Why Are My Cosmos Not Flowering? (How to Solve it)


Cosmos not flowering

Have you ever grown your cosmos from seed only to be dismayed by the lack of flowers? I experienced this myself when I first tried growing them. My mistake?

I planted them in a potting mix composed of well-rotted manure. The potting mix had too much nitrogen, which caused lots of foliage but very few cosmos flowers!

Since my first experience growing cosmos all those years ago, I have become significantly more horticulturally inclined. I even work in a garden center, where I sow cosmos seeds to prepare them for sale.

This article distills everything I have learned from this experience to help you pinpoint the problem and implement the solution.

Looking for the short answer?

Cosmos do not flower 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.

I have seen everything from bad weather, seeds stored for more than a year, too much fertilizer, and slow-draining soil, all affecting cosmos blooms.

The lesser-known fact about the cosmos is that they are unusual in that they require more hours of uninterrupted darkness than daylight to flower (short-day plants), so they may flower later in the season.

Keep reading for why your cosmos is not flowering and for all my tips to promote flowering…

Not Enough Sunlight For Cosmos to Bloom

Cosmos are sun worshippers a love a prime spot in your garden soaking up the sun. You see to make our cosmos bloom, we have to think about how they grow in the natural world…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.

Are your cosmos leggy and lacking blooms? If they were planted in a shady area of your garden, they tend to grow leggy in search of more light and with few flowers.

In fact, from my observations, the amount of sunlight is one of the most influential factors in how well the cosmos flowers. I have tested growing cosmos in a pot with 6 hours of morning sun and cosmos in the same type of pot (to keep the variable controlled) in a location with around 8 hours of direct sunlight.

From this very informal test, I found the cosmos in the 8 hours of direct sunlight had more flowers. I counted 24 flowers in the pot that was in 6 hours of sunlight and 33 flowers in the pot with 8 hours of sunlight with all other variables kept the same to try and make it a fair test.

I think this shows how much of a difference direct sunlight makes to flowering, as even just an extra two hours made a difference to the specific number of blooms on display in my test.

To promote flowering, should transplant your cosmos as soon as possible to a sunnier location. Struggling for more light? I would cast your eyes upwards and look at the canopy. I’ve had to cut back overhanging tree limbs before that were casting shade or, if it’s not too late in the Summer, what I do is sow new seedlings in a garden border with full sun and well-draining soil.

With more sun, cosmos has more energy for growing foliage and displaying flowers.

Nutrient Rich Soil Prevents Flowering

Remember we talked about how our cosmos is native to Mexico? Well, in Mexico, they grow wild and flower in great numbers in sandy soil that is low in nutrients and very well-draining.

As I said at the top of this article, I made this mistake myself when I first started growing my cosmos. So, with some of my other bedding plants, I had always used a potting medium that was composed of some well-rotted manure.

I found nitrogen in the manure filled the growth of my daisies, and they looked great with a great display of flowers.

I did the same thing for my cosmos, and they ended up with lots of green leaf foliage that, to my personal dismay and shock, displayed very few flowers! What had I done wrong?

From research, I learned that my soil is too rich for cosmos to flower.

How to Solve it?: Once I learned that my soil was too high in nitrogen I decided that the best way to grow cosmos so that it flowers is to replicate the soil conditions of the preferred native environment.

How do we do this? When sowing seeds or planting in borders, I recommend adding 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.

Horticultural sand
Horticultural sand.

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.

I have experimented with perlite, grit, and sand, and I’ve found that my cosmos seem to prefer sand the most (in terms of how many flowers are displayed), which is probably (I speculate!) because it replicates their natural conditions most precisely.

One thing I’ve come to appreciate in botany is that 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.

A fascinating survival strategy, don’t you think? Plus, they look gorgeous!

What I find helps is if we 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.

I’ve done a lot of trial and error when growing cosmos plants at my garden center,r and I personally use a compost mix of (when growing both from seed and in pots) 80% peat free compost with 20% horticultural sand.

I try to test different compositions of potting mediums every year, and from my testing, this seems to provide the right amount of drainage and fertility level for the cosmos to perform its best.

(Read my article for why lavender is not flowering).

Too Much Fertilizer Reduces Flowering

As we discussed, cosmos are adapted to low-nutrient sandy soils so 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.

I’m afraid that 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 I have managed to revive cosmos so that it flowers in the Fall! Also, if it is not too late (mid-summer), I’d sow so more seeds!

Watering Cosmos Too Frequently Causes Fewer Flowers

As we already discussed, our cosmos prefers well-draining soil that dries out somewhat after watering or rain, as it is adapted to these conditions. So don’t overwater it!

Cosmos flowering
Cosmos thrive in full sun.

As you can imagine, Cosmos is a native of Mexico, which means it is very hardy when it comes to tolerating drought-like conditions. I’ve found this is a really common mistake when it comes to the cosmos, as people love to treat it like their other bedding plants and really soak the soil, which makes our cosmos an unhappy customer.

If the soil is kept consistently moist, then I’m afraid your plant is going to struggle, turn yellow, and die back or they just not grow very well, depending on the extent of how overwatered it is.

Remember how we talked about emulating the cosmos’ natural environment? Well, the same applies to watering.

We need to scale back our watering of the cosmos so that the soil is slightly dry between bouts of watering.

How do we do this? My method, which I find works well, is to test the soil to a finger depth around the cosmos and adjust the frequency of watering so that the soil feels somewhat dry before watering again.

  • 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, 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 result from less frequent watering but also well-draining soil (with added sand or grit), which should give the best bloom results.

As I mentioned my preferred potting mix for cosmos is 80% compost and 20% horticultural sand. However you may be reading this article from a super rainy climate! In which case, I’d up the quantity of sand to around 30% to increase the drainage, which I’ve found really effective at mitigating the effects of heavy rainfall.

Pro tip: Are you growing cosmos in a rainy climate? Here’s something that I find works really well. Plant your cosmos in a terracotta or clay pot…These materials are breathable, which allows the soil to dry out more evenly after rainfall, which decreases the risk of problems associated with overwatering, and your cosmos is more likely to flower.

Timing of Planting Can Affect Blooms

Do you remember when you sow your cosmos seeds? This is very important for flowering! The optimal time for sowing seeds of cosmos is around March or April or after the threat of frost.

I often plant my cosmos in a seed tray or a pot just in case there is a late frost. Then, if I see a last frost forecast, I can bring my seeds or emerging foliage into my shed.

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. I have sown cosmos as late as July before and still had a good display of flowers, but I was lucky with the weather that year!

This is not always the case…typically I have found 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.

One time, I planted mid-July, the weather so hot that the seedlings weren’t able to grow well or flower as they struggled to contend with heat!

Of course, we need to remember the reverse is true, too. Planting cosmos too early can also risk frost damage from a late frost in the very early Spring, which can prevent flowers.

Success Tip: To ensure your cosmos is safe from the risk of frost in the early Spring, I like 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

So this is a really interesting feature of growing cosmos that I studied when I did my botany diploma. 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. (Cosmos is quite the fickle plant when it comes to light!)

Why does it behave like this? For good reason! 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 usually cosmos start to develop flowers after this date as the days grow shorter.

Do 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.

Are you near any artificial sources of light, such as street lights that are bright after dark? Does your neighbor’s cat trigger the security light at night? These can all interrupt your cosmos period of darkness and prevent them from flowering! We all need our beauty sleep, right?!

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

So the industry wide advice on this is that while 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.

I have tried this a few times as a test, and I found that the seeds germinated surprisingly well! There was only a 15% drop in germination rate for the seeds that were 1 year old (as in seeds that I bought in a packet and didn’t get around to sowing that year) compared to the seeds that I bought that very year.

If your seeds are not germinating, or if they have germinated and they haven’t flowered well, then I would suspect your problem lies in how we stored the seeds.

If the seeds have been stored for a long time in less-than-ideal conditions, this can affect their quality 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 temperature is consistent and does not go below freezing or become too hot. Of course, you need to ensure they don’t get wet. I would even consider whether the room they are in gets a lot of condensation.

My Best Tips for Storing Your Seeds to Make Sure Your Cosmos Flowers: So I have 2 tried and tested methods that I have trialed with success! What I like to do is place my seeds in an envelope that keeps the seeds dry and protected. Another great option is to use an old jam jar and place the seeds in there with a packet of silica gel (as a desiccant to absorb any moisture from the air). The jar is airtight, so the seeds are preserved.

I would caution you against using a plastic bag as they tend to trap moisture.

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, even if you have provided the optimal growing conditions, the weather is just not conducive for the cosmos to flower well. It happens to all of us gardeners! It’s part of the fun, they say!

I’ve had it when, in the Spring or Summer, the temperature has been unusually cold, rainy, or with lots of overcast days without significant sunlight, and my cosmos barely displayed flowers as this was contrary to the cosmos preferred conditions of full sun, warmer weather, and a dry summer to flower its best.

But what did I learn from this experience?

I grow my cosmos in a pot and shield it from excessive rain when it is forecasted, and even move it into a greenhouse.

My cosmos absolutely flourishes when it’s in a greenhouse as the temperature is increased (which is closer to the warmer temperatures in the cosmos’s native Mexico) and I can control the watering again to replicate its preferred conditions.

This made my cosmos flower much better; then, when the weather improved, I was able to return it outside!

Clay Soil Can Prevent Flowering

My advice is to ask yourself, do you have the sort of soil that bakes hard in the Summer and becomes boggy in Winter? This indicates the soil has a high content of clay in it.

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.

As we talked about, if the soil is consistently damp, then cosmos does not necessarily flower.

I can assure you from first-hand experience that amending soil with a high clay content is difficult for growing cosmos, so I always advise people to grow their cosmos in pots, containers, or raised beds.

Why is this? Pots have favorable drainage conditions compared with garden borders, and it is a lot easier to create a well-draining potting mix that suits cosmos and promotes flowering than to amend a garden border.

If you have started growing cosmos in clay soil, it may be possible to transplant the plugs into pots to improve flowering or (in my opinion, this is the better option) 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.

Do you have any questions about cosmos? Or do you have any experiences and perspectives of your own that you’d like to share? If so, please leave a comment below, and I’ll reply! I love hearing from all of you gardeners!

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.

Mark Bennett

Mark Bennett is the writer at gardener report. I'm a qualified botanist, landscape gardener and garden center plant troubleshooter! I grow and care for all the plants I write about so I have lots of first hand experience, in troubleshooting problems with plants and providing step by steps tips for recover. I love hearing from you so please leave me a comment and I'll reply!

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