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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>Eva Emanuel-Keeling</span> Tue, 11/05/2013 - 11:21
I thought I remembered another name for the Cosmos flower. I seem to remember it as ' ? in the mist' Have you a clue?
<span>Amos Spitalhatch</span> Sun, 02/09/2014 - 23:53
Could you be thinking of 'love in a mist'? It also has fine fern-like foliage but not actually a cosmos - a nigella. Although there are white and pink varieties (I believe) the traditional love in a mist are more commonly blue.
<span>Betty Hendrickson</span> Thu, 09/26/2013 - 19:44
First of all, I live in Calgary, Canada where the growing season is short, but it gets to between 25 and 30 degrees Celsius in the summer I grew 60 cosmos plants this year from seed, as it takes too long for them to come up before the frost hits. I had wonderful success with them, some getting over 5 feet tall and full of blossoms. Some were planted directly in the beds and others in pots. Some had full sun and others had partial sun. Everything grew beautifully. I put compost in the soil at initial planting and then only fertilized once with a fertilizer for flower plants. Everything was beautiful until the city got an aphid infestation. I sprayed every week with Safer's Insecticidal soap and that helped a lot. After a while I got tired of spraying, so did a lot of clipping off of branches that had aphids. This an spraying seemed to bring the aphids under control. I water fairly well every day, before they droop, and always in the morning or before supper, and from the top of the plant. I have not had any problem at all with mildew (which I do get on other plants). Mildew comes when the water on the plant's leaves don't get a chance to evaporate, and they stay damp, especially if the temperature dips at night. I will be very sorry to see the end of the season arrive as my yard is full of these beautiful, unkempt looking flowers. When branches have broken I have taken them inside and within a few days any buds have opened and they continue to blossom beautifully.
<span>Bill Mc</span> Sat, 02/08/2014 - 17:06
Aphids can be cleaned up really quickly with ladybugs. They are a natural predator for aphids. They can clean up an infested tree, rose bush, or garden plant in just a few days. You order them, then keep in frig until needed. The challenge is keeping them in your yard. They will leave, eventually, but their offspring, in various stages of development, continue to feed on aphids.
<span>Brayden</span> Fri, 02/07/2014 - 16:08
how long does it take for the cosmos to grow from a seed to a flower?
<span>Tony Conrad</span> Fri, 04/10/2015 - 12:14
I would say six weeks but that is in the UK. Not sure about other climates.
<span>Clair Jones</span> Sun, 09/08/2013 - 21:05
we had 3 cosmo plants that got big enough to do something this year. it was our first time with them and im sure we watered them too much. anyway one flowered several times and died but the remaining two are now like 3 feet tall, very bushy, but not a flower in sight. not even any little buds anywhere on the cosmo trees as i now call them. whats going on with them?
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 09/09/2013 - 08:21

Hi, Clair, Cosmos hate TLC. Treat them poorly! That means almost no watering (once a week) and NO fertilizer or compost and not much care.
Blooming is reduced if overwatered or managed. Soil that is too nutrient rich produces plants that are weak-stemmed and have sparse to no flowers.
Do make sure your soil drains well but, otherwise, soil with average to poor fertility is fine. Cosmos PREFER pool soil so not much soil prep is needed. Plant in a drier area of the garden; they are from Mexico so think of their native climate.

 <span>Kathie Clohessy</span> Mon, 08/26/2013 - 10:04
I live in SE Wisconsin and I have four orange cosmos plants in a big pot that are growing very tall and leggy. There are tons of seed pods everywhere, but fewer and fewer flowers. I thought cutting them back a bit would help, but I don't think it did. Some of the stems I cut back just turned brown and died, while others sprouted new flowers down below. I guess my question is: what is the best way to prune these guys? Is there a rule of thumb (i.e. with roses, you always look for three leaves) or should I just remove the dead flowers and let nature take its course?
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 08/27/2013 - 17:14

(Amended answer) I think you need to wait until the flower dies and the seed head starts forming on the plant before bringing it indoors. If you cut the flowers when blooming and bring them inside they will not form seed heads. Tying a paper bag over the seed heads in the garden is a good idea.

It sounds as if your cosmos grew quite tall; next year, try pinching them back when they're young and they will stay more compact and bushy.