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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>Rich Jeffreys</span> Mon, 09/01/2014 - 18:04
What climate zone do you live in? It is probably too late to plant seeds unless you live in Zones 10 or 11. Otherwise, plant seeds in Spring after last frost.
<span>Sheepdog</span> Thu, 08/07/2014 - 12:16
Question about color genetics on Cosmos. Do they breed true to color or cross pollinate with other colors? Every year I try to increase the amount of white cosmos, but it seems no matter how careful I am in harvesting and separating out my white seed, some purples come out in the beds of what should all be white seed. And what about the more exotic colors/types, like the physco white, and coral bells, etc? Will those breed true from the seed I save this year?
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 08/08/2014 - 11:10

There are a lot of species of Cosmos. Many of them cross-pollinate to create hybrids. You will have a better chance to get true colors if you start out with heirloom or open pollinated seeds.

 <span>Chloe42</span> Sat, 07/26/2014 - 22:00
I have been growing cosmos for years, and never had a single problem with them, until this year. I always like to collect the seeds and plant them the following year, but this year I found a package of candy stripe cosmos and had to have it. They are growing fine in a planter box, and they flower, but as soon as they flower, the stalk right under the flower starts to die, and then I am left with 2 or 3 inch dead section, with a flower hanging off the end. But the rest of the plant doesn't die, and appears healthy. I am wondering if the package I bought has a disease maybe? Or if it's the soil. I tried a new dirt this year, maybe it's too rich? I Can't recall the name, but it's the kind that expands three times it's size and has 6 months fertilizer in it. The others in the yard, and doing fine. Those are last years seeds. Any help would be great. I do have a picture I can email if anyone wants to see.
<span>Guilfoyle</span> Tue, 07/29/2014 - 13:10
Cosmos are not fussy feeders. They need very little fertiliser and can tolerate dry conditions. See the web page on Cosmos hints on growth.
<a title="View user profile." href="/author/editors">The Editors</a> Tue, 07/29/2014 - 09:56

It might be stem caker or it could be a fungi in the soil or it could be something else. The source of the problem is not always in the place that it manifests/appears. More water is not the answer. Here is an (admittedly detailed) page on a wide variety of plant problems that may give you insight. http://www.ext.colostate.edu/mg/gardennotes/331.html

 <span>Suriji</span> Sun, 07/13/2014 - 21:10
I live in a tropical country so it's sunny all throughout. I just bought cosmos that's about 2months old. About a few weeks or so after, the leaves started drying out until one stalk just died on me. Flowers don't bloom completely and some buds just turned brown. I water them everyday, even put the crystal fertilizer once but it's continuing to die on me.
<span>Tonyconrad</span> Fri, 09/26/2014 - 10:53
These plants can thrive in fairly dry soil. Perhaps you are overwatering them. Also they do not like rich soil so leave out the fertilizer.
<span>Lilia</span> Tue, 07/08/2014 - 13:31
Hi Cosmo world! I live in CT and have have head started cosmo seeds indoors in April. I transplanted them outdoors when they were about 3-4 inches tall. My cosmos have not grown too much taller, they are about 6 inches tall and are already flowering! Are they flowering prematurely? Their flowers are only a half an inch wide and I have only had 1-2 bloom. I dead head them in hopes to get bigger and more blooms! What can I do different? Is it a mistake to have them in the same bed as Zinnias?
<span>Sheepdog</span> Thu, 08/07/2014 - 11:54
Cosmos and zinnias are fine together. I've raised them that way for years. Zinnias bloom first, and then about the time fungus starts to take them, the cosmos take over, soaring above the zinnias making a very nice display.