With their feathery foliage and beautiful blooms, cosmos flowers make a lovely addition to any garden. The delicate flowers look fragile but they’re surprisingly easy to grow. Whether you’re an experienced grower or a beginner, learning how to plant cosmos is something anyone can do.
In this guide, I’ll show you how to start cosmos from seed, how to provide the right growing conditions, and how to care for the plants as they grow. I’ll even show you some beautiful cosmos varieties to try. By the end, you’ll be an expert on planting cosmos!
Why Plant Cosmos
There’s a lot to love about growing cosmos. If the attractive flowers aren’t reason enough, here are some more reasons to grow these cheerful blooms.
Beautiful Cut Flowers
Cosmos have long stems that make excellent cut flowers. They look beautiful next to other flowers or all on their own.
Pollinators like bees and butterflies love cosmos too. Planting cosmos near your vegetable garden is a great way to attract these beneficial pollinators.
Easy To Grow
Growing cosmos is easy, even for beginners. Learning how to plant cosmos doesn’t require any special skills, and the plants hardly need any maintenance.
Once they get started, you can pretty much ignore them and they’ll still produce beautiful flowers.
Conditions for Growing Cosmos
You can grow cosmos just about anywhere. They aren’t picky about growing conditions so whatever you have will probably work.
Cosmos grow well in a variety of soil conditions. They prefer moist, well-draining soil, but they tolerate dry soil as well.
Poor soil is better for cosmos than rich, fertile soil. Rich soil can cause the plants to grow very tall and produce a lot of foliage rather than a lot of blooms.
There’s no need to add fertilizer or organic matter to the soil when planting. Cosmos do just fine without it!
Sun and Temperature
Cosmos do best in a warm, sunny location. They thrive in summer, even on the hottest days.
While other plants droop and wilt in full sun on hot summer days, cosmos soak it up and bloom even brighter.
If you don’t have a suitable sunny location, cosmos tolerate partial shade. They probably won’t bloom as much but you’ll still get flowers.
How to Plant Cosmos From Seed
Starting cosmos from seed is simple and affordable! Cosmos seeds sprout quickly and transplant well. You can start seeds indoors or direct sow seeds straight into the garden.
Most cosmos varieties are annuals meaning they’re planted from seed each year and don’t come back on their own.
If left to mature, some cosmos self-seed by dropping their seeds at the end of the season. The seeds sprout in spring for a whole new season of flowers.
How to Start Cosmos Seeds Indoors
Starting cosmos seeds indoors is a great way to get earlier blooms. Start seeds indoors 4-6 weeks before the last frost and enjoy flowers weeks sooner than if you wait to plant them outdoors.
You’ll need some supplies to get started.
Equipment for Starting Cosmos from Seed
At the minimum, you’ll need seed starting trays or small containers to plant seeds in, seed starting mix for planting, and of course, cosmos seeds!
If you don’t already have some basic seed starting supplies, here are some additional things that are helpful. If you plan to start more plants from seed, purchasing good equipment is well worth the investment. It lasts for years and can be used over and over again.
Planting Cosmos Seeds Indoors
Fill your seed trays or containers with seed starting mix. Sprinkle a couple of seeds on the top of each cell and then add a bit more seed starting mix on top to cover them. The seeds should be covered to about 1/4 inch deep. Water thoroughly.
I like to use a spray bottle for watering freshly planted seeds because the fine spray doesn’t disturb the seeds.
Keep the soil moist and seeds should emerge in 7-10 days. Thin the plants to one per cell after the true leaves emerge. The first leaves that emerge are called cotyledon leaves. They look a lot different from the true leaves. You’ll recognize the true leaves by their thin, frilly appearance.
Keep the seedlings in front of a sunny window, or use a grow light to make sure they receive adequate light.
To know if your cosmos seedlings need to be transplanted from seed starting trays to pots, check our guide on knowing when and how to Pot Up Seedlings.
Direct Sowing Cosmos Seeds
Cosmos like it warm, so wait to plant them outside until all danger of frost has passed.
Choose a sunny spot and sprinkle seeds over the soil. Gently work them in with your hands or a cultivator to a depth of 1/4 inch. Water thoroughly and keep the soil moist until seeds emerge. Once they’re 3-4 inches tall, thin the plants to one every 12-18 inches.
You’ll see the first blooms in about seven weeks.
Caring for Cosmos Plants
Cosmos are some of the most low-maintenance plants you can grow. Once they’re established, they don’t need much attention or maintenance at all in order to thrive.
Keep cosmos moist while the seeds are germinating but once the plants are a few inches tall, they don’t need much water.
Rain is usually enough to keep cosmos healthy. If you have a prolonged period without rain, you may need to give them some water but otherwise, they don’t need it.
Since they prefer poor soil, fertilizer isn’t necessary for cosmos. In fact, using a fertilizer can actually be detrimental.
Too much fertilizer leads to lots of green growth at the expense of blooms. To get more flowers, don’t give cosmos any fertilizer.
For abundant blooms, it’s a good idea to deadhead cosmos by removing dead flower heads after they fade. Deadheading encourages the plant to keep producing flowers, giving you continuous blooms all summer long.
There are two ways to deadhead. The first is to use scissors or garden shears to cut away individual flowers as they fade. You can do this every few days during the blooming period.
The second option is to wait until most of the blooms have faded and cut back the entire plant all at once. If going this route, use a pair of long clippers to cut the entire plant back to about 18 inches tall. After a few weeks, you’ll have a whole new set of blooms.
Different Varieties of Cosmos to Grow
Cosmos come in a wide range of beautiful colors and varieties. These are the types of cosmos you’re most likely to find in the United States.
These cosmos have double blooms of small flowers. They come in shades of red, orange, and yellow. The plants usually grow to around three feet tall and produce lots of blooms.
Cosmos Bippinatus have daisy-like flowers with rows of flat petals and a round center. They come in shades of red, purple, pink, white, and yellow. The plants usually grow between two and four feet tall. In rich soil, I’ve seen them grow even taller.
Chocolate cosmos are very dark red with a hint of brown. The blooms smell like chocolate which is where they get their name. Chocolate cosmos are perennial in USDA hardiness zones 7 and above. In other zones, they’re grown as annual flowers. They’re a smaller variety, only growing to about two to two and a half feet tall.
Wrapping Up How to Plant Cosmos
Now that you’ve learned how to plant cosmos you’re sure to have beautiful blooms all summer long. To learn more about starting plants from seed, visit the Seed Starting page on our website.
We have how-to plant guides for dozens of flowers, vegetables, and herbs to get your garden off to a great start! You’ll also find tips, tricks, and product recommendations to help you along the way. Whether you’re a beginner or a master gardener, you’re sure to find some helpful resources there!