Cosmos is a rewarding, free-flowering annual to start from seeds. Easy to grow, cosmos readily reseeds, often producing brightly colored flowers year after year. One of the best flowers for butterfly gardening, cosmos also attracts hummingbirds and bees. Fern-like foliage and bright, daisy-like flowers are a top note in cut flower displays and absolutely stunning in beds and borders.
Here’s how to grow cosmos from seed, including seed selection, planting, care, cultivation and display.
How to Grow Cosmos from Seed
My favorite cosmos variety is Sensation Mixed (Cosmos bipinnatus). “Easy to Grow” is stamped right on the OSC packet and it’s true. I grew these in my last garden and the present owners still get mounds of vivid pink, mauve, carmine and white blooms each summer.
Sensation does well in poor soil and dry conditions, gets about three feet high, and loves heat and humidity. You should find it in local stores. If not, order Cosmos ‘Sensation Mixed’ online from Amazon.
Other types on Amazon include dwarf varieties, or single colors such as yellow, red, or white. Chocolate varieties are often grown in containers, or as perennials in zones rated 7 or above for hardiness. Local suppliers usually stock selections suited for your area so check there first.
I first saw these flowers when my neighbor tossed a handful of cosmos seeds into the bottom of her vegetable garden. The flowers were amazing from mid-summer to first frost, with tons of seed pods. My neighbor gave me some and allowed the rest to reseed: a very easy way to grow cosmos from seed in your garden.
Just let it go!
Cosmos Seed Planting – Timing and Conditions
In cold winter climates, cosmos seeds are planted outside after the last frost. However, self-seeded cosmos sometimes come up with the tulips, defying the odds and doing well.
Jump-start the season by starting cosmos seeds indoors, from a month to six weeks before last frost. Start another set two weeks later for longer blooming time.
Cosmos seeds are small. Plant seeds in flats or pots filled with moist potting soil about three inches deep. Top seeds with 1/8th to a 1/4 inch of soil, tamp down, and keep moist by covering with plastic. Ideally, water seeds from below by placing pots into a container of water and allowing water to wick upwards into the soil. This encourages deep root development. Expect germination in five to seven days.
Cosmos seedlings grow quickly so transfer to individual pots when three to four inches tall.
Harden seedlings by setting outside for a few hours each day before planting.
After your last frost date, transplant seedlings about a foot apart, in a sunny site with average soil. Water, then gently tamp down the soil around each plant. Keep soil moist until the seedlings show new growth.
Remember: cosmos responds to nitrogen by growing lush, feathery foliage. Flowers, not so much. Average, unfertilized soil is truly best for cosmos.
For direct sowing, pick a bare spot in a sunny border and scatter the seeds. Cover lightly with soil and keep seed bed moist until germination. After that, cosmos is an easy keeper, although taller cultivars may need staking.
» Encourage bushy growth by pinching out the third set of seedling leaves.
» Promote plentiful blooms by picking flowers for vases or deadheading spent blooms. Cosmos happily responds by producing more flowers.
» Let some flowers set seed. Harvest seeds when petals are gone and seeds form a small, dry cluster. If some inner seeds are still green, check again in a few days.
» See How to Save Cosmos Seed for a video on saving cosmos seeds and separating the chaff.
» Label and store dry cosmos seeds in paper envelopes, recycled plastic pill bottles or similar storage containers.
Cosmos species – Extensive list of different kinds of cosmos plants (Wikipedia).
Chocolate Cosmos – Perennial Cosmos atrosanguineus variety, with sterile seed, propagated by division (Dave’s Garden).
Deadheading Cosmos: Short video demonstrating how to deadhead cosmos for continuous flowering.