The Complete Guide To Growing Cosmos

pink and white cosmos against the sky

If you’re looking for a garden flower that adds charm and whimsy with a stellar work ethic, then cosmos are for you. These flowers pump out the blooms once the weather warms up for the summer, and they’re easy to grow. Cosmos don’t need much attention other than regularly picking the flowers, making them a fantastic choice for the cutting garden.

If you have a sunny spot waiting for some seeds, keep reading to learn everything you need to know about cosmos.

Common nameCosmos
Botanical nameCosmos bipinnatus, Cosmos sulphureus
Growth habit Warm-season annual
When to start seeds4-6 weeks before last spring frost
When to transplant/direct sowAfter last spring frost
Mature plant height24-62 inches or taller, depending on the variety
Sunlight requirementsFull sun
Water requirementsRegular water until established, then minimal water
Soil requirementsAverage to poor, well-draining soil
Blooming periodSummer until fall frost
AppearanceSingle, semi-double, or double blooms
Colors include white, pink, carmine, dark red, red, orange, yellow
Ferny, lacy foliage
Pests & diseasesAphids, thrips
Fusarium wilt, powdery mildew

How to grow cosmos

Cosmos flowers will be an abundant beauty in your garden with the most basic care. From summer to the first fall frost, cosmos will give you bushels of flowers, putting these flowers at the top of the list for easy-to-grow flowers.

They’ll bloom all summer long as long as you keep them picked or deadheaded. Choose a handful of varieties with varying days to maturity (recommendations below!), and you’ll have a show of blooms for months.

For tips, be sure to read more here: Do Cosmos Bloom All Summer? Tips To Keep Them Flowering

Now, let’s look at the growing requirements of cosmos, which are minimal.

Light

Cosmos are sun-loving flowers native to Mexico and the southeastern United States, growing best in full sun. When cosmos receive eight or more hours of sun per day, they’ll grow to their optimal height and experience fewer diseases like powdery mildew, which can strike when cosmos are planted too close together in shady areas.

To find out how tall cosmos can get when planted in full sunlight, check out this post, How Tall Do Cosmos Grow? (It depends on the variety).

Soil

Many flowers (and vegetables!) enjoy soil that’s well-amended with compost and fertilizer. It helps the plants grow lush and stay healthy and productive for the whole growing season.

Cosmos, on the other hand, prefer average to poor soil that’s well-draining. Rich soil that’s high in nutrients and fertilizer is a detriment to cosmos, and the plants won’t be as productive as they could be. High nitrogen levels will yield abundant leaf growth but few flowers.

If you want leaf growth, perhaps as filler for bouquets, then fertilizing is the way to go for your cosmos. But if flowers are your focus, plant your cosmos in regular garden soil and skip the amendments.

Well-draining soil is crucial for growing cosmos. These flowers won’t tolerate standing water or constantly wet roots, so if you have clay soil that holds a lot of water, try building up a raised bed to encourage drainage. Or you can grow cosmos in containers with a basic potting soil mix.

If you’re finding that your cosmos aren’t blooming the way you expect them to (which should be a lot!), then it’s possible that the soil isn’t ideal for your plants. They may be getting too much nutrition or too much water.

To read more about why your cosmos aren’t producing many flowers, check out this article and get solutions: 4 Reasons Your Cosmos Aren’t Flowering (And how to fix it).

Water

Cosmos are great for the water-conscious garden since they don’t require much irrigation throughout the growing season. As seedlings, cosmos need regular water to the tune of one inch per week, which is standard for most garden plants. But once the plants reach their mature height, you can cut back on the water.

Since cosmos are native to hot and arid locations, they are well-adapted to less water, and it won’t stop them from growing well and producing an abundance of flowers. Too much water can hinder cosmos’ flower production.

One inch of water per week will be plenty to get cosmos through the summer. Depending on your exact climate and location, experiment with watering less. Perhaps try giving a half-inch of water per week if you need to restrict water. If you get a summer rainstorm, you might not need to water that week at all.

How and when to start cosmos seeds

Starting cosmos from seed is a quick task that is best done in the spring once the weather has warmed up for the season. Cosmos are not at all frost-hardy, so if you get impatient and plant early, any surprise frosts can wipe out all your seedlings. Be patient and check the weather forecast before sowing any seeds.

Figure out just when you should get your seeds going with this post: When To Plant Cosmos: The First And Last Planting Dates.

cosmos seedlings in plastic pots

If you’re feeling particularly antsy to get gardening, start your cosmos seeds indoors about a month early. You can get your gardening fix under the protection of the indoors, and you’ll get flowers a few weeks earlier than you would with direct sowing. Win-win!

Starting cosmos seeds is very similar to starting other flower and vegetable seeds. You need a seed-starting mix, seed trays or small pots, and your flower seeds.

Unless you have a cold frame or a very bright window, you’ll also need a shop light to provide artificial sunlight to the seedlings as they grow. Don’t stress about which light to buy or what type of bulbs you need. Any old shop light will do since the seedlings will only be indoors for a month or so.

Start the seedlings 4-6 weeks before you plan to plant them in the garden, and transplant them once the weather is consistently warm and frost-free.

To direct sow cosmos, plant the seeds in bare soil after it has warmed. You can scatter the seeds or plant them in rows. Either way, cover them with a quarter-inch of soil and water the seeds well. They’ll sprout in a week or two and quickly grow as the spring turns to summer.

Get all the details for starting cosmos from seed here: Easy Steps To Plant Cosmos Seeds, Indoors and Out.

If you have any lingering questions, let me answer them for you in this post: Grow Cosmos From Seed: Common Questions Answered.

Why you should grow cosmos

As if producing pretty flowers wasn’t enough, cosmos have several other things going for them that earn them a spot in any garden, including attracting a great many pollinators and beneficial insects.

Bees and butterflies are attracted to the saucer-shaped flowers, which is helpful if you also have a vegetable garden nearby that needs to be pollinated. Beneficial insects such as ladybugs and lacewings will also stop by the flowers, scouring the underside of the petals for aphids while they’re there.

You’ll have no shortage of bouquets when you grow cosmos. As a cut and come again flower, cosmos produce more blooms the more you cut, making them a beautiful, renewable resource for your home and garden.

Cosmos are easy to grow, making them a perfect flower for first-time gardeners and kids. Try starting them from seed with your family and enjoy the blooms all summer long.

For more reasons why cosmos should be on your list of flowers this year, dive into 10 Reasons Cosmos Are Good For The Garden.

Growing cosmos in containers

If you don’t have enough garden space for a patch of cosmos, or you only have a patio for your plants, you can still grow cosmos. You’ll be better off growing shorter dwarf varieties, which stay about two feet tall instead of the standard four feet tall or more.

Like growing them in the ground, growing cosmos in containers only requires basic potting soil, good drainage, and full sun exposure. You can direct sow right in the pot, or you can transplant seedlings that you started yourself or you bought at the garden nursery.

Keep the pots well watered while the cosmos are small. If the container or pot has trouble draining excess water, try putting it up on pot feet or bricks improve the drainage. Standing water in a pot will kill the cosmos planted in it, so be sure to avoid that scenario.

For more details about growing cosmos in pots, jump over to this article to learn all you need to know: Grow Cosmos In Pots When You Don’t Have Garden Space.

Top cosmos varieties to grow

There are dozens of cosmos varieties to grow, so many that you could add a new one to your garden every summer for years on end without repeating any. Check out just a few from Johnny’s Seeds:

While this would be an awesome adventure for your flower garden, let me give you a headstart with some of the most popular varieties to grow that can suit any style.

If you need a list of seed companies to browse, I have a list of my favorites in this post, 10 Best Places To Buy Quality Flower Seeds Online. I particularly like Johnny’s and Swallowtail Garden Seeds for cosmos for their huge selection (Swallowtail has 43 varieties!).

Dwarf/short varieties:

  • Cosimo Collarette: Semi-double petals have one row of full petals and a second row of short, whisker-like petals around the center. Flowers are pink and white bi-color.
  • Apollo Lovesong: Single, ruffled petals in pink, white, and pink picotee make for a charming dwarf variety.
  • Bright Lights: Vibrant orange, yellow, and scarlett blooms are a favorite of butterflies.

White flowers:

  • Physche: This variety is a favorite of florists and flower farmers for it’s long vase life and semi-double blooms.
  • Afternoon White: Bred for cut flower production, the stems and flowers are sturdier than other cosmos varieties to extend the vase life. Single flowers with a large yellow center are adorable in the vase.
  • Purity: Large, single white flowers with a small yellow center make this a favorite for butterflies.

Double or ruffled blooms:

  • Double Click Cranberry: As the name suggests, these double blooms are a deep cranberry red that pair beautifully with other white cosmos.
  • Cupcake Mix: Fluted petals resemble cupcake liners. Flowers come in pink and white with yellow centers.

Simple and sweet:

  • Sensation Mix: One of the oldie-and-goodie cosmos varieties, often called garden cosmos. The plants are tall, reaching 5-6 feet with single flowers in pink, carmine, and white.
  • Daydream: Light pink petals and dark pink centers make a charming flower.

Dramatic petals:

  • Velouette: The petals of this variety are deep red and white striped, with some flowers turning out all red. Some petals stripes resemble Christmas mint candies!
  • Capriola: White petals are edged in pink, as well as a pink ring around the yellow center.
  • Sea Shells: This variety has interesting tubular petals, which is uncommon for cosmos. Blooms are pink, white, and red.

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