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Lilac bushes are terrific specimens in any garden. Lush with green foliage throughout the season, complimented by the gorgeous floral scents in May that give the garden its grandeur.
That is, until you discover your lilac bush is not blooming. When that happens, you need to play detective.
Listed below are all of the possible reasons that can prevent a lilac bush from blooming, buds from opening, or just dying off before they get a chance to flower.
Perhaps It Is Just Too Young to Bloom
Consider the type of bush that has been planted. Dwarf varieties take about two years before they summon the energy to put out buds.
Larger varieties can take as long as five years before flowering. These get their energy mostly from sunlight. Plant food will not speed up bud formation.
Over Feeding It Fertilizer Does Not Promote Flowering
Where you want green, nitrogen-rich fertilizers work. When you want flowers, phosphorous is what to use. A common problem with lilac bushes not blooming is because of lawn fertilizers.
Naturally, if you have your bush planted near the lawn, the lawn fertilizer will find its way into the soil. The signs of overfeeding are easy to identify. Your bush will be crazy green with no flower buds.
Excess nitrogen promotes foliage growth. It does nothing for bud formation. If you are using fertilizer to feed a lilac bush then stop. If you are applying it to your lawn, be more careful around the bush, and any mulch that has been laid.
Late Season Feeding and Pruning
Fertilizing shrubs of any type should stop in the summer to give new growth time to harden off for the winter. When young growth comes in late in the season, the cold weather will kill it. Next season, there’ll be more deadwood to cut away.
What the lilac bush needs protection from is late spring freeze.
These go into dormancy over winter. Warm weather breaks the dormancy period. When a brief cold snap hits after the plant is out of its dormant period, a late spring freeze can kill the flower buds.
Spring freezes will rarely do much damage to the foliage and mature canes, but the buds that formed last year can be damaged too much by a sudden cold snap to be able to flower.
When a cold front is approaching, the buds of a lilac bush need to be protected.
Has It Gone Two Years without Pruning?
Leave a lilac shrub for two years without pruning will tend to stop blooming because basal shoots take over. These are nicknamed “suckers” because they zap the plant of energy, giving nothing in return.
Suckers are the undesirable aspect of growing shrubs, and a big reason for more pruning being required to keep these in bloom.
The basal shoots do not reach as high a height as the main plant. They tend to overcrowd the base of the plant.
When you see the lilac blooms up top, but the bottom of the bush doing nothing, it is likely that the bottom part of the shrub is overcrowded with basal shoots.
These slow the flowering process on the buds up top and prevent the bottom part of the bush from producing buds. The result is less color and scent in your garden.
How to Control Lilac Bush Suckers
The suckers on a lilac bush are connected to the root system. It is one of the few parts not to prune off the plant. Instead, suckers should be torn away to get the most of them detached from the parent root ball.
Just like when you prune any bush, the plant’s response is to put out more shoots. The same happens with the basal shoots. If you only cut it back or keep them in check with a weed whacker, they will rapidly grow back.
The simplest method is to tear the suckers away from the bush. The harder (although more effective) method is to use a sharp shovel to dig them out.
When digging, shovel down as close to the trunk as possible. The aim of digging is to separate the roots of the lilac suckers from the root ball of the parent plant.
To really slow sucker growth down to give your bush the best chance of blooming, there are herbicides that contain napthaleneacetate.
These are plant growth regulators that slow the growth of basal shoots. You can do the same with any herbicide, but it would need to be sprayed away from the root ball.
For lilac bushes with out-of-control sucker growth, apply the specialist herbicides closest to the root system. Further away, where the suckers may be over a foot away, use the regular (and cheaper) herbicides on those.
Aged Lilac Bushes May Be in Need of Rejuvenation Pruning
Rejuvenation pruning is a technique that can be used on any mature shrub that has been neglected. When a lilac bush is left to grow wild, it becomes congested. It gets inward growth; the stems overlap and the overall air flow and sunlight become restricted.
When that happens, a lilac bush stops blooming. Bringing it back takes time.
One of the things gardeners love about the lilac bush is its hardiness. Mature lilac bushes can be cut back to near ground level, and still send up new shoots. Take advantage of that to start from scratch.
When this is most advantageous is if you find one of these bushes neglected in a new home. Rather than getting rid of a neglected lilac bush, it can be cut back to restart the growth stage. Expect it to take a year minimum though.
If you do decide that your lilac bush has reached a monstrosity of a size that it needs to be cut down, do the reduction cuts in late winter. The following season, do not expect blooms.
The first year after drastic reduction in its size, new growth will focus on the woody stems and the foliage that the plant needs for blooming. It can take two to three years before it has enough energy for buds to bloom.
Age Matters for Blooming
The best blooms on a lilac bush happen on young canes. However, young for this type of plant is over five years. On older growth, fewer buds form.
These need to have selective pruning done to keep them blooming. That gets done by selecting the oldest canes, then cutting them back to the branch collar. This makes way for new growth to come through, and keeps air circulation good by preventing the bush from becoming crowded.
Birds Damaging Top Buds
Although being mostly pest resistant, a few insects tend to cause problems on the lilac bush. Mostly, lilac leaves curling.
Borers, leaf miners and mites are the usual suspects. Birds feed on insects so if you have bugs lurking around, the birds are likely after those.
Even if they aren’t though, over the winter, a number of birds will feed on dormant buds. Keep an eye on your bush over the winter because the birds could be eating dormant buds, which, obviously will stop a lilac bush from blooming.
Birds do not eat the flowers. They can either damage lilac bushes by eating the insects that would damage it anyway, or they’ll pinch the dormant buds.
Lilac Bacterial Blight
Only some varieties of lilac are resistant to bacterial blight. If yours isn’t and this disease sets in, flowering will be affected. It will either cause buds to blacken, or fail to open.
If a diseased bud does open, it dies soon after. The flowers do not last as long as you would expect them to.
The biggest risk factor for this disease is cold temperatures following a downpour. Freezing wet foliage sets the stage for fungal pathogens to infect the plant. It tends to affect the foliage first, causing black streaks to develop, lilac leaves turning brown, and the buds turn black.
To treat bacterial blight, selective pruning will be needed. All of the foliage and branches with darkening occurring should be cut away and burned. Failing to do that, the fungal spores can spread to other parts of the bush, eventually killing it.
Don’t Confuse a Lilac Bush Not Blooming with Poor Blooming
The common lilac bush tends to bloom biennially. At least to any decent extent. They have a tendency to put out a burst of flowers one year, then the next year there won’t be as many blooms.
This has to do with the fact the lilac bush is a long-life plant. It will take in as much reserves as it needs. And hold onto the reserves. Putting out a burst of flowers takes a lot of energy.
Considering it can take up to five years for its first blooms to appear, after that, expecting the same type of blooms to appear every year is a bit much.
Only be concerned about a lilac bush not blooming if there are actually no blooms. If it’s only fewer flowers than usual, that’s poor blooming and tends to happen every second year. That’s normal and nothing to worry about.