them 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. Cut these back all the way to ground level. Pruning shears or loppers will generally handle these stems.
- Next, remove any stems thicker than 2 inches in diameter. This regular removal of entire old stems will prevent your lilacs from becoming too tall and cumbersome. Avoid cutting off just the tops of long stems, because this leaves the plant with an odd, unnatural shape. With very large stems, a pruning saw may be the best tool to use, because thick lilac stems can be very tough.
- 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 create 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. Prune out 1/3 of the oldest branches each year for three consecutive years, 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. At this time of year, you can easily see into the shrub and there will be easier access to the base of the stems. 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 simple 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 lilac plant to about 6 to 8 inches above the ground in the early spring. 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, giving consideration to the shape and structure of the plant. Then, encourage branching by cutting back the remaining shoots to just above a bud. Carry on with maintenance pruning after this.
Do Lilacs Need Deadheading?
Deadheading is the practice of removing dead blooms from a plant by hand. With some plants, this practice helps 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 2 to 5 years. While the plants are young, deadheading the spent flowers helps direct the plant's energy into setting more buds. Once the plant has matured, however, it won't need this encouragement, and you'll likely have so many flowers that task would be far too time-consuming.
As with any plant, some years your lilacs will bloom magnificently and some years not so much. Bloom is often dependent on the weather. A pleasant summer during which much new growth develops will reward you 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.
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. They do, however, benefit from deadheading, which is much simpler with these diminutive shrubs. Dwarf lilacs can be pruned when necessary for shaping.
After performing a rejuvenation pruning, fertilize lilac bushes with compost or a balanced fertilizer. This feeding will prompt new growth and speed the shrub's recovery.
Japanese Tree Lilacs
The Japanese tree lilac (Syringa reticulata) is a large shrub or small tree that can grow as high as 25 feet, and it has become increasingly popular in urban environments. It has a vase-shaped crown with spreading branches and produces showy white flowers in June.
Consistent with its more 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. These plants 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 that clutter the interior of the tree. If the tree grows too tall, you can cut back individual branches to a point about 1 foot below the desired height to prompt dense growth at the top.