蜜桃恋人

How to Grow Cosmos: The Complete Cosmos Flower Guide

pink cosmos flower with a yellow center
Photo Credit
Pixabay
Botanical Name
Cosmos bipinnatus
Plant Type
Flower
Sun Exposure
Full Sun
Soil pH
Neutral to Slightly Alkaline
Bloom Time
Summer
Fall
Flower Color
Orange
Pink
Purple
Red
White
Special Features
Attracts Butterflies
Attracts Songbirds

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Planting, Growing, and Caring for Cosmos

Catherine Boeckmann

Cosmos are annual flowers with colorful, daisy-like flowers that sit atop long, slender stems. Blooming from summer through fall, they attract birds, bees, and butterflies to your garden. They’re grown easily from seeds and will even survive in poor soil conditions! Here’s how to plant and grow cosmos.

About Cosmos

鈥淎 member of the Asteraceae family and native to Mexico, this beautiful, flowering plant is also known as Mexican aster,鈥 explains the Cosmos will produce 3- to 5-inch daisy-like flowers in various colors, including pink, orange, red, yellow, white, and maroon. Their flowerheads may be bowl鈥 or open-cup-shaped. These beautiful plants can reach 6 feet tall.

Cosmos grow in both beds and containers鈥攁nd they also make excellent cut flowers and are wonderful additions to a cutting garden.

Read Next

Planting

The soil should be well-draining, but other than that, cosmos don鈥檛 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.

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鈥檙e 3 or 4 inches tall.
  • Young plants (transplants) can be planted outdoors once the danger of frost has passed.

How to Plant Cosmos

  • Just sow seeds lightly鈥攏o more than 1/4-inch deep.
  • Thin to 12鈥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

  • To prolong flowering, you should deadhead the plants (prune off dead/faded flowers). This speeds up flower development and aids branching.
  • 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.
  • Water regularly until plants are established or if it is unusually dry. According to the , 鈥淏e cautious not to overwater your plants.  That and over-fertilizing can lead to the plant producing fewer flowers.鈥
  • Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.
  • 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.
Apart from needing at least a half-day of sunshine, cosmos seem to thrive on neglect.
Apart from needing at least a half-day of sunshine, cosmos seem to thrive on neglect.

Types

  • 鈥楶颈肠辞迟别别鈥&苍产蝉辫;丑补蝉 pretty white flowers with a crimson border around the edge of the flower (some are also flecked in crimson).
  • 鈥楽ea Shells鈥 have white, pink, or red tube-shaped petals.
  • 鈥楶耻谤颈迟测鈥 is a beautiful, classic, white flower that looks spectacular growing against the dark-maroon 鈥楴iger鈥 hollyhock or purple basil.
  • 鈥楥osmos Sonata Series’ looks especially good with Bells of Ireland.
  • 鈥楥丑辞肠辞濒补迟别’ cosmos adds distinction to a white campanula, dianthus, or 鈥楳oonshine鈥 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. 

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鈥檚 best to pick some right when the petals have opened.
  • If you cut the blossoms when they first open, they鈥檒l last more than a week in water. Strip off the lower leaves and put them in a vase.
Cosmos is the heart of a summer garden, and it adds a distinct elegance to bouquets in everything from jelly jars to silver urns.
Cosmos is the heart of a summer garden, adding a distinct elegance to bouquets in everything from jelly jars to silver urns.
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About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it鈥檚 not surprising that she and The Old Farmer鈥檚 蜜桃恋人 found each other. She leads digital content for the 蜜桃恋人 website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann
 

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Comments

ronnie (not verified)

8 years 2 months ago

do i have to cut back cosmos flowers at the end of the season ?
i live in new brighton in england ...
thanks for your advice

Hi Ronnie,

As with most perennials, it is a good idea to cut back cosmos. Cutting this season’s foliage off encourages the plant to go dormant, which is a time in its life cycle when it stores energy (carbohydrates) to be used for next year’s growth.

Liz Farber (not verified)

8 years 2 months ago

How do I get orange cosmos seeds? I have only bought cosmos from the store, Alaska Mill & Feed or elsewhere, but have never seen the orange variety. Also want to give a try to harvesting & planting the seeds from my plants & just this the frilly-petaled variety were sold so I bought some (no white with crimson tips however), just pink and white. Cosmos are my favorite and I seem to have a knack for growing them - their stalks can get to be an inch or more thick and I use NSR Greenleaves liquid food products and everything does fantastic! I even had one come up from self-seeding this summer (I didn't realize they do that). I also began to question if these are cosmos as I noted another flower that looks just like the darker crimson one - just can't remember the name.

karla (not verified)

7 years 4 months ago

did u find the orange ones? I live in florida and they grow here

Hi Liz,

The botanical name for the orange variety is Cosmos sulphureus. You can most likely order it through a large catalog seed company.

Cindy Barr (not verified)

8 years 3 months ago

We have several cosmos plants that were not started from seed. I have had success with them in previous years but for some reason this year, the plants have not had any flowers. Several of the plants are close to 6 ft. And we noticed that the leaves at the bottom are turning brown.
Thank you for your help. Hoping you can help us out.

Hi, Cindy, We have had several questions about flowering plants that failed to bloom this summer. In most cases these have been plants, which, like cosmos, asks for no special attention and thrives in average soil. The culprit seems to be the climate: it has simply been too hot for plants to bloom, so they put their energy into foliage or, as you point out, height.

The brown foliage is merely an indication that the plant’s energy is going elsewhere (up!).

Amir (not verified)

8 years 3 months ago

Hi
I live in Tehran, Iran and we have a temperature of 90 to 100 here. In early summer I planted my cosmos seeds in less than 1inch distances and most of them started to grow. I didn't know that they need dry environment and I kept watering them twice a day from then, Also when I wanted to water them again afternoon, the soil had got completely dry because of the hot weather. They are planted besides a wall and have full sun for near 8 hours. Half of them because of the plants beside them had a soil in shade started to bloom while they are less than 8 inch after 6 weeks and the other half that were completely bare against intense sun started to dry and die. Inverse most of your readers my question is why my flowers have started to bloom when they are too small and don't grow?
Thank you

How nice to hear from a gardener in Iran! Could it be that the soil is dry and compacted
so that the plant is stunted but gets enough of what it needs to bloom?
Maybe the roots are looking for deeper, more loose soil so they can spread a bit more?
Or might they be a dwarf variety....

Sandy (not verified)

8 years 4 months ago

I was not able to plant cosmos seed in time this year (spring 2016)given to me by a friend (with a beautiful cosmos garden!). The seed is from her 2015 summer garden in MA. Can I keep them in their envelope and sow next April, 2017?