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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>Jessalmosthahn</span> Thu, 05/28/2015 - 13:02
I want to plant cosmos for the first time this year. The bed is all prepared: weeds are pulled, soil was tilled and raked flat, and it's finally stopped raining enough that my flower bed isn't sticky clay-mud. But it's pretty late in the season - we had our last frost in early May and it's been a cold, wet, rainy mucky month, so I won't have a chance to get these planted until June first. Will they still germinate, grow, and flower in that time?
<span>Joe X</span> Mon, 06/01/2015 - 11:43
It all depends on when you get your first frost. I planted cosmos, cornflower, zenias and forget-me-nots last year in mid-July, zone 5b. I had flowers in 6 weeks or so up until the first frost, November I think. I just sow the seeds on some loose soil, cover them with mulch then keep them watered.
<span>Hmh</span> Sat, 05/30/2015 - 23:44
Yes but it will take about 7-8 wks...I live In zone 4a-b and seed when there's still frost end of April early May as cold helps cosmos to germinate... My plants are only 4 inches and I only seeded a few weeks ago this year but as soon as it gets and stays hot hot they will have a huge growth spurt! last year I seeded eight weeks before the last frost indoors and it was a waste of time I could've sown straight into the ground and the plants end up the exact same size ! Make sure they are planted in almost sand not a rich organic soil - they do much better this way as they are native to Mexico...and plant very thick -4 to 6 inches apart as this supports the tall stems in a fluffy green hedge ....mine were almost 4 feet high last summer and I do it this way every yr. ....think of it as a burst of colour for end of July when everything else is on the wane or looking faded -they will last right up until it freezes and prolifically self seed for the following year. I also let seeds mature on the biggest and best flowers and collect and keep in a ziplock in freezer to supplement all the volunteers in spring. Otherwise they are spindly and fall over. Ignore spacing guides and plant much closer together and u won't be sorry! Hope this helps...I learned the best way how from Mr. Lim who owns a variety store with a 3 foot deep 20 foot long hedge of cosmos every year -they look so striking and fantastic!! Even if u only have 2sq.' U can fit in 18 or so plants!
<span>Crystal</span> Wed, 08/10/2016 - 14:17
Hurray! Thanks for letting us know we don't have to thin our plants...I planted a bunch in a small area, and I can't bear to pull them out....Do I have to stake them? Just would love a riot of cosmos blooming along the fence....:)
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 05/29/2015 - 14:44

You should be able to get some flowers before the end of season if you plant the seeds now. It all depends on when you get your first frost in the fall. Cosmos are very sensitive to the cold.

 <span>Jacqueline Todd</span> Thu, 05/28/2015 - 10:50
Hello, My cosmos have flowered and some of the flower are dying off. At what point do I collect seeds? I'm not sure what the seeds look like? How will I store them? Sorry I'm very new to all this and just love this flower and would love to plant and nurture again next year. Thank you
<span>Bradley Hatchen</span> Sat, 08/29/2015 - 18:59
Hi iwould be happy to answer this question for you after your cosmos finish flowering and fall of the head will be green the seeds will turn brown or black that will tell you that they have ripen on the flower head grab the whole flower head to pull off the seeds you can store in a paper bag next yeat to dry brad hopes this help
<span>Hmh</span> Sat, 05/30/2015 - 23:50
...they are in the center of the flower in a crescent moon shape -as the flower dries they start to separate and you can see them quite clearly about an eighth of an inch long ....let them dry right on the flower but make sure you get them as they start to open up and separate or They'll self seed the petals will have died back at that point... I actually counted over 4000 seeds from a hedge only a foot deep and about 10 feet long ...I store in the freezer until about the end of April here in zone 4a. Takes about 11/2-2 mos. from sowing to flowers next yr...
<a title="View user profile." href="/author/editors">The Editors</a> Fri, 05/29/2015 - 14:41

Hi Jacqueline,

If you just cut off the spent flowers now you'll get many more blooms to enjoy during the summer months. You can cut some of the stems with flowers to put in a vase and this will also encourage the plant to produce more stems and blooms.Toward the end of summer you can leave the flowers on the plant to go to seed. Just leave them on the stems and after a couple of weeks you'll see the spiky seeds where the flowers were. You can store the seeds in an envelope or container in a dry dark place.

 <span>Tinar</span> Wed, 04/22/2015 - 09:45
I used my cosmo seeds from last year which I planted about 6 weeks ago (approximately), & they were all around 1" tall now. Unfortunately my landlord decided to spray for weeds & sprayed my flowers, since it is already the end of April, is it too late to plant them from seeds again in order to enjoy them through the summer or should I have them replace them with flowers & do you think it is safe to plant again since they sprayed weed killer on the soil? (I did the same with my zinnias)