How to Prune Your Lilacs in 7 Simple Steps

Trying to prune your lilacs this season but aren't quite sure where to start? Pruning your lilacs can be a fun and enjoying experience if you follow the proper steps. In this article, certified master gardener Liz Jaros outlines how to prune your lilacs by following some very simple steps!

how to prune lilacs

Perfectly shaped, full flowering lilacs are a thing of beauty in the landscape. And that’s certainly no accident. If left to their own devices, these spring-blooming, strong-scented members of the olive family will grow thick and wild. They will flower sparsely, and they will have crowded leggy stems. They won’t look like the lilacs pictured on the plant tags, or the ones you see on your neighborhood garden walk. And they’ll probably have issues with pests and disease.

The good news is, it doesn’t have to be that way. Your lilacs’ fates are in your hands, and in your gardener’s tool bucket. They just need to be pruned, promptly and properly! But, where do you start? If you’ve never pruned a lilac before, there’s a few quick tips you’ll want to be aware of before you start.

Lilac pruning really isn’t all that hard to do, it just takes a little patience! No matter the type of lilac you have, the same principles apply. Continue reading to find out exactly how to prune your lilacs, by following these 7 very simple steps!

Start Pruning With a Plan

pruner and a blooming cluster of lilacs on the grass
You’ll want to start with a plan before you start cutting your lilacs.

Before you start lopping off random lilac branches, step back and take a minute to think about why you’re out there. Is it too tall? Too wide? Are they too leggy? Too dense? Have you been seeing fewer and fewer blooms each year? Is there evidence of disease or insect infestation? Would you like it to bloom again this year, if possible?

When it comes to pruning lilacs, you must begin with the end in mind. Each one of these issues will require a different approach and possibly a different technique. You should know what you want to achieve before you make a single pruning cut.

Gather Your Tools

lilac pruning tools
The most commonly used tools for Lilac pruning are the bypass pruner, lopping shears, pole pruner, hedge shears, and a pruning saw.

Since lilacs are typically multi-stemmed with a dense branch structure, you’ll want to protect your eyes with some work goggles, or sunglasses at the very least. If you spread a tarp around your lilac’s base, it’ll catch little sticks and branches that fall during the process, making cleanup a little easier.

Depending on what variety you’re working with, you might be able to perform basic pruning tasks with a small hand tool, but you’ll likely need a little more power if you’re dealing with large stems or branches. Here are the most common lilac pruning tools and their usage:

Bypass pruner

These hand shears are small but powerful, with a thin blade that slides past a thick blade. You’ll use them to remove twigs and stems up to a half inch in diameter. You might also use them to deadhead spent blooms.

Lopping shears

With two long handles that operate in a scissoring motion, lopping shears are typically used for branch widths between a half inch and two inches. They might also be used for smaller branches that are overhead or slightly out of reach.

Pole pruner

This is essentially a hooked blade on the end of a long pole. A long rope attached to the blade is pulled down to initiate the cut against a stationary blade. Use a pole pruner on high branches, but be careful to protect yourself from them when they fall! And watch out for power lines.

Hedge shears

This tool looks like a giant scissors, with long blades and long handles. It’s used to shape small leaf lilacs into hedges and/or shape them into round forms.

Pruning saw

A folding saw swings open to become a long slender saw with large teeth, and it cuts on the pull. It’s used to prune large trunks or branches from your lilacs and is particularly useful in tight spaces. A bow saw is shaped like a triangle and is easier to control, but it needs a bit of space between branches to move and cut properly.

Before you start working, make sure your tools are all clean and sharp. Ethanol or isopropyl can be used to quickly remove pathogens from blades. Or use a solution of 9 parts water to 1 part bleach to disinfect tools that have been used on other plants in the yard.

Have your local hardware store sharpen anything that’s not going to produce neat, perfect cuts, and always keep tools oiled and stored in a dry location.

Prune at The Right Time

woman cutting off lilac flowers
The golden rule when pruning lilacs is to produce them immediately after flowering.

Generally speaking, ‘prune immediately after flowering’ is the golden rule of lilac maintenance. As a spring flowering shrub, lilacs set their blooms in mid to late summer for the following year. If you prune in fall or early spring, you’ll be removing some or all of its buds and you will not get the spectacular flower display you’re anticipating. 

So get out there as soon as your lilac’s flowers have faded, and start doing the prep work for next year’s show. A gardener is nothing if not optimistic.

There are exceptions to the rule, of course. If you’re looking to restore an old, tired, overgrown lilac by cutting it completely to the ground (see rejuvenation pruning below), you should perform these cuts in late fall or early winter when your lilac is dormant. This will cause less stress and reduce its vulnerability to pests and diseases.

Reblooming lilac varieties provide another exception to the ‘prune immediately after flowering’ rule. Most flowers in this category are sold under the brand name ‘Bloomerang’ (Syringa x ‘Penda’) and they are designed to send up a second bloom in midsummer.

While you will want to deadhead the spent flowers on these cultivars to encourage another wave, major pruning efforts should be performed later in the summer after these varieties are completely done for the season.

Remove Suckers

woman pruning the lilac suckers
So that the shoots of lilacs do not draw energy from the roots, they must be removed with a lopper or bypass pruner.

Lilacs are notorious for sending up basal roots, also known as ‘suckers.’ These shoots are a plant’s natural attempt to spread out and colonize an area. You will typically see them popping up out of the dirt near your lilac’s trunk or base.

If left alone to do their thing, suckers will draw energy from the roots and can hinder healthy growth and bloom production, so they should be removed. Dig down as close to the root base as possible and cut them out with a lopper or bypass pruner. Repeat each year as part of your maintenance routine.

Make Cuts To Reduce Density

woman cutting lilac branches to reduce density
‘Renewal pruning’ includes pruning old and tall stems to allow lilacs to get more sunlight and water.

Considered routine maintenance in most lilac shrubs, ‘renewal pruning’ involves cutting out roughly ⅓ of existing stems at the base. Begin with the oldest and tallest stems as well as those that are dead, crossing, or broken. Look for the thickest, woodiest stems to determine the oldest. They will usually be gray and have significantly fewer branches and leaves than newer growth.

Sometimes called ‘thinning,’ this technique will open up your lilac’s center, which will allow more sunlight to reach inner leaves, allow more water to reach roots, and encourage proper air circulation. It’s a good technique for managing fungal disease, increasing bloom production, and keeping shrubs at their optimal size.

Make Cuts to Reduce Height

Woman cutting lilac branches to reduce height
There are three common methods of trimming a lilac to reduce its height: topping off, heading back, and rejuvenation.

Since many lilac species will grow fast and furiously, height reduction is often a lilac pruning goal. While there are several ways to achieve this effect, some will yield a healthier, more attractive plant than others. Here’s a comparison of three common techniques:

Topping Off

If you see someone on your block take a hedge saw or power shears through the middle of lilac to make it shorter, this is called topping off. While the process may reduce your plant’s height immediately and relatively easily, it is not the technique of choice.

Pruning a shrub in this manner will encourage growth from the points just below your cuts, creating a broom-like effect with many smaller shoots at the top of a stump. Side growth below your cuts will be minimal, flowering will be limited, and future shape will be awkward.

Heading Back

Considerably more time-consuming but far more likely to yield desirable results, heading back involves reducing the length of individual branches to reduce overall height. Here’s how it’s done:

Heading Back Steps
  1. Begin with the tallest, centermost branch.
  2. Find your desired height.
  3. Cut it back to a point that is ¼ inch above a bud or intersection.
  4. Cut at a slight angle.
  5. This will encourage wound healing and discourage water collection.
  6. Proceed from the center outward.
  7. Make cuts that are slightly lower on the plant.
  8. Start moving toward outer branches.
  9. This will encourage a rounder, more natural form.
  10. For wider lilacs, snip off the buds on the interior of its branches.
  11. This will encourage an outward spread.
  12. For denser lilacs, snip off the outer facing buds.
  13. This will encourage inward branch growth.

Rejuvenation

If your lilac is growing out of control, has gotten leggy, or has stopped blooming altogether, you can cut the entire plant down and encourage it to start over. This process should be done during your lilac’s dormant period, when it will not be stressed by the procedure. It is an exception to the ‘prune immediately after blooming’ lilac rule.

Before you begin, however, it’s important to note that your lilac will not be blooming again for a few years. This is a long-game strategy for restoring your lilac to its original glory and it requires patience. Here’s how you do it:

Rejuvenation Steps
  1. Start in late winter or early spring.
  2. Use a pruning saw to cut evenly through your lilac’s stems.
  3. Start at a height of 6-8 inches above the ground.
  4. This will encourage multiple new shoots to sprout from the plant’s roots.
  5. In late winter or early spring of the next year, choose 3 or 4 of the largest, healthiest looking shoots to remain.
  6. Prune off all others at the base.
  7. Prune the remaining shoots back to just above a bud or leaf.
  8. This will encourage side branching.
  9. The following year you should see blooms begin.
  10. You can then resume normal maintenance pruning until it fills out.

Make Cuts to Add Shape

Hedge trimming of lilac branches
Lilacs can also be cut with hedge clippers to make a spectacular screen in the garden.

It’s not uncommon to see a landscaper or homeowner tackle a lilac with hedge shears. The process is easy and can be very effective in achieving a wall-like screen. The problem is, this technique encourages bushy outer growth that will eventually reduce light and air to your lilac’s center. A dead zone with no leaves or flowers will form inside and you won’t be able to keep cutting your shrub back to the desired size.

In formal landscapes, where a more rigid structure is desired, shearing can be used to promote shape only if it is accompanied by some thinning or renewal cuts and followed up by a careful heading back. This process will not create a tight, perfect hedge but the effect can be similar, if a little more natural looking. And your lilac should still flower if cuts were made thoughtfully.

Final Thoughts

To encourage prolific blooms, a beautiful shape, and healthy growth, lilacs should be evaluated annually to determine their pruning needs. Keep in mind that this maintenance task will probably not look the same each year. There will be seasons where a light cleanup and thin out will be all that’s required, but there will also be seasons where a full rejuvenation plan must be initiated.

Pay close attention to their needs (and yours!), and make this crucial maintenance task a part of your routine. Your lilacs will thank you by putting on a gorgeous spring show for many, many years to come.

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