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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>Shirin</span> Sun, 06/25/2017 - 21:12
I live in the SF bay area (zone 9b) and planted a few cosmos seeds (Sensation blend) at the end of April and then again in early May this year. It's the end of June now - about two months later - and each plant is just 4 - 5 inches tall. The plants are in almost full sun - they get several hours of sun per day. They look healthy and green but aren't growing much. I'm wondering what could be the problem...I was hoping to have at least some flowers by now. Anyone have any ideas on what might be going on?
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 06/26/2017 - 12:46

Cosmos likes at least 6 hours of sunlight each day. It may take about 85 days for the Sensation variety to reach maturity, so it could be that it just needs more time. Be sure that the soil is well-drained. Also be sure that the plants are thinned to the proper spacing and that there are no weeds nearby that are competing for nutrients, space, and light.

 <span>Annie</span> Tue, 03/21/2017 - 11:54
I was stunned that my pictoee cosmos only took 4 days to sprout from seed as the package said 7 days minimum. That's fast! I started them indoors at the same time as about four dozen other flower and vegetable species, and they're the only ones to break the surface so far. I used soil-less substrate in paper pots, kept just moist enough, and they are under full spectrum artificial lights for several hours a day.
<span>Gb</span> Sun, 02/12/2017 - 13:59
Hi, I have started a few Sensation Mix cosmos from seeds. I would like to know what would be an ideal container size for transplanting these.
<a title="View user profile." href="/author/editors">The Editors</a> Mon, 02/13/2017 - 14:09

If you are growing one plant in each pot they don't need to be too big. A 6-inch pot will do great. Good luck!

 <span>Johanna</span> Sun, 01/22/2017 - 11:51
Hi i am writing to you from the northern cape province in south africa. I am a new beginer to gardening. I just want to thank-you for your wonderful web site. wish I had found it sooner. After much surfing I have at last found an easy to read, to the point,detailed information site.Thank you,thank you. Sincerely, Jo.
<span>Bob</span> Thu, 12/29/2016 - 17:11
Does it take two to three weeks for the cosmos flowers to budd because I read this and I really need them to for my project! Thanks
<a title="View user profile." href="/author/editors">The Editors</a> Wed, 01/04/2017 - 18:45

If you are growing cosmos from seeds, it takes just 5 to 7 days to germinate but about 4 to 6 weeks to first bloom.

 <span>Hayley</span> Fri, 09/16/2016 - 10:31
ok so im doing a science project and i need to know what type of cosmo grows the fastest from seed?
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 09/16/2016 - 13:41

Cosmos will bloom in approximately 50 to 55 days after germination.  So not sure the variety is that much of a big deal. If you wish, go to the site of a retail seed seller (Burpee, Johnny Seeds, American Meadows, etc.) and give them a ring. Keep in mind that growth is also affected by weather, soil temperature, and germination speed.