The reason for lilacs not blooming is usually because it has 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 lilac is pruned in Summer, Fall or Winter the flower buds are removed and the lilac does not bloom in Spring.
8 Reasons for lilac not blooming:
- Pruning to hard or too late in the season which removes flower buds.
- Not enough sunlight for lilacs to flower.
- Hot or humid climates that have mild Winters (lilacs require a cold dormant period in Winter).
- Acidic soils prevent flowering.
- Too much fertilizer (often run off from lawn fertilizer) promotes foliage growth at the expense of flowers.
- Drought stress.
- Lilac shrub is not mature enough for flowering.
- Late frosts can prevent lilacs flowering.
Keep reading for the reasons your lilac (Syringa) is not blooming and how to implement the solutions, so that you lilac can bloom the following year…
(Note that almost all lilac varieties should bloom in late Spring).
1. Pruning Lilac Reduces Flowering
Lilacs display their flowers from old wood rather then new growth and actually flower best if left to their own devices rather then 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.
The flower buds actually 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 then into the formation of new flowers so you end up with a leafy lilac shrub with no flowers.
If you have pruned your lilac back then it can flower again, but 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 it the lilacs 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 is likely to be far fewer flowers.
In deep shade the lilac is likely 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 you lilac is still a fairly small shrub then I recommend transplanting it (in early Spring or Fall) to an area of more sun to stimulate flowering and for a healthier plant.
More more mature, established lilacs 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 wit low humidity and with a seasonal cycle of temperature change throughout the year.
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 climate with mild Winters.
Lilacs are extremely cold hardy but do not tolerate mild winters or high humidity as this is in contrary to the conditions to which they are adapted in their native environment.
Lilacs are best suited to USDA zones 3-7 which has the climate to allow the lilac to more reliably flower.
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 stressed to flower and the growth is likely to be very poor.
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 flowering.
If your lilac is growing poorly and is not flowering then I recommend asking a gardening savvy neighbor about the soils pH as they are almost certainly going to know if the soil is particularly acidic.
However you can also test the soil’s pH your self 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 (Run off from Lawn Fertilizer)
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 concentration then the lilac is unlikely to display many flowers.
A major factor is run off from lawn fertilizer 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 when it rains it can dissolve and run off the lawn into nearby garden boarders where your lilac tree might be 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 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.
Some general, all purpose balanced granular fertilizer can help less mature lilacs become established in the first few years of growth.
6. Drought Stress can Prevent Lilac 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 able to display their flowers.
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.
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.
The way to mitigate drought stress 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 improved the soil structures 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 it can take 2 or 3 years before it flowers properly.
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 the 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 harm the shrubs ability to display its flowers.
- Usually the reason for lilac not blooming is because it has been pruned to 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 then this can prevent lilac from blooming.
- Lilacs require a period of cold dormancy to initiate flowering. If the Winter is too mild then the lilac does 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 which results in poor growth and the lilac not flowering.
- Too much nitrogen in the soil from excess fertilizer or run off 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 whilst 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 then 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 its roots are more established.
- Late frosts in the Spring can turn emerging lilac flower brown and prevent them from flowering properly.