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Planting and Growing Cosmos Flowers

pink cosmos flower with a yellow center
Image Credit:
Pixabay
Botanical Name
Cosmos bipinnatus
Grown as
Annual
Toxicity
Non-toxic
Sun Exposure
Full Sun
Water Needs
Low
Height
1–3 feet
Spread
1–3 feet
Soil pH
Neutral
Bloom Time
Summer
Fall
Flower Color
Orange
Pink
Purple
Red
White
Special Features
Container-Friendly
Cut Flower
Long Blooming
Low Maintenance
Attracts Butterflies
Attracts Hummingbirds
Attracts Pollinators
Attracts Songbirds
Deer-resistant
Rabbit-resistant
Resilience
Drought-Tolerant
Heat-Tolerant
Plant Type
Flowers
Written By: Catherine Boeckmann Executive Digital Editor and Master Gardener

Cosmos are the carefree superstars of the summer garden! With their daisy-like blooms and whimsical, wiry stems, they bring color, charm, and pollinators to even the poorest soils. 

Whether you’re sowing a cottage garden or filling a cutting bed, cosmos bloom with ease and attitude. Here’s how to grow them like a pro!

“A member of the Asteraceae family and native to Mexico, this beautiful, flowering plant is also known as Mexican aster,” explains Oregon State University. Cosmos will produce 3- to 5-inch daisy-like flowers in various colors, including pink, orange, red, yellow, white, and maroon. Their saucer- or cup-shaped flower heads sit atop stems that grow up to 4 feet tall.

Buy Seeds Now

Cosmos grow in both beds and containers—and they also make excellent cut flowers and are wonderful additions to a cutting garden.

Planting

When to Plant Cosmos

  • Direct-sow seeds outdoors once the danger of frost has passed.
  • Alternatively, sow seeds indoors 4 to 6 weeks before your last spring frost date in trays or pots with a good seed-starting mixture. Move them into 5-inch pots as soon as they’re 3 or 4 inches tall.
  • Young plants (transplants) can be planted outdoors once the danger of frost has passed.

Where to Plant Cosmos

The soil should be well-draining, but other than that, cosmos don’t need any special soil preparation. In fact, they like soil that is not too rich, as rich soil will encourage foliage at the expense of blooms. Cosmos are also tolerant of most soil pH levels but grow best in neutral to alkaline soils (pH of 7.0-7.5).

Cosmos can tolerate warm, dry weather very well. They are even drought-tolerant.

How to Plant Cosmos

  • Just sow seeds lightly—no more than 1/4-inch deep.
  • Thin to 12–18 inches apart when seedlings are a few inches tall. Depending on the variety, cosmos can grow anywhere from 18 to 60 inches tall, so plan accordingly.
  • If you are growing cosmos from seeds, be mindful that it takes about 7 weeks to first bloom. After that, your flowers should continue to bloom until the first fall frost.
  • If you let the spiky-brown seed heads blow away during the fall, cosmos are likely to self-sow throughout your garden.
a field of cosmos flowers
Let the spiky-brown seed heads blow where they will, and you may find your garden full of self-sown cosmos flowers.

Growing

Watering

Water regularly until plants are established or if it is unusually dry. According to the North Carolina Extension, “Be cautious not to overwater your plants. That and over-fertilizing can lead to the plant producing fewer flowers.”

Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.

Deadheading

To prolong flowering, you should deadhead the plants (prune off dead/faded flowers). This speeds up flower development and aids branching.

Supporting Cosmos

Because some of these plants can grow really tall, staking may be necessary. Offer them protection from strong winds, encourage balanced branching by pinching out (aka pruning) central shoots or stem tips, or stake them. Growing them against a fence can also provide the necessary support.

Seasonal Care

Cosmos beds may become weedy due to the fact that they self-seed, so remember to remove flowers before they go to seed or to thin seedlings in the spring.

orange cosmos flowers
Apart from needing at least a half-day of sunshine, cosmos seem to thrive on neglect.

Harvesting

Harvesting Seeds and Cut Flowers

  • To harvest more seeds, remember to leave a few flowers on the plant because they will self-seed.
  • You can cut the flowers off anytime after blooming, but it’s best to pick some right when the petals have opened.
  • If you cut the blossoms when they first open, they’ll last more than a week in water. Strip off the lower leaves and put them in a vase.
pink cosmos flowers with a view of the sky
Cosmos is the heart of a summer garden, adding a distinct elegance to bouquets in everything from jelly jars to silver urns.

Pests and Problems

Types

  • ‘Picotee’ has pretty white flowers with a crimson border around the edge of the flower (some are also flecked in crimson).
  • ‘Sea Shells’ have white, pink, or red tube-shaped petals.
  • ‘Purity’ is a beautiful, classic, white flower that looks spectacular growing against the dark-maroon ‘Niger’ hollyhock or purple basil.
  • ‘Cosmos Sonata Series’ looks especially good with Bells of Ireland.
  • ‘Chocolate’ cosmos adds distinction to a white campanula, dianthus, or ‘Moonshine’ achillea.
  • Tall cosmos are best for the back of the garden and filling in mixed borders among perennials such as foxgloves, phlox, delphiniums, and daisies, as well as annuals such as poppies, cleome, and asters.
  • Dwarf cosmos, such as the Ladybird series, are great companions for zinnias, Johnny jump-ups, pansies, alchemilla, or dark-blue morning glory. 

Wit and Wisdom

  • The genus name was derived from the Greek word kosmos, which means order or orderly arrangement.

  • Cosmos is one of the October birth flowers.

Flowers

About The Author

Catherine Boeckmann

Catherine Boeckmann

Executive Digital Editor and Master Gardener

Catherine Boeckmann loves nature, stargazing, and gardening, so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website and is also a certified master gardener in the state of Indiana.

Read more from Catherine Boeckmann

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Comments

<span>Vicky72</span> Thu, 08/20/2015 - 16:10
Thank you that does help. I am just impatient for them to flower. They do look lovely and healthy though.
<span>Meghan Bortz</span> Tue, 07/28/2015 - 17:01
I have a Cosmo that I planted at the end of May, not from seed though. It continues to grow in height, but there still is no sign of flowers getting ready to bloom. Why?
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 07/29/2015 - 11:21

Cosmos usually take a long time to grow the stem and leaves before the buds show. You should see some flowers soon. Remember to dead head the faded flowers to have more buds and flowers through the end of summer.

 <span>Sellis</span> Mon, 07/27/2015 - 10:22
I have cosmos in containers on the deck & while the plants are growing well, the original hot pink coloris all washed out. They are a pinkish gray or dull looking pink. I bought these as sedlings at a nursery in a 4-pak. What blooms they had at the time were very vibrant. Is it not enough sun? Most of the sun is indirect, with the strongest & most direct sun after 2 p.m.
<span>Gumtree</span> Sat, 07/04/2015 - 03:22

The flowers on my cosmos are getting smaller. What am I doing wrong? Thank you!

<a title="View user profile." href="/author/editors">The Editors</a> Thu, 07/09/2015 - 15:45

Cosmost likes full sun. It's daylight sensitive, so if it's not getting enough sun, it may be failing in health. Are you deadheading? Removing spent blooms encourages the plant to put its energy into new flowers, not seeds in old ones.

 

 <span>Debbie435</span> Sun, 06/21/2015 - 15:37
I'm getting ready to plant beginners that are currently in Dixie cups. I'm in eastern Nebraska. The area I will be planting in is mostly shade. Will they grow right or do they need mostly sun.
<span>Gennell</span> Fri, 07/03/2015 - 11:33
I live in Maryland. Last year I put half my seeds in full sun and half in mostly shade. I got tremendous results from the full sun. The mostly shade grew a lot slower, were much smaller, and had a lot fewer flowers. - Good luck!
<span>Hollyann</span> Mon, 06/01/2015 - 18:51
is it advisable to cut the cosmos in half when still growing to promote stronger thicker stems
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 06/03/2015 - 16:05

We would not recommend it. Cosmos do not ask for much: they thrive in fairly poor soil, just make sure the soil drains well (and the plants are not in puddles). Too-rich soil may result in thin-stemmed, weak plants; cutting them will only hasten their demise. Stake tall stems again heavy rain or wind, when forecast. Or plant the flowers closer together than seed packages or plant labels advise so they can support each other. Pinching off flowers encourages blooms. Pinching stem tips can reduce height and encourage branches to grow but it is not necessary.