11 Reasons Your Lilacs Aren’t Blooming and How To Fix it

It’s late spring. The car windows are cracked and the scent of lilacs wafts in as you drive through the neighborhood. You see them everywhere you go. Graceful trees with feathery white blooms in the park. Massive shrubs are so heavily draped with purple panicles that their branches are bending. A smattering of small, but sweet smelling shrubs by the library entrance.

But when you pull into your own driveway and stroll around your own backyard, the lilacs you’ve been growing look less than inspired. It’s downright disappointing. Maybe you’ve only got a handful of blooms on your Charles Joly. Or your Little Miss Kim has decided to take the year off. And now you find yourself mystified, scratching your head and wondering what went wrong.

The good news is, there are reasons for your lilac’s poor performance, and you’ve come to the right place for advice. Here, we look at 11 possible factors affecting your lilac blooms and examine some ways to bring on the buds.

Your Lilac Was Recently Planted

It can take a few seasons for transplanted lilacs to deliver an impressive floral display. After being installed, your lilac will focus most of its energy on establishing a vigorous root system. This will be its priority for at least the first growing season.

On some species, particularly small leaf varieties, you might get a handful of blooms the first year. However, with full flowering, surface-covering panicles are likely a year or two down the road. Some common lilac varieties may even take 2 or 3 years to fully flower, but it’ll be totally worth it. So be patient.

They Aren’t Getting Enough Sun

Lilacs require at least 6 hours of sun and will benefit from a location that is hot in the afternoon. While they will survive in a slightly shady spot, they will not flower well or grow as quickly.

Take a look at the trees in the vicinity of your poorly performing lilac. Maybe they’ve grown significantly in the years since you first landscaped the yard and are now throwing too much shade. Consider hiring a tree service to clean up the canopy and let a bit more sun through. Your lilacs will thank you with a more prolific blooming cycle.

If a row of mature lilacs is growing into each other, consider removing or transplanting every other one. This will allow more light to circulate from roots to tips. If other shrubs or ornamental trees nearby are hogging the sunshine, give them a haircut and your lilac will surely benefit.

You’ve Pruned Too Aggressively

I will say this until I am blue in the face; if your lilacs need pruning, do it immediately after they are done flowering. Do not wait until Fall, when mid-to-late season flowering shrubs will get their cutbacks.

Do not wait until early Spring, when grasses and hydrangeas will be cleaned up for the coming season. Lilacs flower on wood from the previous year, and the buds form very soon after flowering is complete.

They may even be forming as the last of your sweet smelling, pastel florets fade to brown in early summer. So keep an eye out for them, and don’t cut them off.

Lilac buds are large, round, and bright green. Do not confuse them with leaf buds, which are smaller, darker, and slightly pointy. If you see flower buds on your lilac, most likely in a location just below this year’s flowers, it’s too late for a shaping or heading back prune. You will not get flowers next year.

If you’ve determined that your lilac needs to be thinned and you really don’t want to wait until next year, aim to remove about ⅓ of your lilac’s older and crossing stems by cutting them off at the base.

This is called a renewal prune and it will leave ⅔ of your plant to do its lilac thing in spring. The blooms won’t be as consistent or as evenly spaced, but it’ll be better than nothing.

You Cut it Too Far Back Last Season

If your lilac was overgrown, leggy, or out of control last year, you may have given it a rejuvenation prune. This means you cut the entire shrub down to a height of 6-12 inches to get rid of old wood and encourage new growth.

The good news is, lilacs bloom optimally on new wood, and a rejuvenation prune is usually the most effective way to convert a tired, old shrub into a healthier, more prolific flowering specimen.

The bad news is, it’s gonna take a while. Expect to wait 3 years or more for a full, round shrub with normal blooms to return. But again, worth it.

You Forgot to Deadhead

If you’re working with a compact shrub, or if you think your lilac does not need a renewal or rejuvenation prune this year, you can skip it. But you should still deadhead spent blooms.

After flowers have faded, your lilac’s natural tendency is to direct its energy toward nourishing the seed. Removing the spent blooms will tell roots to send up more flowers to facilitate reproduction. In single blooming lilacs, this means buds for next year. In reblooming lilacs, this means buds that will open later in the season.

Deadheading should be done immediately after blooming and repeated if necessary. Use a small pruning tool to make a sharp cut just above the first set of leaves you encounter as you slide your fingers down their stems. Cleaning as many spent blooms off your lilac as possible will encourage a better bloom output next time.

The Roots Are Too Soggy

Lilacs prefer well-drained soil and do not flower well if their feet are wet. Make sure your lilac is not planted in a low area, near a downspout, or adjacent to a plant that requires excessive watering. Also, make sure mulch is properly applied to drain water away from your lilac’s stems and not toward them.

If you’re working with a smaller lilac that’s planted in a container, make sure there are plenty of drainage holes and that they are all clear and open. In the landscape, make sure the soil grade slopes away from them so water does not puddle over their roots.

You might also want to check the soil around your lilac’s stems. If it is sticky and clay-based, work a little organic matter into the area, possibly even a little sand. Strive to create a bed that drains evenly and does not hold standing water.

The Roots Are Too Dry

Conversely, lilacs can get stressed by drought, which can also lead to reduced flower production. While they do not typically require supplemental watering after the first couple of years, lilacs may need extra  irrigation during long dry spells.

Take care to deliver water to your lilacs roots without puddling or overdoing it. A slow soak is always better than a fast flood.

The Roots Have Become Crowded

If you were overzealous when planting young lilacs and did not anticipate the speed with which the genus would grow and expand, your trees or shrubs may be planted too close together. Their root systems are expansive and typically exceed the widths of their canopies. This means your lilacs may be competing for soil nutrients and water.

Consider removing some of them to thin out a border planting. Or, try transplanting anything growing in your lilac’s vicinity that may have an aggressive root system.

Similarly, a container grown lilac will stop flowering if its roots have maxed out their available space. You can transplanting your lilac to a larger container. Or, you can remove it temporarily, trim off some of its outermost roots, and repot.

There’s Been a Late or Early Freeze

As cold weather natives, lilacs are not likely to suffer from extreme winter conditions. Their flower buds, however, are susceptible to damage from quick swings in temperature or unusual weather patterns.

Lilacs require a long, cold, cycle of dormancy in order to flower properly and can be negatively affected when a late or early season warm up is followed by a freeze.

In winter, after your lilac has officially entered dormancy, a prolonged warm spell may trigger an early wakeup and its buds may begin to swell. When freezing temperatures return, these buds are often damaged and a poor flower showing is likely to follow.

Likewise, a prolonged, early Spring warmup can also trigger premature bud opening, and these buds can be damaged when freezing temps inevitably return. Petals may be brown and unsightly, or flowers may not open at all.

While it’s too late to do anything if freeze damage is the culprit behind this year’s lilac bloom failure, if you keep a tarp or cloth handy to insulate your shrubs during abnormal cold snaps, you may be able to prevent it from happening in subsequent years.

Your Soil Has Too Much Nitrogen

Lilacs do not require supplemental fertilization. They are vigorous feeders and absorb nutrients well from the soil. Too often, an enthusiastic gardener will think they’re giving their lilac blooms a head start by blasting the plant with fertilizer, only to find that the flower show is less than inspired.

Turns out, treating lilacs with a fertilizer rich in nitrogen will encourage foliage growth rather than flower growth. So skip it altogether, or use a product with a higher nitrogen to phosphorus to potassium ratio (like 5-10-10)

Since lilacs are typically planted near grassy areas, they are often the unintended recipient of nitrogen in the form of lawn fertilizer. This, too, can have an adverse effect on flowering, so care should be taken to direct chemicals away from their bases and root systems whenever possible.

You Have a Pest or Disease Problem

While lilacs are fairly resistant to pests and disease, there are a few that may cause a reduction in blooms. Scale insects and leaf borers may suck sap from your lilacs leaves and stems, reducing the flow of nutrients to their hungry blooms.

Bacterial blight may turn your lilac buds black and prevent them from opening. A fungal condition such as verticillium wilt may also be nipping your lilacs in the bud.

If you’ve ruled out the more common reasons for lilac bloom failure, take a closer look at the stems and branches near your lilac’s base. This is where the most damaging pests and diseases will first make their presence known.

If damage of this nature is identified, prune affected leaves and stems off to discourage spread, and monitor closely throughout the season.