How to Help Your Lilacs to Flower

Blooming lilac bush near a chain-link fence.
David Beaulieu

The fragrance from lilac flowers is so wonderful, and yet if your lilacs don't flower, you may feel like you're missing out. Having a lilac shrub that doesn't flower is a relatively common problem, and it doesn't necessarily mean the gardener is doing anything wrong. Lilacs can be slow to bloom, and there may be some environmental factors that impact flower production. But the good news is that they're worth the wait.

Lilac Bushes Failing to Bloom: What's Up With That?

What is the reason, exactly, why these classic plants fail to bloom? Well, rather than zeroing in on a single explanation, look at several possible reasons behind this problem, which include (in addition to diseases and pests):

  1. Pruning at the wrong time
  2. Cold weather killing the flower buds
  3. Planting your lilacs in the wrong place
  4. The shrub in question is either too old or not old enough to produce flower buds

Notice that, while numbers one and three speak to the gardener's having done something wrong, reasons number two and four are nobody's fault.

Why does the precise time that you prune matter? Lilacs are shrubs that bloom on old wood. This means that the flower buds for the next spring's blooming period are set on the growth produced during the prior year. When you prune off this growth, you lose the flower buds—and, by extension, the flowers that they would have brought. That is why you are advised to prune lilac bushes right after they are done flowering (before they have set bud for next year).

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Click Play to Learn How to Prune Lilacs

The common lilac (Syringa vulgaris) is a very cold-hardy plant, but if a hard frost or a freeze comes along just as the flower buds are about to open, they can be damaged. This results in the loss of blooms for that year. There is not much that you can do to prevent this; just accept the loss and appreciate next year's blossoms twice as much.

As with most plants, where you have planted your lilac bush very much matters. Lilacs are full-sun plants that want well-drained soil. If you made a mistake on either of these fronts when you first installed your plants, you may be paying for it now—in the form of your lilac not flowering. But there is an easy remedy to the problem: transplant your shrub to a more suitable spot.

Regarding reason number four, be aware that, while these bushes are long-lived, their flower production does tend to peter out over the decades. The solution to this problem is to perform a rejuvenation pruning on your lilacs (do not expect immediate results, though). The opposite is also true sometimes: namely, that your plant is simply too young to bloom. Give it time.

How to Get Lilacs to Flower

But while you are waiting, is there anything that you can do to speed up their blossoming? The general advice for getting plants to bloom is to apply phosphorus (the middle number in the NPK sequence found on fertilizer bags) to the soil. If you are not very good at chemistry, you may want to write that down, because it has to be phosphorus, not nitrogen. Applying nitrogen can lead to the growth of a lot of vegetation, but with few flowers.

There is another trick, however, that you can try to speed up flowering on lilacs. The time to try this trick is when your lilacs are in a period of dormancy (which occurs from late fall to early spring):

  1. Fill a bucket with a gallon of water, and stir two ounces of Epsom salts into it.
  2. Then pour this solution onto the plants.