The reasons for cosmos, not flowering is because of too much shade, overwatering, or soil that is too nutrient rich which promotes foliage growth without flowers. Cosmos are short-day plants and require at least 12 hours of darkness to produce flowers.
Bad weather, seeds that have been stored for more than a year, too much fertilizer, and slow-draining soil can all affect cosmos blooms.
Cosmos are unusual in that they require more hours of uninterrupted darkness than day light to flower (short-day plants) so they may flower later in the season.
Keep reading for why your cosmos is not flowering and for the best practices to promote flowering…
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Not Enough Sunlight For Cosmos to Bloom
A lot of the varieties of cosmos that are cultivated by gardeners originate in Mexico and in the Southern states of the US where they flower en mass with at least 6 hours of sun per day.
If your comos have been planted in a shady area of your garden then they tend to grow leggy in search of more light and with few flowers.
In fact, the amount of sunlight is one of the most influential factors for how well your cosmos flower.
To promote flowering you should transplant your cosmos as soon as possible to a sunnier location, cut back any overhanging tree limbs that could be casting shade, or if it’s not too late in the Summer, sow new seedlings in a garden border with full sun and well-draining soil.
With more sun the cosmos has more energy for growing foliage and displaying flowers.
Nutrient Rich Soil Prevents Flowering
Cosmos are native to Mexico where they grow wild and flower in great numbers in sandy soil that is low in nutrients and very well-draining.
If your garden soil is rich in nutrients and has had amendments such as compost, or manure added to it recently then it is likely that the soil is too rich for cosmos to flower.
In rich soils, cosmos tends to grow lots of strong and impressive foliage but with far fewer flowers.
How to Solve it: The best way to grow cosmos so that it flowers is to replicate the soil conditions of the preferred native environment.
When sowing seeds or planting in borders, add some horticultural grit or sand (available from garden centers and on Amazon) to the soil.
Horticultural sand or grit are great for improving drainage and they are inorganic materials that do not add nutrients to the soil.
Therefore the added sand helps to balance the nutrient profile of soil or compost so that it is not quite as fertile and effectively emulates the soil profile of the soil to which the cosmos grow (and flowers) in the wild.
Plants often need some stress stimulus to start flowering (otherwise they tend to grow lots of foliage and no flowers) so a slight nutrient deficit stimulates flowering in plants such as cosmos which are adapted to growing in harsh conditions.
Think of the soil conditions as similar to Mediterranean plants such as lavender and rosemary and with a nice well-draining sandy soil mix your cosmos should flower abundantly.
(Read my article for why lavender is not flowering).
Too Much Fertilizer Reduces Flowering
As cosmos are adapted to low-nutrient sandy soils they do not appreciate the use of fertilizer as this can result in lots of foliage growth with no flowers.
So if your cosmos is tall and healthy looking with lots of foliage without flowers then the use of fertilizer organic feed or even mulch is often responsible.
There is not much you can do to promote flowers after applications of fertilizer, however, if your cosmos is in full sun and it is cared for appropriately (well-draining soil, not too much water) then it is possible that the cosmos could flower in the Fall.
Watering Cosmos Too Frequently Causes Fewer Flowers
Cosmos prefers soil that is well draining and dries out somewhat after watering or rain as these is the conditions to which it is adapted.
Therefore if cosmos is to flower, you should refrain from over-watering the soil.
Cosmos is very hardy when it comes to tolerating drought-like conditions. If the soil is kept consistently moist then this can promote healthy foliage with few flowers.
Cosmos thrive in conditions that are fairly harsh as it is adapted to sandy soils in arid climates.
To promote flowers for cosmos that may not flower due to over-watering, you should scale back the watering so that the soil has a chance to dry out between bouts of water.
To ensure the soil has a chance to dry out, test the soil to a finger depth around the cosmos and adjust the frequency of watering.
- If the soil is still damp then skip watering for several days until the soil is dry the soil is somewhat dry but…
- If you can still detect moisture then wait a day or so until the soil feels dry, at which point, give the cosmos a good soak to encourage the roots to grow and establish.
It is important to acknowledge that dry soil should not just be a result of less frequent watering but also well-draining soil (with added sand or grit) which should give the best results in terms of blooms.
Timing of Planting Can Affect Blooms
The optimal time for sowing seeds of cosmos is around March or April or after the threat of frost.
If you sow seeds after the optimal window of March and April then this can delay flowering for some weeks, however with patience and the right conditions the cosmos can still bloom.
Sowing in the middle of Summer can often be detrimental as the cosmos has less time to germinate, and has to contend with particularly high temperatures as it grows and the flowering period may be late enough in the year that there could be a threat of frost which damages the flowers.
Planting cosmos too early can also risk frost damage from a late frost in the very early Spring which can prevent flowers.
To ensure your cosmos is safe from the risk of frost in the early Spring, it is a good idea to sow the seeds indoors in a greenhouse to protect them whilst they are still tender and give the seedlings time to grow in the early Spring so they can flower as early as possible and for a long time in the Summer.
Short Day Flowering- Cosmos Only Flowers When Days are Shorter than Nights
Cosmos can often flower best in late Summer or Fall as they tend to flower best when the day length is less the 12 hours (short day length plants).
Whilst full sun is beneficial for flowering if cosmos are in bright light for longer than 12 hours then this can impact flowering as they require a period of darkness.
This is due to the cosmos being adapted to sense seasonal change by the pattern of changes in day length so that the plant knows when to flower.
In the northern hemisphere, the longest day is on the 21st of June, so cosmos start to develop flowers after this date as the days grow shorter.
If you have a source of light at night that may light up around your cosmos (and other short-day plants) then this can reduce flowering even if it is just for a few minutes.
Think of artificial sources of light such as street lights or security lights that are bright after dark as cosmos require 12 hours of consistent darkness to flower.
In Northerly latitudes the day length tends to be longer which causes the cosmos to flower later in the Summer and perhaps into the Fall.
If you can successfully block out light sources (with horticultural fleece or something similar) so that the day length and hours of light are less than 12 hours then this can help to promote flowers at any time during the Spring or Summer or you can wait until late Summer or Fall for flowering.
Old Cosmos Seeds Do Not Flower as Well
Whilst seeds of many plants can be kept a long time before sowing, the germination rate can be lower if the seeds are more than a year old.
Generally, it is best practice to sow Cosmos seeds that you have bout from the garden center that year.
If the seeds have been stored a long time in less than ideal conditions then this can affect the quality of the seed and potentially impact flowering.
When storing seeds ensure that they are in a nice, cool, dry place such as a garage or shed where the temperate is consistent and it does not go below freezing or become too hot.
If your cosmos did not flower then I recommend buying seeds from a reputable brand and sowing them in the following Spring in full sun, well-draining soil without too much watering and they should flower well.
Bad Weather Can Impact Blooms
Sometimes the weather is just not conducive for cosmos to flower well even if you have provided the optimal growing conditions.
If the Spring or Summer has been unusually cold, rainy, or with lots of overcast days without significant sunlight then the cosmos is less likely to display flowers as cosmos prefers full sun, warmer weather, and a dry summer to flower its best.
If you are growing cosmos in a pot then you can improve the display of flowers by shielding the cosmos from excessive rain and perhaps moving the pot to a greenhouse and the sun, high temperatures and control over watering can promote flowering.
Clay Soil Can Prevent Flowering
Clay soil (particularly heavy clay) is not well suited for growing cosmos as it is rich in nutrients (which promotes foliage at the expense of flowers) and does not drain very well and cosmos prefers well-draining sandy soil.
If the soil is consistently damp then cosmos does not necessarily flower.
It is difficult to amend soil with a high clay content for growing cosmos so a better solution is to grow your cosmos in pots, containers, or raised beds.
Pots have favorable drainage conditions compared with garden boarders and it is a lot easier to create a well-draining potting mix that suits cosmos and promotes flowering than it is to amend a garden boarder.
If you have started growing cosmos in clay soil it may be possible to transplant the plugs into pots to improve flowering or if there is still time left in the growing season then you can grow some more cosmos but generally, the optimal window for growing from seeds is around March or April to give cosmos enough time to grow and flower before frost.
Key Takeaways:
- Cosmos do not bloom if they are in too much shade, consistently boggy soil, or with too much nitrogen fertilizer. Cosmos require well-draining, low-nutrient soil with lots of sun and not too much watering.
- If the day length is more than 14 hours then cosmos cannot display flowers.
- The optimal time for planting is March/April. Late sowing of seeds can cause cosmos not to flower.
- Seeds that have been in storage for more than a year are less likely to germinate and flower.
- Cold, rainy, windy weather with overcast days can reduce flowering as cosmos prefers warm weather and dry soil.
