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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>Donna Morse</span> Fri, 02/09/2018 - 18:51
Next to wild white Daisies with the yellow eye, Cosmos are a favorite. I too have a sandwich bag of Cosmo seeds. I am excited to get winter over so i can get the seeds growing. Thank you for the information. My question is," can i mix wild daisies in the same garden with the Cosmos?" Thanks again.
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 05/16/2018 - 14:02

This advice is too late for you, but hope it helps others. We'd sow our cosmos seeds in March or April. Harden off the seedlings before planting out in a sunny spot. Many flowers mix well with cosmos. We haven't tried daisies but I'm sure they'd grow. It depends on the color of the cosmos. Against a white daisy, we probably would not pick a white cosmos; it would be better to have a deep magenta cosmos.  However, it's a personal preference!

 <span>John</span> Sun, 02/04/2018 - 16:16
i have a sandwich bag full cosmo seeds. plz sumone tell me how many seeds i need to plant per plant.
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 02/06/2018 - 09:07

Hi John,

Plant Cosmo seeds 2 feet apart to give them adequate space. This distance also gives them the benefit of leaning on each other when they are mature.

 <span>Pat Serio</span> Mon, 10/23/2017 - 13:38
Should the Cosmos be cut back? pulled out? left long/leggy? or what? Thanks. BTW - mine grew to at least 8' tall - Monarch, Painted Lady, and Skipper butterflies, and bees LOVE the flowers!!
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 10/24/2017 - 14:40

Cosmos is an annual, so it will not continue to grow noticably (and will die) this growing season. However, it sometimes self seeds, so you may get more of the same...which sound spectacular! Kudos on the cosmos, Pat!

 <span>Danna</span> Thu, 08/10/2017 - 14:58
My grandmother is 90 and has dementia. I help take care of her and while going thru some old junk drawers I came across a package of cosmos seeds. I don't know how old they are, when she bought them, or if they are still any good or not but in the article it says they can be "self-sowing" so if this next spring I were to just take them out and scatter them in a bare place in the yard and water it occasionally, when I do the rose bushes say, do you think they might grow? Also she is wanting to plant something in her hanging baskets on the back porch is there anything we can plant now, in the mid-summer in zone 6 without going to the garden center and spending a lot on already half grown flowers? Thanks for any help!
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 08/11/2017 - 15:20

Cosmos seeds may last between three and four years in a cool, dry environment, so your grandmother's seeds may still be viable. It's worth giving them a try, in any case! Maybe buy a supplemental packet, too, if you can.

As for your hanging baskets: It's a tad late to be starting most flowers from seed, unfortunately, however you do have a couple options. Pansies are probably your best bet if you want to start from seed. You may be able to find a few cheap impatiens, portulaca, and petunia transplants on sale at this time, too.

 <span>Kitty</span> Wed, 08/09/2017 - 12:21
I live in southern NH and was wondering if I scattered Cosmo seeds in my wildflower field in September would they give me flowers next spring? Any special variety? TX
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 08/09/2017 - 16:35

Hi, Kitty. Cosmos are best planted in the spring and need 7 weeks to grow, and will not survive the first fall frost.