8 Reasons for Lilac Not Blooming- Lilac Not Flowering

Why is my lilac not blooming

I love lilac flowers, and I look forward to seeing my tree in bloom each year! However, to my dismay, for one year, it flowered far less than usual. So, I did my research and reached out to some experts at work (I work at a garden nursery) to find out what was going on…

They told me that the most common reason for lilacs not blooming is because they have been pruned too heavily or at the wrong time of year.

Lilacs typically flower best when they are not pruned. The flower buds develop on old wood, so if the lilac is pruned in Summer, Fall, or Winter, the flower buds are removed, and the lilac does not bloom in Spring.

When I spoke to specialist growers at work, they gave me 8 reasons for lilac not blooming:

  1. Pruning too hard or too late in the season removes flower buds.
  2. Not enough sunlight for lilacs to flower.
  3. Hot or humid climates that have mild Winters (lilacs require a cold dormant period in Winter).
  4. Acidic soils prevent flowering.
  5. Too much fertilizer (often runoff from lawn fertilizer) promotes foliage growth at the expense of flowers.
  6. Drought stress.
  7. Lilac shrub is not mature enough for flowering.
  8. Late frosts can prevent lilacs from flowering.

Keep reading to learn why your lilac (Syringa) is not blooming and how to implement the solutions so that your lilac can bloom the following year

(Note that almost all lilac varieties should bloom in late Spring).

1. Pruning Lilac Reduces Flowering

This is the classic mistake that I see a lot of people make with lilac trees.

Lilacs display their flowers from old wood rather than new growth and flower best if left to their own devices rather than consistent annual pruning.

If you need to prune your lilac, the best time to prune is in Spring immediately after flowering, otherwise you can prevent your lilac flowering the following year.

I learned that the flower buds start developing in Summer to be displayed the following year, so if you prune too late in the season (Summer, Fall, or Winter) you remove the flower buds and prevent the lilac from displaying flowers for a year.

Lilacs flower more when not pruned. Pruning promotes the growth of new foliage to replace the lost growth rather than into the formation of new flowers, so you end up with a leafy lilac shrub with no flowers.

The most elegant and plentiful flowering lilac tree I have ever seen was last pruned 30 years ago, according to its owner!

If you have pruned your lilac back then it can flower again, but I must caution you it requires some patience as it may be a year or so for the lilac to put on more growth from which the flowers should display the following year.

Note that whilst pruning lilacs is discouraged if you want blooms, regular deadheading of spent flowers is best practice and can increase the number of flowers as the lilac’s energy is redirected from producing seeds to producing more flowers for that season.

2. Not Enough Sunlight for Lilacs to Flower

Lilacs require full sun (6 hours of sun or more) to produce the most flowers. Lilacs can, however, grow and flower in light shade, although there are likely to be far fewer flowers.

So, if your lilac is in deep shade, it is unlikely to flower at all, and the growth is likely to be poor.

Lilacs are native to the Balkan peninsula in Southern Europe where they grow in rocky hills with typically full sun.

If your lilac is still a fairly small shrub, then I recommend transplanting it (in early spring or fall) to an area with more sun to stimulate flowering and create a healthier plant.

For more mature, established lilacs, what I recommend is to cut back any other tree limbs or shrubs that may be casting shade on your lilac so that it can flower more in the following Spring/Summer.

3. Hot Climates or Humid Conditions Prevent Lilac Flowering

Lilacs are adapted to climates with low humidity and with a seasonal cycle of temperature change throughout the year.

I learned that it is this cycle of sequential temperature change, specifically the cold period of Winter is essential for the lilac to produce flowers.

A period of cold-initiated dormancy is critical for producing flowers and bud development of lilacs. Therefore lilacs often struggle to flower in climates with mild Winters.

This is what happened to my lilac. I discovered the reason it did not flower particularly well, because the Winter was unusually warm.

Lilacs are extremely cold and hardy but do not tolerate mild winters or high humidity, as this is contrary to the conditions to which they are adapted in their native environment.

Lilacs are best suited to USDA zones 3-7 which have the climate to allow the lilac to more reliably flower.

My own lilac was able to flower better the following year as we had a cold Winter.

4. Acidic Soils can Prevent Flowering

If your lilac is not flowering, this could be because of overly acidic soils. Lilacs are adapted to growing in well-draining, chalky soils that are typically pH neutral or alkaline. If the soil is too acidic, then the lilac is likely to be stressed to flower, and the growth is likely to be very poor.

In my conversations with specilast growers I found that lilacs are capable of tolerating slightly acidic soils and can flower very well. If the garden soil is particularly acidic (lower then pH 6) then the acidity can prevent the lilac from uptaking certain nutrients which can prevent your lilac from flowering.

If your lilac is growing poorly and is not flowering, then I recommend asking a gardening-savvy neighbor about the soil pH, as they are almost certainly going to know if the soil is particularly acidic.

However, you can also test the soil’s pH yourself with a soil test kit or send a sample of your soil away for testing to determine the soil’s acidity and whether this is the cause of your lilac not flowering.

(Read my article, why is my lilac dying).

5. Too Much Fertilizer Prevents Lilacs Flowering (Runoff from Lawn Fertilizer)

This is a classic mistake I see from well meaning gardeners…Too much fertilizer stimulates the lilac to grow lots of foliage at the expense of flowers. If you have applied fertilizer too often or in too high a concentration, then the lilac is unlikely to display many flowers.

I have personally seen at a garden that I used to manage that runoff from lawn fertilizer can be a problem as lawn fertilizer typically contains a high concentration of nitrogen, which is the nutrient responsible for causing the lilac to grow foliage at the expense of flowers.

Nitrogen is water soluble, so what happened was, when it rained, the nitrogen dissolved and ran off the lawn into nearby garden boarders where the lilac tree was planted.

Once the lilac has had too much fertilizer, there is not much you can do to stimulate flowers this year, but if you scale back the fertilizer, the lilac should be able to flower the following year,

If the lilac is planted in good soil and the particular shrub is relatively mature, then the advice that I had was that lilacs typically do not need much, if any additional fertilizer. The more mature a lilac, the more develop its roots system is and therefore the greater access to nutrients in the soil.

However, I recommend some general, all-purpose, balanced granular fertilizers to help less mature lilacs become established in the first few years of growth.

6. Drought Stress can Prevent Lilacs from Blooming

Lilacs do not flower if the soil is too wet or too dry. Lilacs require a balance of moist soil that is well-draining to be able to display their flowers. It can be tricky to get the balance right, can’t it?

If there is a drought in the Spring then this tends to affect the flowers but a significant drought in the Summer can affect bud develop for the flowers the following year.

As we discussed, typically, drought stress does not affect the older, more established lilac shrubs, but immature or recently planted shrubs can be affected and display far fewer flowers.

Pro tip: The way to mitigate drought stress by preventing flowering is to give the lilac a really thorough watering and apply a 2-inch layer of mulch around the base of the shrub (ensure the mulch is not in contact with the wood as this can cause rot).

Mulch materials such as compost, leaf mold, or well-rotted manure are ideal as they help to conserve moisture and improve the soil structure so that it is more favorable for lilacs to grow and display flowers in the Spring.

7. Lilac isn’t Mature Enough to Flower

Lilacs are relatively long-lived shrubs, with some varieties capable of living 100 years or more!

However, when a lilac is immature or has recently been transplanted, then from my experience, it can take 2 or 3 years before it flowers properly.

The experts told me, this is because in the first 2 years of growth the lilac invests its energy into establishing its root system as this is key to immediate survival and long term prosperity.

When the roots are established, there is some more mature growth (and the conditions are favorable with full sun, etc.), then the lilac can redirect its energy from immediate survival after planting to producing a lovely display of flowers.

8. Late Frosts can Prevent Flowering

Lilacs flower in late Spring, and they try to time the emergence of their flowers after the last frost.

If there is a late, heavy frost whilst the flowers are just emerging, this can turn the blooms brown and prevent a good display of flowers.

Lilacs are very cold hardy, but the emerging blooms can be more sensitive to the cold, and frost harms the shrub’s ability to display its flowers.

I’m afraid that the only thing you can do in this case is to wait until the following year. But I find my lilac tends to flower more so if the flowers were frost damaged the year before!

Key Takeaways:

  • Usually, the reason for lilac not blooming is because they have been pruned too hard or at the wrong time of year. Lilacs flower on old wood. If lilacs are pruned after Spring, then this removes the developing flower buds, and the lilac can not display its blooms in Spring.
  • Lilacs require at least 6 hours of sun to stimulate flowering. If the lilac is in too much shade, it can prevent it from blooming.
  • Lilacs require a period of cold dormancy to initiate flowering. If the winter is too mild, then the lilac will not flower.
  • Lilacs grow best in slightly acidic, neutral, or alkaline soils. If the soil is too acidic, this prevents the roots from uptaking certain nutrients, resulting in poor growth and the lilac not flowering.
  • Too much nitrogen in the soil from excess fertilizer or runoff from lawn fertilizer results in a lilac with abundant foliage but no flowers.
  • Lilacs require the soil to be evenly moist. If the soil dries out when the flowers are emerging or while the flower buds are developing in the Summer, this can prevent the lilac from flowering.
  • Lilac shrubs do not flower if they are immature (less than 3 years) or recently planted. The lilac redirects its energy from displaying flowers to growing its roots for the first few years as it prioritizes its survival. Lilacs usually flower after 3 years when their roots are more established.
  • Late frosts in the Spring can turn emerging lilac flowers brown and prevent them from flowering properly.

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