How to Grow and Care for Cosmos Flower

If you want to grow a cheerful and colorful flower that will bloom all the way through summer and to the first frost, cosmos are a great option to go with.

Cosmos are annuals that are easy to grow and will reward you with bright colors and consistent blooms. They pair well with many other annuals and perennials in the garden and can also be grown in containers.

Here’s more about how to grow and care for cosmos flower so that you can brighten up your garden with their pretty faces.

What is Cosmos Flower?

Cosmos are bright and attractive cottage garden flowers that bloom with daisy-like petals. The plants are native to Mexico, other parts of Central and South America, and southern regions of the U.S.

Most gardeners grow cosmos as annuals, although they can be perennials in USDA hardiness zones 9-10. There’s also a perennial variety of cosmos that grows from tubers much like dahlias.

The two main species of cosmos grown in the garden are Cosmos bipinnatus and Cosmos sulphureus. Both seed themselves freely, although C. sulphureus can become a little more weedy if not kept under control.

The many varieties of cosmos range in height from about 1 foot tall to almost 6 feet tall. Their typical spread is 1-2 feet, and flower blooms get 3-5 inches across.

Cosmos have lovely daisy-like flowers and bloom from summer until frost. They will be one of the most reliable annuals in your garden and come back year after year from seed.

Depending on variety, flowers are a range of yellow, orange, red, white, pink, and maroon. Some are bi-colored and others have a double layer of petals. Bloom time typically starts in early to mid summer and goes all the way until the first fall frost.

Why Grow Cosmos?

One of the biggest benefits of growing cosmos is that they are an extremely low maintenance plant. Drought and heat tolerant, plants grow happily in poor soils and will actually flower better in soils with low fertility.

Even though they aren’t perennials, cosmos reseed themselves and will give you lots of free plants the following spring. You’ll rarely need to buy more plants unless you want to try new varieties.

Cosmos also attract bees, butterflies, birds, and various other pollinators to your garden. They make an excellent addition to a butterfly garden or a cut flower garden.

It’s extremely easy to save the seeds from cosmos plants if you want to try your hand at starting your own seeds the following year. Or you can just regularly harvest the flowers to put in vases and flower arrangements.

The long flowering season of cosmos is another benefit. They’ll be happy and blooming in your garden even in the middle of summer heat when many other plants start fading.

Cosmos are pollinator friendly and will attract bees, butterflies, and birds to your garden. They’re also extremely low maintenance, drought tolerant, and make great cut flowers.

Cosmos Varieties to Grow

You’ve got lots of choices when it comes to size and color of cosmos, including some dwarf varieties great for small spaces. Here are some top picks:

• Seashells Mix– A pretty pastel mix of white, light pink, and dark pink blooms. Flowers grow 3 inches across, and the petal edges roll up like seashells. Foliage is fern-like, and plants grow 3-4 feet tall.
• Double Click– Flowers in this series are large and double or semi-double. You can get pure white, pink, deep purple, or a mix. Plants grow about 3 ½ feet tall.
• Cupcakes and Saucers Mix– A unique new mix, pinwheels of small petals are inside larger petals that look like cupcake liners. Flowers are a mix of pink and white, and plants grow about 4 feet tall.
• Picotee– This variety is truly stunning. Flowers are pure white with a crimson border all around the petals. Some are also flecked with red. Plants flower profusely and grow 3-5 feet tall.
• Bright Lights Mix– If you like warmer colors, this is a mix of bright orange, red, and yellow. Foliage is lacy and plants grow about 3 feet tall.
• Cosmic Orange– One of the best orange varieties, this is a cultivar of Cosmos sulphureus. Plants grow only 1-2 feet, flower abundantly, and rarely have pest or disease problems.

Picotee and Peppermint Candy show off white blooms that are outlined in either red or purple. You can truly find some standout cosmos varieties.

• Sensation Mix– This mix has large blooms (3-4 inches) of pale purple, white, pink, and magenta. Plants grow 4 feet or taller.
• Ladybird Series– This is a dwarf series that grows 1-2 feet tall. Flowers are semi-double and shades of orange and yellow.
• Peppermint Candy– Magenta and white blooms explode on this cultivar with no two flowers exactly alike. Plants grow 1 ½ – 2 feet tall.

How to Grow Cosmos Flower

You can pick up cosmos transplants at your local garden center or nursery, but they’re also extremely easy to grow from seed. Starting seeds gives you a lot more choice for variety and also saves you money.

Starting Seeds Indoors

To get a jump on the growing season, start your cosmos seeds indoors about 4-5 weeks before your last average frost date in spring. You can sow them in plug trays or cell packs.

Use a good quality soilless seed mix and get it damp before filling your trays. Fill them just below the top of the cells. Cosmos seeds should be sowed shallowly: about ⅛ – ¼ inch deep. You can make indents in each cell with your finger before dropping your seeds in.

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Water your seeded trays and cover them with plastic domes to keep moisture in while the seeds germinate. Place them somewhere warm, and sprouts should appear in 1-2 weeks.

Remove the plastic domes after your seedlings pop up, and put the trays under grow lights or by very sunny windows. Ideally, you want your seedlings to get 10-14 hours of light each day. You can also run a fan a few times a day for good air circulation.

A week before you plan to plant, take your seedlings outside during the day to harden them off. Keep them in a shady spot so they don’t get scorched by the sun and bring them back in at night.

Starting Seeds Outdoors

It’s also easy to sow your cosmos seeds directly in your garden. Wait until all danger of frost has passed and the soil has warmed (cosmos are very sensitive to freezing temperatures).

Before putting your seeds out, prep your soil by weeding it, getting rid of rocks and debris, and loosening the top inch or two. Cosmos don’t really need any soil amendments, but you can mix in some compost if you want.

Sow your seeds shallowly or simply scatter them on top of the soil. You can either space your seeds out as you plant them or plan to thin them later.

Cosmos seeded directly in the garden will be a little bit behind those started by seed indoors. It typically takes them 7-8 weeks to bloom after germination.

Keep the seeded area moist while seeds germinate, which should take 1-2 weeks. Once your seedlings are 3-4 inches tall, you can thin them to the proper spacing for the variety you’re growing. Pull up or cut off seedlings you don’t want.

While most gardeners will put their cosmos seeds out in spring, you can get a headstart by sowing them in the fall. 

Wait until a few hard frosts have come through, then scatter seeds where you want them to grow in the spring. Unlike the plants, cosmos seeds won’t be hurt by the cold, and it will keep them from germinating until next year.

Planting Your Cosmos

Cosmos are very un-picky plants, but paying attention to a few important details will really ensure that they grow well.

Growing Conditions

Probably the most important tip for growing cosmos is to plant them in full sun. They will still grow in partial shade but will have much fewer blooms and struggle to thrive. The one exception is areas that get prolonged periods of extreme heat where plants may need afternoon shade.

Cosmos are a unique plant in that growing them in rich and fertile soil is not the best choice. Poor to normal soil will give you the best blooms, whereas rich soil will give you lots of foliage and few flowers.

Full sun and good drainage will give your cosmos just about all they need to grow well. Avoid planting them in very rich soil because it will give you lots of foliage and few flowers.

You can grow cosmos in a wide range of soil types, but they don’t like wet feet, so try to grow them somewhere with good drainage. They do well in dry and arid soil.

How to Plant

You’ll want to plant cosmos seedlings well after all danger of frost has passed in the spring. Don’t rush it since these flowers are very frost sensitive.

The right spacing for your plants will depend on what variety you’re growing and how big its mature size is. Shorter varieties (mainly C. sulphureus) can be spaced 10-12 inches apart and taller varieties (mainly C. bipinnatus) can be planted 1-2 feet apart.

Dig holes that are about the size of the root balls of your plants. Gently lift one plant out at a time and loosen the roots with your fingers before putting it in a hole.

Fill back in around your seedlings with soil and firm the top of it with your hands. You want the top of the root ball to just be buried under a thin layer of soil. Water your new seedlings well, but try to avoid getting the leaves wet when you water.

Planting in Containers

Cosmos can make a good plant for a container garden, although they won’t grow as large as they would out in your garden. You’ll probably have the best success with dwarf varieties.

If you’re short on space or don’t have a garden, you can grow cosmos in containers. Dwarf varieties like Cosmos sulphureus hybrids will do the best.

Use medium to large containers with drainage holes in the bottom. Fill them with a good quality potting soil and mix in water until the soil is damp. Plant your cosmos after the danger of frost has passed (although you can always pull your pots in at night).

The main thing to keep in mind with containers is that they need to be watered more frequently than plants in the ground.

Cosmos Care

The good news is that you won’t have to do much to take care of your cosmos once they’re planted.

The biggest maintenance task is to deadhead spent blooms to keep your plants flowering for longer. This is entirely optional, but it will cause your plants to branch out and produce new blooms.

You can leave the brown seed heads on in fall to feed the birds and so that they will reseed for next year.

Once established, cosmos are drought tolerant, so they won’t need watered unless you have a month long drought. However, you will need to water your seedlings during dry spells until they send down deeper roots. Plants in containers will also need watered when the soil is dry.

Despite their delicate appearance, cosmos are very hardy and will do fine with long periods of neglect. It’s better to underwater rather than overwater them and leave them to themselves.

Taller varieties of cosmos may need staking or some other kind of support (like a fence) to keep them from falling over on windy days.

If you don’t deadhead regularly, most cosmos will self-seed. This is good if you want new plants, but you’ll want to keep on top of pulling out seedlings that are growing where you don’t want them to.

Pests and Problems

Cosmos rarely have any serious pests or diseases, which is one of the things that makes them so low maintence.

Occasionally, aphids or thrips may make their way to your plants. You can simply hose them off or use an organic insecticidal soap for more serious invasions.

Plants can develop fungal diseases or gray mold when they are crowded together and the weather is damp. Space them out properly to avoid this.

Deer and rabbits will at times munch on cosmos, especially small ones, but they aren’t their first choice.

Harvesting Flowers and Seeds

Cosmos make an excellent and long-lasting cut flower. You can cut them off anytime during the growing season by using a pair of garden clippers to cut off healthy stems. Harvest stems where the flowers have just opened for them to last the longest.

Cosmos are beautiful as a cut flower and will last for a long time if they are cut right after the blooms open. You can also harvest the seeds to plant next spring.

It’s also easy to collect seeds in the fall to sow again next year. Just keep in mind that hybrid varieties may not come true from seed.

All you need to do is clip off seed heads when the seeds are brown and dried. Separate the seeds from the rest of the flower head and store them in airtight containers over the winter. Make sure you put them somewhere dark, dry, and cool.

Enjoying Cosmos Flower in Your Garden

Cosmos work well with so many other flowers. Here are a few design ideas:

• Orange and yellow cosmos look great when paired with purple flowering plants. Place them next to purple coneflower, purple asters, balloon flower, or liatris.
• Grow cosmos as part of a wildflower garden with other natives like bee balm, milkweed, butterfly weed, Joe Pye weed, cornflower, and calendula.
• Pair with perennials like black-eyed Susan and agastache or other annuals like zinnias, dahlias, cleome, or cornflowers.

No matter how you use them in your garden, these cheerful flowers will brighten up any space. Be sure to look for some cosmos volunteers to pop up next spring, since they don’t like to leave once invited in!