How to Grow Cosmos From Seed

If you’ve never sown cosmos from seed, let this be your year! They’re great for beginners and easy to do at home. Join organic farmer Jenna Rich as she walks us through how to start these pretty wildflowers from seed.

Written by Jenna Rich Horticulture review by Sarah Jay Last updated: March 6, 2024 | 5 min read

A kaleidoscope of Cosmos unfurls beneath the summer sun, captured from above. Feathery purple, pink, white, and yellow petals dance in the gentle breeze, their vibrant hues a stark contrast to the lush green carpet below.

Contents

Cosmos are an incredibly resilient and adaptable annual flower that is among the earliest to bloom and last to peter out in the fall. These daisy-like flowers bloom in shades of white, pink, lavender, violet, yellow, and orange. Some varieties are edible, and others can reach six feet tall!

Cosmos are easy to grow from seed and great for beginners. I recommend trialing several different types to find your favorite. I guarantee you’ll want them throughout your garden after seeing how fun they are to grow.

Let’s get into how to grow cosmos from seed.

Gather Supplies

A close-up reveals a gentle hand cradling brown cosmos seeds, their future potential waiting to unfold. Beside them, a vibrant yellow cosmos stands tall, a testament to the beauty these seeds hold. — To start cosmos seeds indoors, gather trays, a heat mat, a soil mix, water, labels, and light.

Supplies needed:

Cosmos seeds
Cell trays if starting indoors
Optional heat mat
Seed-starting mix
Access to water
Labels
Light; natural or artificial

Sowing Seeds Indoors

A black plastic seed starting tray with 12 cells rests on a warm wooden table. Brown, organic soil, damp and rich with potential, fills each compartment. In some cells, tiny miracles emerge - bright green cosmos sprouts reaching for the sunlight. — Ensure speedy cosmos seed germination by sowing in trays and maintaining warmth.

Cosmos are fun to grow from seed because they are quick to germinate, especially on a heat mat. Start them indoors about four weeks before your last frost in warmer climates and five to seven weeks in cooler climates. Any sooner, and they may become leggy if they don’t have proper access to light.

Sow seeds into a cell tray. Fill trays with soil and gently drop them to tamp the soil down. Sow seeds horizontally about a half inch down, and cover with soil. Water them well and keep the soil moist until germination.

Cosmos will germinate best when temperatures are between 68° and 72°F (20° and 22°C), and it may take up to two weeks. Using a heat mat to supply consistent temperatures may encourage quicker germination, sometimes in just a few days.

Sowing Seeds Outdoors

A close-up of vibrant green leaves unfurling from thin, reddish stems, reaching for the sun in black plastic pots. Moist brown soil, flecked with organic matter, cradles their hopeful beginnings. — Cosmos are directly sown into garden beds in the fall for early spring blooms.

You can also grow cosmos from seed by direct sowing them into prepared garden beds. In warmer climates, sow in fall for early spring blooms or once temperatures have warmed up to at least 60°F (16°C) in the spring.

Pro tip: Flowers that readily self-seed indicate they’re a good option for winter sowing.

Growing On

A single Cosmos seedling stands proudly in the foreground, its delicate cotyledons like twin emeralds reaching for the light. Behind, a soft blur hints at the bustling community of fellow sprouts, all pushing through the soil. — Trays go under light after germination; pinching after true leaves ensures robust growth.

Place the trays under light as soon as you see germination. They can live in a heated greenhouse, under artificial light, or in a sunny place in your home.

Pinching after the first true leaves emerge will encourage side shooting and a strong root system.

Hardening Off

Sunlight bathes a vibrant tapestry of Cosmos seedlings, bursting from various plastic pots. Tiny emerald leaves unfurl, whispering promises of blooms, while established sprouts reach skyward, their slender stems adorned with delicate buds. — Gradually acclimate seedlings outdoors when temperatures reach 65-70°F (18-21°C) during the day.

Slowly harden off seedlings once outdoor temperatures have reached about 65-70°F (18-21°C) during the day.

Transplanting and Spacing

A close-up view reveals a cluster of Cosmos sprouts bursting from rich, brown soil. Their thin, emerald leaves, delicately veined and translucent at the tips, unfurl like tiny dancers reaching for the sun. — Seedlings are planted 9-12 inches apart when soil temperatures reach 65°F (18°C).

Transplant once soil temperatures reach 65°F (18°C). Space out seedlings at 9-12 inches with about six inches between rows. Three rows fit comfortably in a 30-inch wide bed. Cover as needed in cooler climates after spring transplanting.

Soil and Sun Requirements

Two golden Cosmos stars stand proud, their delicate petals in a captivating display of yellow hues. From pale sunshine to deep honey, they radiate beauty, surrounded by a soft blur of verdant leaves. — They excel in full sun and are hardy in zones 2-11.

Cosmos aren’t fussy to grow from seed and easily adapt to all soil types. They prefer soil that’s well-draining but not overly rich, and they perform well when neglected. They thrive in full sun and are hardy in zones 2-11.

Offering Support

A single, pristine white cosmos flower basks in soft focus, its delicate petals gently unfurling. Behind it, a slender bamboo stake provides subtle support, while the background melts into a warm blur of sun-dappled brown. — Certain cosmos cultivars may require horizontal support due to their height and robust blooms.

Cosmo stems are quite robust, but the height of some cultivars may require support. Netting works great and can be installed easily when seedlings are transplanted using a few stakes in the corners of the garden plot.

Water Requirements

Close-up of Cosmos sprouts bathed in soft light. Tiny green heroes stand proudly, first true leaves unfurling like miniature flags. Blurred background whispers of growth, fertile earth cradles their potential. — Water young seedlings weekly, allowing them to establish without overwatering.

Young seedlings should be watered once a week, but no more. Just enough to allow them to establish. They’ll become drought-tolerant over time but should receive water to decrease unnecessary stress.

Mulching

Tiny green sprouts, their stems slender and leaves delicate, push through soil enriched with black and brown rice hulls. The contrasting textures and colors showcase the raw energy of new growth. — Mulch around spring-emerged cosmos seedlings to suppress weeds and retain moisture.

If you’ve direct sown cosmo seeds in the fall, mulching around the newly emerged spring seedlings will help suppress weeds and retain moisture.

Successive Sowing

Vibrant cosmos flowers in various colors blanket a bee meadow, their delicate petals buzzing with activity as bees flit between them. The string tied between wooden fence posts creates a gridded enclosure, protecting the floral haven and its busy inhabitants. — Cosmos are resilient against pests and diseases, blooming throughout the season.

Most cosmos varieties are long-lasting bloomers, so successions aren’t necessary. However, if you have specific aesthetic garden goals, sowing new cosmos seed varieties every four weeks will give you a constant supply of fresh blooms.

Cosmos rarely suffer from pest and disease pressure and under ideal conditions, they bloom all season.

Pruning

A mesmerizing close-up of a Cosmos Bipinnatus flower, highlighting the delicate dance of its pink and purple petals, deepening inwards to a rich wine. Tiny sunshine-yellow dots paint the heart of the bloom, a beacon against the blurred backdrop. — Plants are trimmed to a third of their height in mid-summer for fresh blooms.

Cut back your plants to about a third of their height in mid-summer to freshen them up and encourage new blooms. Keep them watered and get ready for a brilliant fall flush of flowers. Do this in stages if you have several patches of cosmos.

Harvest

Sun-kissed yellow cosmos burst into life in a field, their delicate petals glowing golden under the warm rays. A sea of vibrant blooms unfurls, with hints of deep orange centers peeking through. — Harvest cosmos with budding color for a longer vase life.

Harvesting when you notice color and the bud beginning to crack open before pollination has occurred should add a few days to its vase life. Cut stems low and strip the foliage from the bottom before adding it to clean water.

Cool them right away to arrange later, or bring them inside to enjoy. The foliage adds a whimsical touch to wildflower bouquets if blooms aren’t available yet.

Collecting Seeds to Save

A close-up of a pile of cosmos flower seeds on a woven billow. The seeds are various shades of brown and white, and some have a fuzzy texture. Textures hint at future blooms, while sunlight catches the billow's natural fibers, creating a warm, inviting scene. — Easily save cosmos seeds by drying stems and storing them in labeled containers.

It doesn’t get much easier than saving these seeds. Hang freshly harvested stems to dry in a cool, dark area. Secure the blooms around a gently tied brown bag with holes poked in it for airflow and to catch the drying seeds. After several weeks, place the dried seeds in an airtight container and label them for next season or gift them to a friend.

If cosmos are left in place, they’ll drop seeds for the following season. Remove all plants and debris if you don’t want them to germinate there.

Recommended Cutting Varieties

Cosmos bipinnatus are popular annual flowers in cutting gardens. Here are a few of my favorites.

‘Sensation Blend’

This is a well-trusted and reliable mix that was bred specifically for cutting. Their single blooms are consistently two and a half inches across with striking pink and white shades and yellow centers. It’s wildly productive and will be the last thing blooming in late fall.

‘Sensation Blend’ will be productive in short days and adapt to being grown in a greenhouse. They should be staked due to their extreme heights, reaching over six and a half feet.

Harvest when you barely see color and cool right away for the best vase life. Their foliage can be used as a bouquet filler before blooms are available.

‘Double Click Blend’

Close-up of a mesmerizing "Double Click Cranberries" flower. Layers of frilly pink petals, tipped with maroon, unfurl in a captivating spiral. The golden heart shines bright, while blurred green foliage creates a dreamy backdrop. — This cultivar presents elegant double and semi-double ruffled blooms.

‘Double Click‘ offers elegant double and semi-double ruffled blooms with a gorgeous mottled and often striped appearance. The colors range from soft pink to eggplant purple, with various purples and magentas in between.

They can reach four feet tall and mature in 75-90 days. Incorporate these into cottage gardens or in mass for a pop of brilliant color.

‘Fizzy White’

It doesn’t get more perfect than this for a summer wedding. These crisp white cosmos have a unique ruffled center, and blooms can reach up to four inches across. Their petals have clean lines resembling pleats, and plants can reach up to five feet tall.

Recommended Edible Varieties

Only Cosmos sulphureus (cultivated) and Cosmos caudatus or Ulam Raja (wild) are edible. Studies show the latter has potential medicinal benefits. They’re shorter with smaller blooms and come in shades of bright yellow and orange.

‘Bright Lights Blend’

A sunbeam spotlights a dazzling yellow Cosmos sulphureus. Its delicate petals unfurl like a star, each tip sparkling with a tiny diamond of dew. Wispy florets, pure sunshine in color, sway ever so slightly, while a verdant tapestry of leaves peeks through the background — The ‘Bright Lights Blend’ heirloom cosmos showcases vibrant golden-orange and reddish-toned blooms.

This heirloom boasts vibrant golden orange and reddish-toned 2 ½- inch blooms. Plants attract hummingbirds, bees, and butterflies and will reach about three feet. Mature plants are drought-tolerant and are extremely low maintenance.

Add these to a summer salad, cocktail, oil, and vinegar blend, or garnish a season charcuterie board with them. They offer a slight tang.

Final Thoughts

Cosmos are a fun and joyful annual to grow. Grow them from seed with a basic seed-starting setup. Beginners and expert gardeners alike will enjoy adding these pops of color to their gardens.