Cosmos do not bloom if they are in too much shade, in 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 than 14 hours, then the 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 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.
Planting Timings Affect Flowering
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.
Lack of Sunlight Can Affect Cosmos Flowering
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 cosmos 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.
Daylight Hours: Short Day Flowering Cosmos on Flower on Shorter Days
Cosmos can often flower best in late Summer or Fall when as they tend to flower best when the day length is less than 12 hours (short day length plants).
Whilst the 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 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 the 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 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 flowers to bloom later in the Summer and possibly into the Fall.
If you can successfully block out light sources 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.
Too Many Fertilizers Can Reduce Cosmos 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 much water), then it may flower in the Fall.
Over Watering Can Result in Fewer Flowers
Cosmos prefers soil that drains well and dries slightly after rain or watering, as these are the conditions to which it is adapted.
If the 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.
Blooms Can be Affected by Bad Weather
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 of 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.
Clay Soil Can Prevent Cosmos Flowering
Clay soil, especially heavy clay, is not suitable for growing cosmos. It is high in nutrients and drains poorly. Cosmos prefer sandy soil that drains well.
If the soil is constantly damp, the 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 borders. 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 in March or April. This gives the cosmos enough time before the frost.
Nutrient-Rich Soil Can Prevent Blooming
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, the cosmos tend to grow a lot of impressive and strong foliage but far fewer flowers.
To grow cosmos in a way that flowers, you must replicate the soil conditions found in the native environment.
Add some horticultural soil grit 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 where the cosmos (and flowers) grow in the wild.
Plants need some stress stimulus 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.
If Cosmos Seeds Are Old, They May Not Flower 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 purchased from the garden center 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 the next Spring in full sunlight and well-draining soil. They should then flower well.