How to Plant and Care for Cosmos—a Whimsical Cut Flower That Blooms All Summer Long

It's easy to see why gardeners love cosmos. These delightful flowers burst with cheerful blooms in vibrant shades, from white to purple to pink. They're perfect in a meadow or English cottage garden, but they also look beautiful growing in a highly curated landscape.

Even better? Pollinators love these full-sun beauties, which have a long growing season, filling gardens with color from midsummer until the first frost in fall. Cosmos grow tall and full, adding vertical depth to your landscape, and have thick foliage that helps keep weeds at bay. Here, our gardening experts share their tips for growing and caring for cosmos so you can enjoy their beauty all season long.

When to Plant Cosmos

Plant cosmos once the risk of frost has passed and soil and air temperatures are consistently warm, says flower farmer Julia Keel. "Here in north Florida, this is around mid-March, but in colder climates this will be much later," she says, warning that cold temperatures can negatively affect plant growth. To know exactly when to plant cosmos, consult the USDA gardening zones map, which will tell you when the last frost in your area is.

How to Plant Cosmos

You can start cosmos indoors five to seven weeks before your last spring frost, or plant them directly in the ground. Plant cosmos in an area with well-draining soil that receives full sun.

1. Scatter seeds over the soil.
2. Lightly rake the seeds into the soil so they are about 1/16 to 1/8 inch deep.
3. Sprinkle a light layer of soil and vermiculite over the top of the seeds.
4. Gently, but firmly, press the seeds into the soil to ensure good contact.
5. Water the soil deeply, keeping it consistently moist, but not soggy.
6. Thin the seedlings out so they're 12 to 24 inches apart once they germinate.

Types of Cosmos

Some gardeners love to plant a variety of cosmos, resulting in a lovely blend of colors, heights, and styles. "It is important to read the descriptions of the cultivars you are choosing carefully to make sure that the plant height is a good fit for your intended use," says Keel.

Explore the options at your local garden center or in your favorite seed catalogs to find cultivars that suit your preferences. These are a few of the most popular species among home gardeners.

• Cosmos sulphereus: These flowers are typically yellow and are native to meadows and grasslands. Flowers from this species grow to be 4 to 7 feet tall.
• Cosmos bipinnatus: A mix of purple, pink, and white flowers, the blooms of C. bipinnatus range from 1 to 6 feet in height.
• Cosmos atrosanguineus: Also known as chocolate cosmos, this species features purple-brown flowers with a chocolatey fragrance.

How to Care for Cosmos

Cosmos aren't difficult to grow, but there are some things to keep in mind to ensure optimal results. 

Light

Cosmos prefer full sun and should be planted in an area that receives at least six hours of sunlight daily, says Keel. Avoid sowing cosmos flower seeds in an area covered by buildings, trees, or fences.

Soil

Cosmos aren't picky about soil as long as it's well-draining. If your soil is very sandy, it will need to be amended, says Keel. "On our farm, we incorporate compost into our sandy soil prior to planting," she adds.

Water

Keep the soil moist, but not soggy, watering just before the soil dries out. Cosmos don't need excessive watering, and are somewhat drought-tolerant once established, says Joray.

Temperature

Cosmos prefer moderate summer temperatures. "Cosmos need a warm environment to germinate, and will germinate fastest when the soil temperature is between 70 and 72 degrees Fahrenheit," says Joray. "Since they will die in a frost, we start them indoors about five to seven weeks before planting them outside, and only plant them outdoors once the soil temperatures have reached 60 degrees Fahrenheit, similar to basil and tomatoes." 

Fertilizer 

Cosmos are generally low-maintenance and usually won't require fertilization. "Oversupply of nitrogen can result in tall, spindly growth and reduced flowering," says Keel. 

How to Prune Cosmos

Pruning cosmos during the growing season can prevent them from becoming unwieldy. "Cosmos grow very thick stems, and we prefer thinner stems for design work, so we 'pinch' the plants back when they have four to five pairs of leaves or are about 12 inches tall," Joray explains. "We cut above the top pair or two of leaves, which allows the plants to branch above the cut, producing many more stems per plant." Cosmos are very prolific, so you don't need to deadhead individual flowers to encourage more blooms, but you can if you want a cleaner look.

How to Harvest Cosmos

Cosmos look beautiful in flower arrangements, adding a pop of color and whimsy. When harvesting cosmos to display in a vase, Joray recommends cutting single varieties once they are just budded up and showing color. If you're harvesting double varieties, wait until the blooms are about half open.

Tips for Growing Cosmos in Containers

If your outdoor space is limited, you can easily grow cosmos in containers on your porch or balcony. When growing cosmos in containers, choose varieties that are a little shorter so they look more proportionate and won't require staking. Choose a pot with a drainage hole for excess water to escape. Place the container in an area that receives full sun, checking the soil moisture regularly, as container plants dry out faster than plants grown in the ground.

Common Problems 

Cosmos are generally pest- and disease-resistant, so you usually won't find many issues with this flowering plant. That said, there are a few common pitfalls to look out for.

Lack of Support

One of the most common mistakes gardeners make is not giving their cosmos support. "Although they look wild and beautiful with no support, the plants are sometimes so tall and wispy that they do benefit from a light corralling or at least a wall or fence to lean against," says Joray.

Insects

It's possible to find insects like aphids or spider mites on your cosmos, but it's typically a limited issue. "Occasionally, we will find a Japanese beetle or other insect chomping on their leaves," says Joray. Check with your local extension for guidance on how to treat these pests.

Air Flow

Like many other plants that spread thickly into an area, cosmos can develop powdery mildew if there is too much congestion. "They do like room for air to circulate among their fennel-like leafy fronds," Joray says. Thinning the plants out as seedlings can help increase airflow.