How to Grow and Care for Cosmos Flowers

When I create my summer garden, I like to add elements that make it feel like a peaceful oasis and that are very low maintenance.

Cosmos flowers fit both of these requirements perfectly – they don’t require much care, but wow, do they ever make the garden feel like a haven of life and beauty!

In fact, one of my favorite ways to relax on summer days is to watch bees, butterflies, and even hummingbirds foraging from the showy blooms on my cosmos plants.

Sweetening the deal, these gorgeous flowers produce loads of blooms from early summer all the way up until the first fall frost.

In this article you’ll learn everything you’ll need to know to grow these lovely flowers – so that, if you choose, they can become one of your summer highlights too.

Here’s what we’ll cover:

What Are Cosmos?

Cosmos are herbaceous plants cultivated for their gorgeous flowers. They’re most often grown as summer annuals.

These flowers are available in a wide assortment of cultivars comprising many different colors and featuring different types of blooms.

Blooms are held on tall, branching stems, while flowers have petals in shades of pink, purple, white, lavender, red, orange, or yellow, as well as bicolors, which are beautifully spaced around a central disk.

Of the 20 or so species in the Cosmos genus, two of these are commonly grown in home gardens.

C. bipinnatus, also known as “garden cosmos,” is most often sold in a mix of white, pink, purple, and burgundy. The foliage is delicate and wispy, and bloom colors tend toward cool shades.

C. sulphureus, also known as “sulfur cosmos,” comes in warm shades of yellow, orange, and red and has foliage that you might mistake for marigold or chrysanthemum leaves.

A third species that is somewhat less common in the garden landscape is C. atrosanguineus, also known as “chocolate cosmos.”

You’ll learn more about these species and their cultivars – as well as some tips on where to purchase them – later in this article, so keep reading!

Cultivation and History

Plants in the Cosmos genus are native to the Americas, ranging from Washington State in the Pacific Northwest of the United States all the way to South America.

However, the three species most commonly cultivated in gardens are all native to Central America.

These plants belong to the huge Asteraceae family, and are related to other garden beauties such as coneflowers, dahlias, and marigolds.

The genus Cosmos takes its name from the Greek word “kosmos,” a word with many different translations, ranging from “adornment” to “universe.” In the case of this flower, the word is best translated as something akin to “beauty.”

Although they are most commonly referred to by their genus name, these flowers are also known by the common name “Mexican asters.”

In most parts of the United States, Mexican asters are considered annuals, though gardeners in USDA Hardiness Zones 9 and 10 can treat them as perennials.

Propagation

You can propagate garden and sulfur cosmos by sowing them directly into the soil, or by starting them indoors and transplanting out the seedlings. For chocolate cosmos, the most reliable propagation method is to divide tubers.

Outdoors from Seed

Garden and sulfur cosmos are extremely easy to grow from seed, so if you are a beginning gardener (or just revisiting some of the basics) these are excellent flowers to start with.

In fact, these seeds germinate so easily that they tend to self-sow, growing back from seed year after year, and they are known for becoming a bit weedy.

(However, I must point out that a plant’s status as a “weed” is entirely up to one’s point of view. I look at cosmos volunteers not as “weeds” but as welcome “bonus” blooms.)

Does it sound like I’m digressing? I’m trying to make a point about growing these flowers from seed, and you’ll see where I’m going in just a moment, so bear with me…

Before you start sowing, you’ll want to know the best time to plant these seeds.

And now we’re coming to the point. Every other source you consult will most likely recommend waiting to sow cosmos seeds until after the last average spring frost for your location.

However, I’m not one to follow horticultural advice to the letter while ignoring what’s transpiring in my own garden, or the veggie plots and flower beds of many other gardeners.

The number of volunteer cosmos seedlings that sprout before the last spring frost every year and survive to brandish their glory throughout the summer are proof that waiting may not be necessary.

Taking a cue from what was happening among my volunteers, lately I have started sowing these seeds in early spring along with my cool-weather crops, just to see what would happen.

As I suspected – and much to my delight! – my early-spring-sown seedlings survive light frosts, providing me with early summer blooms, a valuable asset in my region where the growing season is short.

I’ll admit, though, that I consider this approach to sowing warm-season flowers to be an experiment.

So if you want to play it safe, wait until one to two weeks after your last average frost date when the temperature of your soil has reached at least 60 or 70°F.

If you’re willing to take a risk, try planting some of these at the same time as your spring garden seeds. You can always use floating row covers to protect the seedlings if your weather forecast predicts a hard freeze.

When you’re ready to sow, prepare your beds by mixing in some compost, push aside a little soil to use for covering the seeds, then smooth the soil down so it’s level.

Water the soil in with a gardening wand so the soil surface is moist.

Place a group of three seeds every nine to 12 inches, then cover them very lightly with soil. The seeds should only be covered by about 1/16th of an inch.

Expect seedlings to emerge in five to 21 days after sowing.

While the seeds are germinating, keep the soil moist, watering daily if needed.

Taper off the frequency of your watering as the seedlings that sprout become established. You’ll learn more about how often to water established plants later in this article.

When the seedlings are a couple of inches tall, thin them so you have only one every nine to 12 inches.

Those of us in colder zones where the ground stays frozen throughout winter might also try pre-seeding our gardens with cosmos seeds in the fall or sowing them in the winter so they come up the following spring.

Both of these methods will expose young seedlings to early spring frosts, so keep your row covers at the ready in case any hard frosts are forecasted.

Indoors from Seed

If the early sowing method I just described sounds a little too risky for your taste but you still want to get a head start on summer, you can sow these seeds indoors and then transplant them out after your last spring frost.

You’ll want to get the timing right when using this method so you don’t end up with plants that are too big to manage indoors when the weather is still frigid outside.

Plan to sow seeds indoors four to six weeks before your last average frost date.

For indoor sowing, you’ll need a sterile seed-starting medium, two-inch pots or six-celled seed starter trays, a spray bottle, and of course, seeds for the varieties of your choice. You may also want to use a grow light and a heat mat.

Fill the pots or cells with growing medium, leaving about half an inch to an inch of space between the surface of the soil and the rim of the pot. This way, you’ll have room to water without spilling soil over the edge.

Spray the growing medium with water to moisten it without allowing it to become soggy. Place one to three seeds in the center of the pot or in each cell, cover them very lightly with soil, then spray the surface of the soil with the water bottle.

If you have a very bright south-facing window, you can grow your seedlings next to this light source. Alternatively, you may want to consider using grow lights, which will give the seedlings the light they need and prevent them from becoming leggy.

If using sunlight as a light source, rotate the pots daily to prevent lopsided growth.

If more than one seedling has germinated in each pot or cell, thin the weakest-looking seedlings by snipping them off with a pair of scissors.

You can transplant the seedlings one to two weeks after the last frost, but be sure to check your weather forecast first for any dips in the temperature.

While direct-sown seeds may have some resistance to light frosts, don’t count on that protection for seedlings grown indoors. These hothouse flowers will be much more vulnerable and not yet able to put up with extreme weather fluctuations.

Harden seedlings off before transplanting to help them build up some resilience.

You can find guidance on hardening off and planting transplants in our complete guide to starting annuals indoors.

From Division

Propagation via division is only appropriate to use for C. atrosanguineus, the species commonly called “chocolate cosmos,” which doesn’t grow reliably from seed.

To divide this species, you can follow the same procedure as you would for dividing perennials.

You can learn more about the propagation of C. atrosanguineus, as well as specific growing and care recommendations, in our complete guide to growing chocolate cosmos. (coming soon!)

How to Grow

This summer annual is easy to grow and doesn’t require any coddling. In fact, giving it too much love might cause problems.

Here’s what you need to know in order to provide these flowers with the best possible care:

Sun

These plants should be grown in full sun. That’s to say, they should receive at least six to eight hours of direct sunlight.

If summer heat is extreme in your location, a bit of shade can help, particularly afternoon shade. Just make sure that they get their full quota of sunlight.

Water

When seedlings are young, they will require more frequent watering.

However, once plants are established, they have only moderate water needs, with sulfur cosmos requiring even less water than other varieties.

In fact, overwatering can result in plants with fewer blooms.

So when it comes to irrigating these flowers, water infrequently but deeply, preferably showering the surface of the soil rather than the tops of the plants in the absence of rain.

These annuals are fairly drought tolerant, but during hot summer weather, look for wilting as a sign to increase watering.

Soil

These low-maintenance plants grow best in sandy or loamy soil with a pH ranging between 6.0 and 8.0. The soil should have excellent drainage.

Overly rich soil can cause plants to become leggy and droop over, so this is truly a case where too much love won’t win you any favors.

Growing Tips

• Plant in full sun.
• Water deeply but infrequently.
• Grow in well-draining, sandy or loamy soil.

Pruning and Maintenance

Cosmos plants require very little maintenance, especially if you don’t mind seeing dead flower heads mixed in among the blooms. Here’s what you need to know:

Weeding

While young plants are getting established, weeding around the small fern-like seedlings can help so they won’t be crowded out.

If weeding seems to take up too much of your gardening time, be sure to read our article on how to spend less time weeding.

However, keep in mind that once cosmos are well-established, they won’t be impacted as much by neighboring weeds.

Fertilizing

Fertilizing is another point where too much love can cause problems for these members of the aster family.

Giving them too much help in the way of fertilizer can make them overly tall and lanky.

Instead of fertilizing them during the growing season, just mix some compost into the soil before you sow your seeds or set in your transplants, and your fertilizing is done!

If you offer these plants nothing but compost, they’ll be fine. But if you introduce any other amendments, be aware that fertilizers heavy in the plant nutrient nitrogen will encourage the production of foliage, but not flowers.

Mulching

Mulching around these annuals will help prevent water loss from the soil and extend the time in between watering visits.

Place mulch about two to three inches thick, taking care to leave a couple of inches bare around the stems of these plants, to avoid creating conditions where fungal pathogens could flourish.

And when it comes to mulch, think beyond wood chips and pine straw – there are many other options you can use to cover the soil. Check out our article on mulching to learn more.

Pruning

Cosmos don’t require pruning, so if you are taking a low-maintenance gardening approach like I am, you can skip this part.

However, if you prefer the look of smaller, bushier plants, these can be cut back to a third of their height in midsummer. Just be sure to use sterilized garden pruners to avoid unintentionally spreading disease between plants.

Another reason you might want to do a bit of pruning is if you want to keep spent flower heads off of your plants.

Personally, I love seeing these plants with flowers at all stages of their development – new buds, opening blooms, glorious flowers, and fading petals, finally turning into seed heads.

However, if you don’t like seeing the spent heads or want to prevent re-seeding, you can deadhead them by snipping the faded blooms off with a pair of scissors or garden snips.

Staking

When grown densely in mass plantings, these annuals won’t need any help staying upright – each flower’s neighbors will provide structural support to help keep it from leaning.

However, if your plantings aren’t quite dense enough to offer structural support, you may want to consider staking these plants.

It’s best to provide support before the plants are eight to 10 inches tall, so you don’t damage their root systems when inserting stakes into the ground.

If you only have a few cosmos plants, you can support each one individually with a bamboo stake and some twine.

However, if you have a large planting of these flowers, you may want to use a staking system similar to that used for tomatoes, such as the Florida weave.

You can read our article to learn how to create your own Florida weave.

Species and Cultivars to Select

As mentioned earlier, there are three main species of Cosmos available to gardeners. We’ll look at the features of each of these, consider their differences, and explore a few cultivars of interest.

Atrosanguineus

Commonly known as “chocolate cosmos,” C. atrosanguineus is a species native to Mexico that has dark, brownish-red blooms with even darker centers.

True to their common name – yes, it’s true! – these one-and-a-half-inch flowers have a chocolatey fragrance. Blooms are single, making them friendly to pollinators.

The leaves are small and lance shaped, and the flowers are held on long stems above the foliage, making them wonderful for cut arrangements.

These plants have a mounding growth habit, growing up to 30 inches tall with an 18-inch spread.

Since most varieties of this species don’t have seeds that will germinate reliably, they are typically grown from tubers instead.

C. atrosanguineus is often grown as an annual in Zones 2 to 6, but the tubers can be lifted and stored like dahlias for the winter.

Pests

Mexican asters are generally pest free, and you are not likely to encounter a problem. However, it’s always good to know what to be on the lookout for in case pests do move into your flower patch.

Aphids

If you notice ants crawling all over your cosmos flowers, take a closer look and see if you spot any aphids as well.

While the activity of the ants will be fairly obvious, the aphids may be harder to spot. Ants collect honeydew from aphids, which is why you might find both insects living on your plants.

Check stems and the undersides of leaves for aphid populations.

Luckily, cosmos also attract beneficial insects such as green lacewings, whose larvae are voracious predators of aphids.

Spider Mites

If you notice yellow stippling of your plant’s foliage, check the undersides of the leaves – your plants might have spider mites.

When infestations are severe, you might also notice webbing around the foliage.

These small arachnids suck nutrients from plants and can cause stunting.

You have many options for dealing with such an infestation – learn more about detecting and controlling spider mites in our article.

Thrips

You’ll be most likely to see thrips feeding on flowers rather than foliage.

These insects are so small that you’ll need a hand lens to properly identify them.

While thrips won’t cause much direct damage to your plants, they can transmit viruses which can cause a lot of harm.

Learn more about identifying and controlling thrips in our guide.

Disease

You aren’t likely to find signs of disease among your cosmos, but in the rare event that you do, here are some the most likely problems you might encounter:

Aster Yellows

If your cosmos are infected with aster yellows, you’ll most likely notice deformed flower heads on your plants.

These deformations are caused by phytoplasmas spread by an insect, the aster leafhopper.

Since the only cure for aster yellows is prevention, it can be helpful to know what to look for and what steps to take to prevent it.

To learn how to prevent this disease as well as how to manage an existing infection, check out our article on aster yellows.

Powdery Mildew

Have you noticed large white or gray patches on the foliage? If so, powdery mildew may be to blame.

While patches of mildew on leaf tops are the first sign, affected leaves will eventually turn brown and shrivel up.

Risk of encountering this fungal disease is highest when the weather is humid at night and less so during the day. If you live in a humid climate, you can help prevent this disease by ensuring there is adequate airflow around your plants.

If infection has already set in, remove any infected leaves and dispose of them to prevent further spread.

Read our article to learn more about how to prevent powdery mildew, as well as how to and treat it using natural, homemade remedies.

Smut

Smut, another fungal disease, can make itself known in your ornamentals with the appearance of small yellow spots on foliage.

These spots will develop into pustules that eventually rupture, spreading fungal spores.

Smut is best prevented through good gardening habits such as allowing adequate spacing between plants, watering the soil surface instead of showering the tops of plants, and taking care to sanitize garden tools between uses.

You can learn more about smut in our article.

Best Uses

Cosmos can be used in the landscape in countless ways.

Use them as part of your landscaping plan for a tall, wispy floral accent or create mass plantings for a strong visual statement.

In mixed flower beds, plant tall varieties at the back of borders or as a central focal point, and use smaller varieties in the midground or foreground.

Taller varieties can also be used to create a vegetative screen.

For planters and containers, make sure to use dwarf varieties since full-sized specimens will crowd out the other plants, and their roots will dominate available soil.

Since they are easy to grow, these are a great option for children’s gardens, and they are also a welcome addition to butterfly gardens or cut flower gardens.

In the veggie garden, cosmos make valuable insectary plants to attract beneficial insects such as bees, predatory wasps, and hoverflies – and they can be grown as companion plants among vegetable crops such as artichokes, cabbage, or kale.

Blooming right up until the first frost, and sometimes longer, these flowers are excellent for late summer color.

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