How to Grow Cosmos From Seed

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 

Sowing Seeds Indoors 

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

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

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

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

Transplanting and Spacing 

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 

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 

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 

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

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

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

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 

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

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’

‘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’

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.