“Cosmos” is the Greek word for harmony or ordered universe and is directly opposite of “chaos,” which is how one might describe the riotous explosion of color these attractive flowers bring to the home landscape.
Spanish priests in Mexico named these cheery little blooms because of their evenly placed, orderly petals.
Indeed, the plant’s pink, red, white, lavender, orange, or chocolate petals are beautifully spaced around a (usually) yellow central disk.
What You’ll Learn
These plants belongs to the huge Asteraceae family, and are cousins of daisy and marigold. Of the 20 or so species of cosmos, two are most commonly grown in home gardens: C. sulphureus (sulphur and yellow cosmos, Mexican aster) and C. bipinnatus (common and garden cosmos and also – because, of course – Mexican aster).
C. sulphureus is native to Mexico, Central America, and northern South America. It has naturalized in most other parts of the world. C. bipinnatus, also native to Mexico, has found its way into most corners of the globe, as well.
These pretty flowers liberally reseed – to the point where they are sometimes referred to as weeds. In the event that you have too many blooms, cut the flowers and put them in vases.
Hot, dry conditions, along with poor to average soil, are optimum conditions for this plant. Usually grown from seed, you may also find starts at a nursery.
In most parts of the United States, this plant is considered an annual, though gardeners in zones 9 and 10 may find them to be perennial.
And with a long growing season, they add vibrant color to the autumn garden.
You’ll find a wide variety of cultivars and mixtures available from seed suppliers. For a brilliant red, orange, and yellow collection of C. sulphureus, try this packet of 500 seeds from Mountain Valley Seed Co., available via True Leaf Market.
These plants will reach about 12 inches tall and produce semi-double, 2- to 3-inch flowers that bloom throughout the season.
Or, choose classic cosmos in a rainbow of colors in this seed mix from David’s Garden Seeds, available via Amazon.
You’ll get 10 open-pollinated varieties in this mix of 500 seeds that will grow from 45 to 54 inches tall.
David’s Garden Seeds also offers a stunning deep red variety, also available on Amazon. ‘Tetra Versailles Red’ flowers open wine-red and gradually fade to a bright pink.
You’ll get 500 non-GMO, hand-packed seeds that produce 2- to 3-foot plants that bloom in about 80 days.
Care and Maintenance
This low-maintenance plant tolerates fairly poor, well-drained soil with a pH between 6.0 and 8.5. You don’t want to fertilize much because you’ll end up with lots of foliage and not many flowers.
Sow seeds outdoors at a depth of 1/16 inch, or indoors five to six weeks before the expected date of last frost.
These 1- to 6-foot-tall plants can become top heavy and floppy, so plant them in masses and they’ll support each other.
These cosmic gems flower best in full sun, but the plant will tolerate a bit of shade. It requires only a moderate amount of water.
Plant tall varieties at the back of borders or as a central focal point, and use smaller varieties as mid-sized plants in mixed beds.
For planters and containers, make sure to use dwarf varieties. Full sized specimens will crowd out the other plants, and their roots will dominate available soil.
Deadhead spent flowers to encourage more blooms, but leave some in place to develop seeds – they’re a favorite of visiting winter birds!
Pests and Diseases
For both of these pests, try insecticidal soap, such as this one from Garden Safe, available via Amazon.
Also look out for aster yellows and powdery mildew.
The only cure for aster yellows is prevention. Use diatomaceous earth or insecticidal soap to kill the insects that spread the bacterial disease. Plants infected with aster yellows must be pulled up and destroyed.
Treat powdery mildew with a fungicide, such as this one from Safer Brand, available via Amazon.
A Little Order, A Lot of Beauty
If your garden universe doesn’t include this worldly annual, perhaps it should. This easy-care plant thrives on sun and heat, offering glorious color in return.
Look for seeds online, plant indoors a few weeks before the last freeze, or outdoors in early spring. Keep diatomaceous earth and insecticidal soap on hand and you’ll be set.
Cosmos Quick Reference Chart
Are cosmos growing in your garden? In the comments section below, tell us about your experience with this galactic favorite. And learn about another Asteraceae family member in this article about Swan River daisy.
Don’t forget to Pin It!
Photos by Lorna Kring, © Ask the Experts, LLC. ALL RIGHTS RESERVED. See our TOS for more details. Product photos via True Leaf Market, David’s Garden Seeds, Safer Brand, and Garden Safe. Uncredited photos: Shutterstock. With additional writing by Lorna Kring.
About Gretchen Heber
A former garden editor for a daily newspaper in Austin, Texas, Gretchen Heber goes through entirely too many pruners and garden gloves in a year’s time. She’s never met a succulent she didn’t like and gets really irritated every 3-4 years when Austin actually has a freeze cold enough to kill them. To Gretchen, nothing is more rewarding than a quick dash to the garden to pluck herbs to season the evening meal. And it’s definitely time for a happy dance when she’s able to beat the squirrels to the peaches, figs, or loquats.