How to Plant and Grow Cosmos

There is hardly any annual flower that is as easy to recognize as cosmos with its saucer-shaped pink, magenta, or white flowers and bright green, feathery foliage. Cosmos is a day-sensitive plant that blooms best in late summer when the days get shorter. 

If you decide to plant cosmos, you are spoiled for choice. There are more than 25 species of cosmos and numerous varieties and cultivars. Cosmos cultivars offer a larger number of flower colors than the cosmos species, and there are even bicolor varieties. The choices don’t end here. Cosmos come with single, semi-double, or double flowers, and in sizes ranging from dwarf to tall.

Where to Plant Cosmos

Plant cosmos in a location with full sun and well-drained soil with a neutral to slightly alkaline pH.

Match the planting location with the cosmos variety. Tall cosmos are best for the back of the garden or a flower bed whereas short, compact types should be planted in the front where they won’t be obstructed by other plants. Or, plant petite varieties in containers for a pretty splash of color on the patio. Cosmos flowers are the most striking when planted in groups or drifts.

Pair cosmos with three or four other easy-to-grow annuals in a cut flower bed and enjoy weeks of freshly cut flowers right from your own garden. Anchor the cutting garden with multiple varieties of cosmos. You may also want to include other annuals like zinnias, sunflowers, larkspur, bells of Ireland, and bachelor's buttons. Because cosmos flowers are so easy to grow, they make a fun choice for a children’s garden.

How and When to Plant Cosmos

Directly sow cosmos seeds in the garden after the last spring frost and when the soil temperature is at least 65 degrees F and ideally between 70 to 75 degrees F. Scatter the seeds on the soil surface and lightly rake them into the soil. The seeds should not be covered with more than 1/8 inch of soil otherwise they won’t germinate. Keep the soil evenly moist. The seeds will germinate in 7 to 14 days.

Alternatively, start the seeds indoors 6 to 8 weeks before your last frost date in spring and transplant the seedlings outdoors after the last frost date.

For spacing, leaving about 1 foot between transplants or thinning the direct-sown seedlings accordingly allows the plants to support one another without being overcrowded.

Cosmos Care Tips

Cosmos is a tough annual plant that is easy to grow and requires minimal care.

Light

A location that receives at least 8 hours of full sun is a must. Too much shade will lead to poor bloom and spindly growth.

Soil and Water

Cosmos grows in a wide range of soil types. Soil with average to poor fertility and a pH between 7.0 and 7.5 is best. Excellent soil drainage is a key requirement.

Keep the soil moist at all times during the seed germination period and water as often as needed. After the seedlings emerge, water them deeply about once a week. Once the plants are established, no watering is required—cosmos thrives in dry soil.

Temperature and Humidity

Cosmos is a warm-climate plant native to Mexico that needs temperatures above 60 degrees F. It is not frost-tolerant. Growth is stunted in temperatures below 53 degrees. High humidity is not an issue.

Fertilizer

Do not fertilize cosmos. This is a plant that thrives in poor soil. Rich soil produces floppy, weak plants, as well as a sparse bloom.

Pruning

Deadheading the spent flowers promotes re-blooming and also cuts down on the self-seeding. If the plants are getting tall and spindly, cutting them back to a height of 12 to 18 inches in midsummer encourages a bushier growth habit.

Potting and Repotting Cosmos

Cosmos makes a great container plant. Choose shorter, compact varieties as the tall varieties are more prone to topple over. Plant cosmos in a pot filled with well-draining potting mix. Make sure the container has large drainage holes.

Like all potted plants, cosmos in containers require more frequent watering than inground plants, as often as daily in hot summer weather.

Repotting won’t be required as cosmos are annuals.

Pests and Problems

Cosmos is rarely affected by serious pests or diseases. The plant may attract aphids. Potential diseases include powdery mildew, gray mold, and stem canker. Proper spacing to ensure good airflow helps to prevent diseases.

How to Propagate Cosmos

Cosmos is propagated from seed and easily reseeds itself. If you want cosmos in a different location, you can also collect the seeds, provided that you are growing a cosmos species, The seeds from a cultivar, on the other hand, won’t necessarily produce a cosmos that is true to the parent plant. If you want exactly the same cosmos, you will need to purchase seeds from a seed company every year. Follow the instructions for planting cosmos above.

Types of Cosmos

'Sonata White' Cosmos

Cosmos bipinnatus 'Sonata White' bears pure-white flowers on sturdy, 18-inch-tall plants.

'Cosmic Yellow' Cosmos

Cosmos sulphureus 'Cosmic Yellow' is a compact selection with double yellow flowers over dark green foliage. It grows 12 inches tall.

'Cosmic Orange' Cosmos

This cultivar of Cosmos sulphureus bears many double orange flowers that are great for cutting. It grows 12 inches tall.

'Sensation' Cosmos

This popular mix of Cosmos bipinnatus bears large, 4-inch-wide flowers in a mix of lavender, pink, red, and white on plants that grow up to 5 feet tall.

'Versailles' Cosmos

The strong stems and large flowers in shades of pink, white, and red make this mix of Cosmos bipinnatus especially good for cutting.

'Candystripe' Cosmos

Cosmos bipinnatus 'Candystripe' has pure white petals edged in deep pink. It reaches 3 to 5 feet tall.

Chocolate Cosmos

Cosmos atrosanguineus ‘Choca Mocha’ is a short, compact cultivar with chocolate-colored flowers and fern-like foliage. The sturdy stems grow up to 12 inches tall.

Cosmos Companion Plants

Celosia

There are few annual flowers as showy as celosia. Whether you plant the plumed type, which produces striking upright spires, or the crested type, which has a fascinating twisted form, you'll love using celosia in bouquets. The flowers are beautiful as fresh flowers, but you can also dry them easily. And they bloom in all the colors of a glowing sunset. Plant established seedlings in spring after all danger of frost has passed. Celosia likes rich, well-drained soil with moderate water. In hot, dry weather spider mites may become a problem.

Dusty Miller

Dusty miller is a favorite annual because it looks good with everything. The silvery-white color is a great foil for any type of garden blossom and the fine-textured foliage creates a beautiful contrast against other plants' green foliage. Dusty miller has also earned its place in the garden because it's delightfully easy to grow, withstanding heat and drought like a champion.

Salvia (Sage)

There are few gardens that don't have at least one salvia growing in them. Whether you have sun or shade, a dry garden or lots of rainfall, there's an annual salvia that you'll find indispensable. All attract hummingbirds, especially the red ones, and are great picks for hot, dry sites where you want tons of color all season. Most salvias don't like cool weather, so plant them outdoors after all danger of frost has passed.