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How To Keep Cosmos Blooming Strong – From Spring Until Fall!

Cosmos are one of the easiest and beautiful annual flowers to grow from seeds, and with just a few basic tips, you can keep them healthy and blooming all season long. 

These daisy-like flowers are quick to impress. They can go from seed to flowering plant in just two short months. But without a little help, their blooms can be greatly reduced as time goes along, or in some cases, they can even stop blooming altogether.

Thankfully, the task of figuring out the issue and remedy when the plant slows its blooming is fairly simple and straightforward. And that is exactly what today’s article is all about!

pink and purple cosmos blooms with the green foliage in the background
Cosmos blooms have a daisy-like appearance and sit atop thin long stems.

A Little About Cosmos

Cosmos are a beautiful flowering annual that blooms from late spring until the first frost with proper care. They are both drought and heat-resistant. In addition, they are often passed by deer and other animals, making them a great addition to any property. 

Cosmos can grow anywhere between 18 inches to 6 feet tall depending on their variety. In addition, plants can also be anywhere from 1 to 3 feet wide. 

Cosmos can be started indoors or directly sown during the early springtime. They take around 7 to 21 days to germinate. While this time frame might seem like a long time, you won’t have to wait long after germination before you are rewarded with stunning blooms!

Those daisy-like blooms can come in nearly every color in the rainbow, including red, orange, yellow, purple, pink, white, and even brown. In addition, their colorful flower heads are packed full of pollen, which makes them excellent for attracting bees, birds, butterflies, and other pollinators to your garden. 

A butterfly sitting atop a light purple cosmos bloom.
The gorgeous blooms of cosmos are perfect for attracting pollinators to your property. The seed heads are packed with pollen, making them quite attractive to bees and butterflies.

Taller varieties look stunning when planted with equally vertical plants such as black eyed susan and coneflowers. These varieties work well for borders and flowerbeds while the shorter varieties are excellent for pots and containers. (For even more easy-to-grow flowers, check out How To Grow Annual Flowers From Seed.)

A great advantage to planting cosmos on your property is that even though they are annuals, they are also self-seeders. That means that with just one planting, you can enjoy their beautiful blooms year after year with some basic maintenance. 

How To Keep Cosmos Blooming Strong

There are many different reasons why your cosmos might not be blooming to their full potential. Take the following tips into consideration not only when you go to plant your cosmos seeds, but also throughout the growing season as well. You’ll have your cosmos blooming beautifully in no time at all!


One of the most common reasons why your cosmos might not be flowering strong is the lack of pruning and deadheading. 

A bright pink and light purple cosmos bloom along side several spent blooms
Be sure to deadhead any spent blooms in order to promote continual flowering.

When plants are around 18 inches tall, prune off all stems until they are around a foot tall. This will allow the cosmos to branch off in many directions as opposed to creating one long central stem. More branches equal more options for flowering.  

In addition to pruning, be sure to deadhead old or decaying blooms from the stems throughout the entire blooming period. Use pruning shears to cut between the stem and a leaf section, but don’t cut too low on the stem since it will take longer to grow back flowers. The process of removing these spent blooms will promote the plant to produce more flowers all throughout the growing season.  

If you want to have your plants self seed at the end of the growing season, leave a few of the dried blooms on to allow them to drop their seeds.

Excess Water

Since cosmos are native to Mexico, they are used to dry, humid soil conditions. In fact, they thrive in drought-like conditions. So overwatering is often a big reason for a reduction in blooms. 

If the soil is kept moist, the cosmos plants will focus on growing foliage as opposed to putting out beautiful blooms. It can also cause the plants to rot from excess moisture on the roots. You’ll likely only have to water established plants in case of a severe drought. 

Use a finger to feel down a few inches into the soil. If it feels damp, hold off on watering and check again in a few days. If your finger is dry, then give the cosmos a good soaking and wait to water again until the soil is dry.


As mentioned above, the cosmos plants do not require fertile soil. The use of fertilizer will actually cause the plants to produce more foliage as opposed to blooms. While your plants might look lush and healthy, you will be left with little to no flowers to enjoy. 

A cosmos plant that is full of mostly foliage and hardly any white blooms
Using fertilizer will actually cause your cosmos to produce a massive amount of foliage but hardly any blooms.

Unfortunately, there isn’t much you can do if you’ve already applied fertilizer to the plants. Just be sure to try to keep all of the other aspects mentioned in this article in check to get your plants to flower as much as possible. 

If your plants do look like they are struggling to survive, you can try giving a low dose of fertilizer to see if that helps perk them up. Just make sure to use a type that is lower in nitrogen levels since this will cause plants to become tall and spindly with few blooms.  

Giving Cosmos The Best Chance For Success! – How To Keep Cosmos Blooming Strong

The Right Type Of Soil

Unlike most seeds that you plant in your garden or in containers, cosmos actually prefer less than ideal soil fertility. Since cosmos are native to Mexico, they thrive in sandy, well-draining soil. 

If your garden soil is nutrient-dense and has been amended with compost, this will actually cause your cosmos plants to produce fewer blooms. The reason is that the plants will actually put more resources into growing their foliage as opposed to using it for blooming.

In order to help out your nutrient-rich soil, you need to amend it with sand to create a soil that is more similar to the cosmos’ native soil. Adding sand will not only help to improve drainage (another plus for cosmos), but it will also create more of a balance between the organic and non-organic materials in the soil.

Clay Soil  

Heavy soil such as clay will also cause cosmos to struggle to take root and produce blooms. As mentioned above, the soil needs to be well-draining in order for the cosmos to have their best success. 

A boot pushing a shovel into dense and saturated clay soil.
Clay soil is notorious for holding excess moisture as well as being nutrient-dense. It’s not the best growing medium for cosmos flowers.

Clay soils tend to retain too much moisture. This causes the roots of the cosmos plant to be constantly damp. These issues make clay soil the total opposite of the ideal growing conditions that cosmos prefer.     

Again, you can amend the heavy soil with sand. However, you might be better off growing cosmos in pots, containers, or raised beds since you have more control of the drainage and type of soil used in those situations.   

Your Planting Date 

The best time to plant cosmos seeds is in the spring after the threat of frost has passed. Waiting until your soil has warmed to around 60° Fahrenheit (16° Celsius) is actually ideal. For several locations in the United States, that tends to be around March and April. If you plant too early and receive a late frost, your cosmos are unlikely to survive.  

On the flip side of that, don’t wait too long to sow seeds in the ground. If you do, your plants will also struggle. Cosmos are known as short-day plants. This means that they bloom their best when daylight hours are 14 hours or less.

Since cosmos take a long time to germinate, planting too late in the spring or early summer will cause them to have maximum growth when daylight hours are the longest in summer. 

If you have shorter springs, you might want to consider starting seeds indoors to give cosmos plants a head start. You can then transplant them into the garden once the weather warms up. This will allow the flowers to flourish while the daylight hours are still shorter than they are in the middle of summer. 

Two dark pink cosmos blooms covered in frost and drooping downward.
Cosmos plants are unlikely to survive a late spring or early fall frost. Timing their planting is critical to still take advantage of cool weather, but not put them in harms way of a frost or freeze.

Age Of Seeds

As with all plants, you need to make sure you aren’t planting older seeds. If seeds have been improperly stored for a long period of time, this will cause a direct impact on the plant’s health and blooming ability. 

It’s best to use seeds that have not been stored for more than a year or two at most. Check your seed packet since most have the date they were packed printed on the label.

When storing seeds, be sure to keep them in a cool, dry location and away from light. For more information on properly storing seeds, check out our article How To Store Extra Seeds Properly.

Proper Sunlight

Cosmos require at least six hours of daylight per day in order to bloom to their full potential. While plants can survive in partial shade, you will have a great reduction of blooms if there is not enough sunlight. Plants located in a shady area will likely become leggy as well. 

Consider transplanting cosmos to a sunnier location if your plants spend too much time in the shade. If transplanting isn’t an option, try to cut back any tree limbs or foliage that might be causing the cosmos to become shaded. 

Factors Out Of Your Control

Sometimes, Mother Nature can simply make it a bad year for cosmos. An unusually cold spring or summer can cause your cosmos to not bloom as prolifically. In addition, unusual amounts of rainfall or multiple cloudy days may also be to blame. Excess winds might even cause weaker plants and that directly affects the plant’s health as well. 

So if you have one bad year of growing cosmos on your property, don’t give up! They are definitely stunning flowers worth giving a second shot!

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