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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>Amos Spitalhatch</span> Mon, 02/10/2014 - 00:53
We live in New Zealand and must be doing something wrong. My wife is in charge of the cosmos, which she grows from seedlings purchased locally, although there are also some self-sown plants here and there as well. I planted the seedlings about a foot to 18 inches apart; my wife scattered all-purpose fertiliser pellets among them. The cosmos formed dense bushes about 6 feet tall with dozens of flowers. They are watered daily at ground level and are thriving. According to the postings here, we are doing it all wrong, but we've actually had passers-by call in to express their admiration of the plants! We've been growing them for about ten years now and have always been very successful with them - but will now bear in mind that we should cut back on the watering if they do start to look sickly. They are not dead-headed - the plants are grown specifically to attract goldfinches who love the seeds. There are not many butterflies in this country, but the few we do have like the flowers as well.
<span>Raewyn J</span> Sun, 03/30/2014 - 04:39
Yes, me too. I live in New Zealand and my cosmos have reached great heights with a few with flowers reaching nearly 7 feet tall and others around 6 feet an 4 feet wide. Like you, I don't deadhead, and as I'm an amateur gardener who knew nothing about cosmos, was delighted when babies started shooting up, so am hoping to grow some more large ones. Did any of your second-generation not grow tall? I'm thinking it was because I left them to grow naturally and did not give them the TLC I gave the others, so am doing that with the new ones coming on. Now I have so many I don't know what to do with them! However, am loathe to get rid of them as I love them so much. Do they still grow during winter? What happens to those who are still large?
<span>Amos Spitalhatch</span> Sun, 03/30/2014 - 05:43
Good to hear of your experience with cosmos, Raewyn. We too have little chaps sprouting up all over the place - and they don't seem to get any higher than about 6 inches. The sad part is that they all die back as soon as there's a frost and in no time at all, there's just a dried up white skeleton. The babies come to nothing despite their healthy beginnings. There's always next year and a plentiful supply of seedlings at the plant shop - and we always seem to end up with the taller varieties!
<span>Tonyconrad</span> Fri, 09/26/2014 - 11:03
I'm in the UK and bought a packet of wild flower seeds. The only thing that relly stood out were the wonderful tall light purple cosmos. I am looking forward to next year.Yes they only flower until the frost so maybe they keep going in hotter countries?
<span>Allie Zem</span> Fri, 10/04/2013 - 21:50
The cemetery where my father-in-law is buried gave us some wildflower seeds they said were perennials to plant as a memorial. They turned out to be cosmos which grew into a very tall (over 6' & still growing), spidery bush with long arms and gorgeous hot pink flowers. I'd never seen anything like it and absolutely love it! I'm very disappointed to read here that it's an annual! Any chance it could be a perennial?
<span>Jruth</span> Fri, 05/09/2014 - 14:37
There may be a very good chance the Cosmos can be a perennial. I live in a colder climate and get Cosmos back every year. Just let the flowers go to seed and drop in the fall.
<span>Jruth</span> Fri, 05/09/2014 - 14:45
I meant that let the seed heads drop in the fall!
<span>Tonyconrad</span> Fri, 09/26/2014 - 11:08
Doesn't that mean they are annual if it's the seed that keeps them going. Perennial means they grow again from the root doesn't it?
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 09/26/2014 - 15:31
  • Most of the Cosmos varieties are annuals and are started from seed in spring. There are a few perennial varieties of cosmos that have rhizomes and can be propagated by division.
<a title="View user profile." href="/author/editors">The Editors</a> Sat, 10/05/2013 - 03:24

Cosmos are an annual.  There are also perennial species in cultivation, but it wouldn't be grown from seed. Cosmos comes from the Greek word kosmos, meaning “beautiful" and is a member of the aster family. Though it's an annual, cosmos has been known to self-seed and bloom again. If you do not deadheaded (remove the spent flowers), it will self-sow in warm regions.