Why Aren't My Lilacs Flowering?

What to Do When These Pretty Bushes Fail to Bloom

Picture: Bloomerang lilac.
David Beaulieu

"I fondly remember the lovely lilac shrubs growing in my grandmother's yard when I was a little girl," emailed reader, Carol, "which is why I am so mad that my own lilacs are not flowering. Their fragrance is so wonderful, and I feel that I am missing out on it. What am I doing wrong? Or, better yet, what can I do to correct the problem and finally enjoy some long overdue blooming?"

I emailed her back, beginning, "Take heart, Carol: this is a common question. You may not be doing anything wrong, at all." I proceeded to offer her some tips. But I realized that this problem is, in fact, common enough to justify an article, so that everyone can profit from the advice that I offered Carol, which went something like this:

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

"Why aren't my lilacs flowering?" is a cry heard far and wide among the gardening public. Yes, lilacs often infuriate people, because they are so slow to bloom. You have to be really patient with them. That is the bad news. The good news is that, yes, they are worth the wait.

But what is the reason, exactly, why these classic plants fail to bloom? Well, rather than zeroing in on a single explanation, we must speak of 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 #1 and #3 speak to the gardener's having done something wrong, reasons #2 and #4 are nobody's fault. Let's take a closer look at each of these potential reasons for a lilac not flowering:

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).

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 a 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 #4, 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. Believe me, I feel your pain. I waited five years for a "sucker" to bloom that I had transplanted from an old stand of lilacs. At the end of this agonizingly long wait, I was finally rewarded with flowers, without having done anything specific to get the bush to bloom -- I just waited.

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 2 ounces of Epsom salts into it.
  2. Then pour this solution onto the plants.