Cosmos are delicate annual daisy-like flowers that brighten your yard with glorious color all summer. The gorgeous blooms are a favorite with bees and butterflies and will attract these important pollinators to your garden in early summer.
Even the feathery leaves and slender stems are attractive and will add softness to your garden. Cosmos flowers are easily grown from seed and will spread freely if allowed to self-seed for the following growing season.
In other words, they’ll self-sow themselves so you’ll have more flowers the following growing season.
However, they are considered somewhat invasive in the southern parts of the US like Florida, so it’s best to grow them in pots if you live in this region and don’t want them to self-seed all over the garden.
History
All the commonly grown cosmos in gardens originate from Central America. Their botanical name Cosmos comes from the Greek word “kosmos” which has numerous different meanings. The most common ones are adornment, universe, and beauty.
These blooms have been grown for many centuries. In fact, it is believed that ancient civilizations in Latin America used the yellow and orange cosmos flowers to make dye.
In the late 1700s, a Spanish botanist by the name of Casimiro Gomez de Ortega brought some seeds from the Cosmos bipinnatus species to England.
Plant Facts
| Scientific name | Cosmos sulphureus, Cosmos bipinnatus |
| Common names | Cosmos, Mexican aster |
| Genus | Cosmos |
| Family | Asteraceae |
| Height | Up to 6 feet |
| Width | Up to 3 feet |
| USDA Plant Hardiness Zone | 2 to 11 |
| Origin | Northern South America, Central America, Southern North America |
| Flower colors | White, yellow, pink, red, magenta, orange, chocolate |
| Blooming season | Early summer until the first fall frost |
| Plant/Flower special features | Cosmos are perfect for adding to cut flower arrangements and the cosmos flower attracts pollinators and other beneficial insects. |
How to Plant and Grow Cosmos Flowers
Cosmos grow happily out in the garden. They are great for planting en masse to create a beautiful and colorful display.
They are more often grown from seeds and if you leave the last blooms on the plants to go to seed, you’ll have them popping up again the following year.
Alternatively, cosmos grow in pots too. On average, you want to have one plant per gallon of your container size. Use a fairly heavy pot and make sure that it’s at least 12 inches (60 cm) in diameter. Make sure that the potting mix you use is not too rich, otherwise, the plants will grow too tall and leggy.
When growing cosmos in pots, you might find that you have to stake the blooms. Especially if you’re going to grow some of the taller cosmos flower varieties.
How to Propagate Cosmos Flowers
The easiest and best way for growing cosmos in your garden is to use seeds. However, it is also possible to propagate cosmos flowers using stem cuttings. We’ll explain how to either grow your cosmos from seeds or how to propagate new plants from stem cuttings.
How to Sow Cosmos Seeds Outdoors
You can either start your cosmos seeds indoors around 4 to 6 weeks before the last frost or sow them directly into the garden once the danger of frost is over. Here’s how to sow your seeds outdoors after purchasing them from the garden center:
- Wait until the temperature is at least 75 degrees Fahrenheit (24 degrees Celsius).
- Make sure that the soil is quite friable to a depth of around 8 inches (20 cm).
- Sow the seeds by scattering them over the soil and covering them lightly with fine sand. It’s important not to plant them too deeply or they won’t have enough energy in the tiny seeds to break the surface of the soil.
- Water the soil to ensure that it’s just moist.
- Each cosmos seed should germinate in 7 to 21 days and each cosmos plant will start to produce blooms in around 50 to 60 days.
- Once the seedlings have emerged, you might want to thin them out a little so that you have one plant every 9 to 12 inches.
Starting Cosmos Seeds Indoors
If you want to start your seeds indoors to get a head start on your spring planting, you should sow these around 4 to 6 weeks before the last frost date. Here’s the best method to start your seeds indoors:
- Fill small pots or seedling trays with a good-quality seed-raising mix.
- Spray the mix with water to moisten the surface.
- Place around 2 to 3 seeds in each small pot or sow cosmos seeds over the mix in your seedling tray.
- Cover these cosmos seeds very lightly with some more seed-raising mix.
- Spray the soil again to moisten it.
- Place your pots or seedling trays in a very bright south-facing window and turn them on a daily basis to ensure that your seedlings don’t grow in just one direction.
- If you don’t have a south-facing window or the inside of your home is quite dark, you might want to invest in a grow light.
- You might also want to place your cosmos seedlings on a heat mat in order to facilitate good germination.
- If too many cosmos seeds have germinated and you have an overabundance of seedlings in each small pot, just thin out the weakest ones. It’s best to just cut these at the soil level with a small pair of scissors in order to avoid disturbing the roots of the stronger seedlings.
- Remember to harden off your cosmos seedlings before planting them out in the garden in early spring. You do this by placing your pots outside for a short time on the first day and increasing that time day by day for around a week.
Propagating Cosmos Flowers from Stem Cuttings
Here’s what you need to do:
- Fill a pot or two with a high quality well-draining potting mix.
- Using a pair of sharp and sterile secateurs for pruning shears, look for a straight stem that has around 3 to 5 leaf nodes. Make your cut under the bottom leaf node ensuring you have a decent section of stem under that leaf node.
- Remove the leaves growing around the lowest leaf node.
- Make a hole in the potting mix with a pencil.
- Place the stem in the hole but ensure that the bottom leaf node is just above the soil level. Firm the soil around the stem so that it is well-supported.
- Dampen the soil in the pot and make sure that you keep it nice and moist until you see new growth appearing.
- After around 3 weeks or so, the cuttings should have produced new growth and a solid root structure. You can now plant them out into the garden taking care not to damage the root ball.
Propagating Chocolate Cosmos By Division
Chocolate cosmos (Cosmos atrosanguineus) does not have seeds that germinate readily. Therefore, it’s much more reliable to propagate this species by dividing the underground tubers.
Here’s what you need to do:
- Once the plants have stopped flowering in the fall and the foliage has died down, dig up the clump of tubers carefully.
- Using a sharp knife or secateurs, divide the clump into individual tubers that are nice and plump and look healthy.
- You can store these tubers in a cool, dry spot over winter and then plant them back out into the garden in spring when the danger of frost has passed.
Cosmos Flower Care and Maintenance
Cosmos flowers are very low-maintenance. The seeds can be sown into established garden beds and they make excellent cut flowers. These plants are fairly drought tolerant once they’re fully established and will even grow in poor soil conditions.
Soil
Cosmos will grow quite happily in poor soil where many other plants may not. They do prefer neutral to alkaline soils with a pH level between 6.0 to 8.0.
Cosmos don’t require special soil preparation. In fact, it’s wise not to grow cosmos in rich and fertile soils because this may cause the plants to grow too tall and fall over.
In order to stop the plants from falling over if your soil is too rich, you might need to stake them. Alternatively, you can grow them very near to other plants that will offer some support.
Water
As mentioned, cosmos flowers are drought-tolerant once established. Therefore, they don’t need too much supplementary water unless your area suffers from a long period of dry weather.
You just need to ensure that young plants receive enough moisture for a few weeks until they’ve become fully established. However, during the heat of summer, if you see your plants starting to wilt, water them deeply at the soil level.
Fertilizer
You do not need to fertilize your cosmos plants. In fact, too much fertilizer can encourage foliage growth and not very many blooms. If your soil has been enriched with a little compost before planting, this should contain all the nutrients that your cosmos flowers need.
Sunlight
Cosmos will flower the best when grown in full sun. Ideally, your cosmos flowers should receive around 6 to 8 hours of direct sunlight daily.
However, they will flower in part shade but not as profusely. These hardy plants will even thrive quite happily in the hottest conditions when exposed to full sun.
You’ll find though that most seed packets will suggest that you grow cosmos in full sun soil.
Temperature and Humidity
Cosmos flowers love the heat. In fact, they grow best in hot conditions and also don’t mind a bit of humidity.
Mulching
Mulching is always a good idea for annual cosmos because it will prevent moisture evaporation and inhibit weed growth. However, you want to ensure to keep the mulch around 1 to 2 inches away from the plant stems.
Staking
If you plant your cosmos flowers fairly densely, they shouldn’t require staking because they’ll support each other. However, if your plantings are a little more sparse, you might want to stake individual plants to stop them from falling over.
Thin bamboo stakes are ideal for this because they’re not too intrusive but will provide enough support for each cosmos flower. However, if you have a large planting of these gorgeous blooms, you might want to consider using a trellis or other suitable support system.
Pruning
In general, the plants themselves don’t require pruning. However, you should remove the dead flower heads regularly in order to keep the plant producing more blooms right through summer and into the fall.
If you haven’t had the time to deadhead your cosmos flowers regularly, you can give the plants a new lease of life by cutting the whole lot back by around one-third. If you do this, you’ll find that your plants will produce a second flush of gorgeous blooms.
When the blooming season has finally finished, you can just cut the plants right down to the ground or even pull them out. However, if you want some more cosmos flowers to grow the following spring, you should leave the plants until they’ve produced seed heads.
This will allow the plants to disperse their seeds and you won’t have to reseed them again in the spring.
Cosmos Flower Pests and diseases
Fortunately, cosmos flowers are fairly hardy and don’t have too many problems. However, there are a few concerns you might need to watch out for.
Fusarium Wilt
This fungal disease will cause your plants to wilt and the leaves to become discolored. Once your cosmos has succumbed to fusarium wilt, there’s nothing you can do to save it. It’s best to dig up the plant and dispose of it in the garbage to stop the infection from spreading further.
Powdery Mildew
Powdery mildew is a common problem for many different types of plants in the garden. It can also infect your cosmos plants. This is a fungal disease that is spread by spores through the air.
If your cosmos plants are infected with powdery mildew, you’ll find that the leaves have a white powdery-type coating.
You can prevent your cosmos plants from getting powdery mildew by growing them in full sun and making sure that they have good air circulation around them.
It’s also a good idea to only water your plants at the root level and avoid getting the leaves wet.
Many gardeners have had success controlling powdery mildew by spraying their plants with a mixture of milk and water or you could try a proprietary fungicide.
Aster Yellows And Bacterial Wilt
This is a disease that is spread by leafhoppers. It can be quite common for plants in the Aster family. Evidence of this disease appears as yellow mottling on the leaves and stunted or distorted blooms.
There is nothing you can do to treat this disease so it’s best to just pull the plants out and dispose of them in the garbage.
Aphids
Like many plants in your garden, your cosmos flowers may also attract sap-sucking insects such as aphids. Luckily, cosmos flowers also attract beneficial insects such as lacewings that will help to control the aphid population by feasting on them.
Therefore, you want to be quite careful about how you treat any insect infestation on your flowers so as not to harm the beneficial insects that offer biological control in your garden.
Thrips
Thrips feed on the flowers but generally won’t harm the plants. To control these, you can use an oil-based product such as neem oil.
Spider Mites
These annoying pests can usually be found on the undersides of the leaves and can easily be identified by the tiny webs that they create. They are best treated with an oil-based product such as neem oil.
Cosmos Flower Uses
Primarily, cosmos flowers are used as ornamentals in the garden to provide plenty of gorgeous color in summer. These flowers are ideal for planting in drifts along pathways or in sunny flower beds.
Ideally, you want to plant tall cosmos at the back of the flower bed and plant shorter growing flowers in front of them.
They also make excellent cut flowers and this makes them popular with cottage gardeners. For longer-lasting flowers, be sure to add some sugar to the water in the vase.
Because these lovely flowers are so good at attracting pollinators, they can also be planted as a border around your veggie garden. Or, as companion plants in and around fruiting crops.
See more: Cosmos flower meaning
Common Varieties and Cultivars of Cosmos Plants
There are three species of cosmos flowers that are grown by home gardeners as well as many different cultivars.
Some perennial cosmos varieties will have large and striking single flowers on long stems while others will have gorgeous double blooms.
Cosmos sulphureus
Cosmos sulphureus (sulfur cosmos) has golden yellow flowers that can be single or semi-double flowers. These plants are tall cosmos and can grow to around 48 to 72 inches tall. However, many of the recent cultivars have shorter growth than the taller varieties and their flowers tend to be more orange in color. Cultivars in this species tend to be the most heat tolerant.
Cosmos bipinnatus
Cosmos bipinnatus (garden cosmos) come in a variety of different flower colors including white, orange, yellow, purple, pink, and red. Cosmos bipinnatus is also native to Mexico and is the most commonly grown garden variety.
Cosmos bipinnatus can grow to a maximum height of around 4 feet. Therefore, this species is another tall cosmos. It is not quite as heat tolerant as Cosmos sulphureus but it will still thrive in bright, sunny spots in your garden.
Cosmos atrosanguineus
The chocolate cosmos is native to Mexico. This species has dark red flowers that have a delicious chocolate scent. This particular species actually grows from underground tubers and requires a little more care and maintenance than the other two primary species.
You’ll also find that these flowers do not grow reliably from seeds. Therefore, the best way to propagate this species is by division of the underground tubers.
Some of the more popular cultivars include:
- Cosmos ‘Bright Lights’ is a tall cosmos with a mix of colorful blooms in red, orange, and yellow.
- Cosmos ‘Peppermint Candy’ is a tall cosmos with flower petals striped in red and white.
- Cosmos ‘Cosmic Orange’ is a tall cosmos with amazing orange flowers that have a semi-double form.
- Cosmos ‘Sea Shells’ a tall cosmos with unique tubular petals that come in a range of pastel colors.
- Cosmos ‘Ladybird’ with shorter stems and blooms in colors of yellow, gold, orange, and red.
- Cosmos bipinnatus ‘Versailles Tetra Red’ is a tall cosmos with rusty red single blooms that have a cheery yellow center.
Conclusion
Cosmos flowers can be a colorful and beautiful addition to your summer garden. Ideally, they should be grown in full sun and don’t need rich soil. They’ll respond to a little extra watering while they’re young but are drought-hardy once they’ve become fully established.
Not only are cosmos flowers easy to grow, but they are also excellent at attracting pollinators to your garden. This makes them ideal for growing near your veggies and other fruiting plants. And, to top it off, cosmos also makes really good cut flowers so you can enjoy them indoors as well as in the garden.
So, if you’ve wondered “are cosmos easy to grow”. then the answer is definitely yes.For more flowers to grow, see our list of flowering plants.
