Why is My Cosmos Not Flowering?

The reasons why cosmos are not flowering are due to too much shade, too much watering, or too rich in nutrients that promote foliage growth but no flowers. Cosmos are short day plants and require at least 12 hours of darkness to produce flowers.

Bad weather, long-term storage of seeds, excessive fertilizer, and slow draining soil all can affect the cosmos’ ability to bloom.

The cosmos is an unusual plant because they need more darkness than day light to bloom (short day plants). It may flower later in season.

Continue reading to find out why your cosmos has stopped flowering and the best practices for encouraging flowering …..

Not enough sunlight for the Cosmos to bloom

Many of the cosmos varieties that are grown by gardeners originate in Mexico. They also grow in the Southern US states, where they bloom in masses with at least 6 hours of sunlight per day.

If your comos were planted in a shaded area of your garden, they will tend to become leggy as they seek more light and produce fewer flowers.

In reality, the amount of sunlight is the biggest factor in how well your cosmos flowers.

To encourage flowering, you should move your cosmos to a sunny location as soon as possible. If it is not too late in Summer, plant new seedlings in a garden border with full sun and well-draining soil.

The cosmos is more energetic when there’s more sunlight.

Cosmo flowers

Nutrient-rich Soil Prevents Flowering

Cosmos can be found in Mexico, where they are found in large numbers in sandy soil. This soil is rich in nutrients and well-draining.

If your garden soil has been amended with compost or manure recently, it is possible that it is too rich in nutrients for cosmos to grow ..

In rich soils, cosmos tends grow a lot of impressive and strong foliage but far fewer flowers.

How to Solve It: To grow cosmos in a way that flowers, you must replicate the soil conditions found in the native environment.

Add some horticultural soil grit (available at garden centers or on amazon) to your soil before sowing seeds.

Horticultural sand and grit can improve drainage. They are organic materials that don’t add nutrients to the soil.

The addition of sand helps to balance soil or compost’s nutrient profile so it is not as fertile. It effectively replicates the soil profile to which the cosmos (and flowers) grow in the wild.

Plants need some stress stimulus in order to flower. Otherwise, they tend to grow lots and lots of leaves. So a slight nutrient deficiency stimulates flowering in plants like cosmos that are adapted for growing in harsh environments.

Think about the soil conditions as being similar to Mediterranean plants like rosemary and lavender. With a well-draining sandy mix , your cosmos will bloom abundantly.

Too Many Fertilizers Reduce Flowering

Cosmos don’t like fertilizer because it can cause excessive foliage growth without any flowers.

If your cosmos looks healthy and tall with plenty of foliage, then fertilizer or organic food or mulch may be necessary.

After fertilizer applications, there isn’t much you can do to promote flowers. However, if the cosmos is in full sunlight and is properly cared for (good drainage soil, not too many water), then it may flower in the Fall.

Watering Cosmos Also Frequently Causes Less Flowers

Cosmos prefers soil that drains well and dries slightly after rain or watering, as these are the conditions in which it is adapted.

If cosmos is to bloom, it is important not to overwater the soil.

Cosmos can tolerate drought conditions well. Healthy foliage can be promoted by keeping the soil moist.

Cosmos can thrive in harsh conditions because they are adapted to sandy soils in dry climates.

To encourage flowers for cosmos, which may not be flowering because of overwatering, reduce the amount of watering. This will allow the soil to dry between waterings.

To ensure that the soil is dry, you should test it to the depth of your fingers around the cosmos. Then 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 recognize that dry soil does not necessarily mean less frequent watering. However, well-drained soil with added sand and grit should produce the best blooms.

Planting Timings Can Impact Blooms

The optimal time for sowing seeds of cosmos is around March or April or after the threat of frost.

If you plant seeds in the wrong timeframe, such as March or April, it can delay flowering for a few weeks. However, with patience and the right conditions, the cosmos can still blossom.

Sowing in the middle of summer can be dangerous because the cosmos has less time for germinating and must deal with high temperatures. The flowering period could also be too late in the year to be at risk of frost damage.

Planting cosmos too soon can lead to frost damage in the very early spring, which could prevent flowers.

To ensure that your cosmos is protected from frost damage in the spring, it is a good idea for you to sow the seeds indoors in an enclosed greenhouse. This will allow the seedlings to develop in the Spring and then flower in the summer.

Bad Weather May Impact Blooms

Sometimes the weather is not ideal for cosmos to bloom well, even if they have the best growing conditions.

If the Spring or Summer was unusually cold, wet, or with lots overcast days, the cosmos will not display its best flowers. The cosmos prefers full sunlight, warm weather, and dry summers to bloom their best.

If you grow cosmos in a container, you can enhance the appearance of the flowers by protecting them from excess rain. You can also move the pot to a greenhouse where the sun, high temperatures, and control over watering can encourage flowering.

Short-Day Flowering-Cosmos Only Flowers when Days are Shorter than Nights

Cosmos can often flower best in late Summer or Fall when 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 then 12 hours then this can impact flowering as the require a period of darkness.

This is because the cosmos has been adapted to sense seasonal changes by the pattern in day length. The plant then knows when it should flower.

In the northern hemisphere the longest day is on the 21 st of June, so cosmos start to develop flowers after this date as the days grow shorter.

If you have an area of light that is bright enough to light your cosmos at night (or other short-day plants), this can help reduce flowering, even if it’s only for a few seconds.

Think of artificial sources of light such as street light or security light that are bright after dark as cosmos require 12 hours of consistent darkness to flower.

In Northerly latitudes, the day length tends be longer which causes the cosmos flowers to bloom later in the Summer and possibly 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 is less then 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

Seeds of many plants can still be stored for a while before sowing. However, the germination rate may be lower if they are older than a year.

It is a good idea to sow Cosmos seeds you have purchased from the garden centre in that year.

Seeds that have been stored in less than ideal conditions for a prolonged period of time can cause problems with their quality and possibly impact the flowering process.

Seeds should be stored in cool, dry places such as garages or sheds where the temperature is constant and does not drop below freezing.

If your cosmos didn’t flower, I suggest buying seeds from a reputable company and sowing them in the next Spring in full sunlight and well-draining soil. They should then flower well.


Clay Soil May Prevent Flowering

Clay soil, especially heavy clay, is not suitable for growing cosmos. It is high in nutrients and drains poorly. Cosmos prefers sandy soil that drains well.

If the soil is constantly damp, cosmos will not flower.

It is hard to amend soil that has a high clay content to grow cosmos. A better option is to plant your cosmos in containers, pots or raised beds.

Pots have better drainage than garden boardsers. It is much easier to make a potting mix that promotes flowering and is well-draining than to amend a garden border.

If you have already started to grow cosmos in clay soil, you may be able to transplant the plugs into containers to increase flowering. However, if you still have time in the growing season you might be able to plant more cosmos. The best time to start growing seeds is usually March or April. This gives cosmos enough time before the frost.

Key Takeaways

  • Cosmos do not bloom if they are in too much shade, consistently boggy soil, or with too much nitrogen fertilizer. Cosmos need well-draining soil that is low in nutrients. They also require lots of sunlight and little watering.
  • If the day length is more then 14 hours then cosmos cannot display flowers.
  • The optimal time for planting is March/April. Cosmos can not flower if they are sown too late.
  • Seeds that have been in storage for more then 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.


Went from a bad gardener to a half-decent one over 10+ years. Super happy to share my tips and tricks with you :)