How to Grow Cosmos

Cosmos Flowers (Cosmos bipinnatus)
Cosmos is a great cottage garden flowers. It's airy, a profuse bloomer and it self-sows. ooyoo / Getty Images

Cosmos are freely flowering annual plants that are ridiculously easy to grow. If you're looking for a flower that will stay in bloom for months and can be grown by simply scattering seeds, cosmos are a great choice.

The flowers sit atop long slender stems and form a cloud of color that not only looks attractive throughout the summer but also attract bees, butterflies, and birds to your garden. Cosmos grow as easily in beds as they do in containers and they also make great cut flowers.

Cosmos can handle drought, poor soil conditions.and general neglect. They even self-sow, but not to the point of becoming a nuisance. This is a truly low maintenance plant

  • Flowers: Cosmos flowers are daisy-like with ray florets surrounding the center disc of florets in a shallow cup. There is a broad range of color - more every year - including white, pink, magenta, orange, yellow, reds and chocolate.
  • Leaves: The leaves grow opposite and are either deeply lobed, pinnate, or bipinnate and feathery, depending on the variety. Since few pests bother cosmos, the plants look good all season.

Botanical Name

Botanical names are important when you want to be certain you are growing the exact plant you are after. There are 2 types of cosmos typically grown in gardens.

  • Cosmos sulphureus is a North American native with golden yellow blooms. It is very drought tolerant and loves hot weather. They grow 1 - 2 feet tall and come in double and semi-double flowers, however, some of the more recent cultivars tend to be shorter, more orangy and with smaller flowers.
  • Cosmos bipinnatus are the colorful daisy-like flowers that come in white, pinks, reds and orange. There are quite a few series of hybrids and although they are not quite as heat tolerant as Cosmos sulphureus, they will grow well just about anywhere.

Common Names

There are many common names, including Cosmos, Mexican Aster, and Cut Leaf Cosmos

Using Cosmos in Your Garden Design

Cosmos are quintessential cottage garden flowers and mix well with just about everything. The taller varieties look good in the middle or rear of the border, with spiky flowers, like Agastache and Goat's Beard, and with rough-textured flowers such as coneflowers and black-eyed Susans.

Shorter varieties make very colorful, airy edging plants. Cosmos also grow well in containers and make good cut flowers.

Cosmos Growing Tips

Soil: A neutral soil pH of 6.0 to 6.8. is best, but cosmos don't really care. They will grow in poor soil, although adding some organic matter will give them the well-draining soil they prefer. Don't make the soil too rich, or you'll get thin, scraggly stems and fewer blooms.

Sowing: You can start seed indoors, 4 - 6 weeks before your last frost, but cosmos are quick to start growing if you direct seed them outdoors and will catch up to those started earlier. Cosmos are also widely available as seedlings if you don't want to or forgot to plant the seed.

Planting: Wait until all danger of frost has passed, before sowing or transplanting outdoors.They grow very quickly but can be killed by a late frost, so don't rush it.

Seed packets usually recommend something absurd, like 2 ft. spacing, but you will get a better display if you simply scatter the seed and let the plants support each other, as they grow. You can always thin if you need to, and move the extra plants to another part of the garden.

Sun Exposure

For the best flowering, choose a site that gets full sun. Cosmos will grow in partial shade, but along with fewer blooms and less vigorous plants.


There are cosmos species that are perennials, but the common garden cosmos are grown as annuals. Plants may reseed and it is also very easy to collect the dried seed at the end of the season, to save for next year. However, if they are hybrids, you never know what you may get.

Days to Maturity

Blooming will depend on variety, but most cosmos plants start setting buds within 50 - 55 days. That's why they are perfect for direct sowing, although starting them indoors earlier will speed flowering by a couple of weeks.

Mature Plant Size

  • Height: 2 - 5 ft.
  • Spread: 1 - 2 ft.

Caring for Your Cosmos Plants

Very little maintenance is required. You should not need to water your cosmos plants unless there is a prolonged drought, and even then, the water would be better used elsewhere.

Unless your plants seem to be struggling, there is no need for fertilizer. Cosmos can handle poor soil and they only need to survive one season.

Tall cosmos varieties can flop over. If you are not growing them closely or near other plants that can support them, you may need some type of staking.

The only real maintenance cosmos plants need is deadheading. This will prolong the flowering season. If you fall behind, simply shear the plants by about 1/3, when most of the flowers have faded. You'll get a second flush of leaves and flowers.

Some Great Varieties of Cosmos to Grow

  • Bright Lights Mix - a blend of exuberant yellows, oranges, and reds.
  • "Cosmic Orange" - a brilliant, clear orange flower with great drought tolerance.
  • "Peppermint Candy "- an award-winning variety with petals splashed in magenta and white.
  • Seashells Series - comes in a pretty mix of colors or individually, with distinctive tubular petals.
  • "Chocolate" Cosmos (Cosmos atrosanguineus) - the dark red flowers do indeed smell like chocolate. This is a perennial that is hardy to USDA Zone 7, but it is much more finicky than the annual cosmos.

Pests and Problems of Cosmos

These tough cookies are virtually pest free. Even the 4-footed pests don't bother them.