Cosmos flowers have been growing in the partial shade of their much flashier cousins, zinnias, for too long. Zinnias, with their double blooms, steal most of the sunlight, but cosmos are just as easy to grow as zinnias. They’re every bit as beloved by pollinators like butterflies and hummingbirds as zinnias. They enjoy as long of bloom time as zinnias.

They come in as many beautiful colors as zinnias—everything from delicate pink to the bright yellow of sulfur cosmos, and even a candy stripe in between. And thanks to their sturdy stems, cosmos actually make better, much longer-lasting cut flowers than zinnias.

Cosmos flowers might not be as flashy, but they can add a whimsical charm to your garden with their simple, daisy-like flowers and feathery leaves. 

Here’s your guide to growing cosmos flowers from seed to cut flower harvest. 

How to Grow Cosmos Flowers. Image of blue sky with clouds and dark pink cosmos flowers with green foliage growing.

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WHEN TO PLANT COSMOS

Cosmos are frost-sensitive annuals, so it’s a good idea to wait a couple weeks after the danger of frost has passed to sow seeds. Ideally, the soil temperature should be above 60°F for the best germination. For example, my last frost date here in the Austin area is mid-March, so our cosmos planting time would be the end of March or early April. We can keep sowing cosmos seeds until we’re just 2 months out from our first frost date in November.

It’s typically recommended to direct sow your cosmos outdoors as soon as the weather is right, but if you wanted to get a head start on a short growing season, you could start cosmos indoors about 4-6 weeks before your last frost in the spring. Follow the tips in our seed starting guide, and make sure to wait until your last spring frost has come and gone before you move your cosmos seedlings outside.

Cosmos are from Mexico and Arizona, which is great news for those of us who live in warm climates. Cosmos thrive over the hot summer months.

pink lemonade cosmos flowers blooming

WHERE TO GROW COSMOS

Cosmos aren’t too picky about their soil. Actually, soil that’s too rich can encourage these plants to grow tall and floppy or produce lots of lush foliage instead of any actual flowers. I literally have cosmos growing in the gravel walkways around my pollinator garden thanks to seeds that were dropped last year—and they’re just as healthy and productive as the ones growing in my garden beds. With that in mind, you can grow cosmos in the ground, in pots or containers that are 12+ inches deep (make sure there’s at least one good drainage hole in the bottom), or in your raised garden beds.

The most important consideration is choosing a sunny spot with well-draining soil. Cosmos are hardy little flowers that can survive a lot, including not getting enough sun, but they’ll give you more blooms if you give them full sun, or at least 8 hours of direct sunlight.

One thing to be mindful of is how tall cosmos can grow (up to 6 feet tall). Make sure not to put tall varieties in the front of your beds, where they might block too much sunlight from other plants. 

Pro Tip: 

There are tons of fun cosmos varieties to grow. I love all the high-quality seed options from Renee’s Garden and Botanical Interests

dark pink and light pink cosmos flowers blooming in a raised bed garden

2 EASY WAYS TO PLANT COSMOS SEEDS

Remember, wait to plant cosmos in your garden until all threat of frost has passed. Cosmos seeds look like little brown crescents, and they’re easy to hold in your hand and separate out. 

You have two options when it comes to sowing cosmos seeds. The first option is to sprinkle the seeds over the soil by shaking them from the palm of your hand. This method is a fun and efficient way to plant for in-ground beds, but keep in mind that you might need to come back and thin overcrowded plants by cutting them at the base later. 

The second option is to place the seeds where you’d like them to grow, giving each seed about 6-12 inches to spread out. I prefer this method for cosmos that will grow in my raised beds.

Once you’ve got all your cosmos seeds scattered or placed, use either your hand or a little hand rake to cover the seeds up with a light layer of soil or compost. These seeds shouldn’t be buried at all. You just want to barely cover them so that birds don’t come and steal them. 

Water the planting area well to tell the seeds it’s time to sprout. Cosmos generally germinate in about 5 to 10 days. 

up close image of cosmos seeds

COSMOS GROWING TIPS

Cosmos are overall super low-maintenance. Once they’re up and growing, they can handle drought and pretty severe neglect. Like I said, I have cosmos growing in my gravel pathways. I never water them or fertilize them. The only care I give them is to avoid stepping on them, and they’re doing just fine.

Your main tasks when growing cosmos will be basic care like watering and pruning, if you feel like it. If you don’t, that’s cool, too. Over-watering them and fertilizing them can actually lead to fewer flowers, not more. 

Water

Water young plants regularly until they’re a couple inches tall. After that, mature plants really only need supplemental watering if you’ve been experiencing prolonged drought. It’s best to give cosmos one deep soak that will last for several days, if not the whole week, rather than several short waterings. Make sure to aim water at the base of the plant. Wet conditions on the leaves can cause fungal diseases like powdery mildew. 

Prune

You can do what’s called “pinching off” the first blooms on your plants as they begin to form. The idea is to encourage your plants to branch out a bit more and produce more flowers in the future. You’ll also want to prune, or deadhead, flowers once they’re looking past their prime. To prune flowers, simply cut the stem right above a leaf node (a pair of leaves) with a clean pair of pruners or scissors. Cosmos are cut-and-come-again flowers, so the plant will regrow what you take.

If you don’t have time to prune individual flowers, you can also just shear the plants about 1/3 of the way down once most of the flowers are looking spent. Your plants will produce new growth, including more blooms, shortly. 

If you leave dried seed heads on the plants, they will likely drop seeds in your garden, aka self sow. That means you’ll get new cosmos plants popping up next year. 

Feed

You really don’t need to fertilize cosmos at all. If you want to give something to all the flowers growing in your cut flower garden, I recommend applying a balanced fertilizer every 4 to 6 weeks during the growing season. I like MicroLife Maximum Blooms, which is a slow-releasing liquid fertilizer.

Protect

Cosmos aren’t prone to pest issues, but you may occasionally spot some aphids or flea beetles. Give your plants a strong spray with your watering hose or treat with insecticidal soap or Neem oil as needed.

Support

Taller varieties might need to be staked to prevent them from blowing over in strong winds. I like to pack my cosmos plants in a little tighter than the back of the seed packets recommend so that they can support each other as they grow.

HOW TO HARVEST COSMOS FLOWERS

Cosmos typically take about 50 to 60 days to bloom once they’ve sprouted, depending on the weather and the variety you’re growing.

If you’d like to enjoy cosmos as cut flowers, harvest in the morning. Select flowers that are just beginning to open up—that’ll give you the longest vase life. Harvest the flowering stem by cutting at an angle above a leaf node with a clean pair of pruners or scissors. Make sure not to cut any stems that are branching out from the same base—those will soon produce flowers for you.

Take your pretty little blooms inside, strip off the lower leaves, and put them in a jar or vase filled with fresh water. Better yet, bring a jar or vase with you to harvest cut flowers from your garden to prevent wilting. Your blooms should last at least a week this way. 

At the end of the season, harvest any remaining cosmos blooms, thank the plants for the joy they brought you, and remove the plants from your garden space before your first frost. 

dark pink and light pink cosmos flowers harvested and placed in a glass jar with water.

COSMOS FAQs

Is cosmos a perennial plant?

Cosmos are annual flowers, which means they grow from seed, produce blooms, and complete their life cycle within one growing season (unlike perennial plants, which can live for several years). Even if they could live longer, those first frosts will take them out.

Are cosmos safe for dogs?

Cosmos are not toxic to pets or their humans. Most cosmos varieties are edible, but it’s a good idea to double check the back of your seed packet to make sure. You can use flowers as pretty little garnishes for desserts, charcuterie boards,  salads, and veggie trays.

white cosmos flower with yellow center and small pollinating insect resting on the flower.

Your spring-planted cosmos should bloom all through the summer and right up until your first frost. You won’t regret adding these cheerful, fuss-free flowers to your garden space, not once you see for yourself how ridiculously easy they are to grow!

Let us know if you have any other questions about growing cosmos!

Looking for more flower growing tips?

Check out my good friend Stephanie’s new book on growing flowers in small spaces.