Cosmos Flower: How to Grow Cosmos From Seed
A florist's guide to the cosmos flower: when to sow cosmos seeds in the UK, caring for cosmos seedlings, pinching, staking and deadheading, the best varieties, chocolate cosmos and cutting for the vase.
By Magda Callery, interior designer and florist
Cosmos is the flower I recommend to anyone who wants to grow their own cut flowers for the first time. It's cheap and easy from seed, grows into a cloud of feathery foliage and daisy-like blooms, flowers from summer until the first frosts, and the more you pick it, the more it produces. A single row can keep your house in flowers for months.
In this guide I'll cover everything you need to know about the cosmos flower: when to sow cosmos seeds in the UK, how to care for cosmos seedlings, planting out, pinching, staking and deadheading, the best varieties for cutting, pots and borders, chocolate cosmos, common problems, how to cut cosmos for the vase and what cosmos means.
Cosmos at a Glance
| Cosmos (Cosmos bipinnatus) | |
|---|---|
| Plant type | Half-hardy annual |
| Sow indoors | March to April, about 6 to 8 weeks before your last frost |
| Sow outdoors | Late May to June, once the soil is warm |
| Plant out | After the last frost, usually from late May |
| Spacing | About 30 to 45cm apart |
| Position | Full sun and free-draining, not too rich soil |
| Height | From about 60cm for compact varieties to over a metre for tall ones |
| Flowering | Summer into autumn, until the first frosts |
| Colours | White, pink, magenta, crimson, apricot and bicolours |
What Is a Cosmos Flower?
Cosmos are members of the daisy family, native to Mexico and the wider Americas. The cosmos most of us grow, Cosmos bipinnatus, has fine, feathery leaves and open, saucer-shaped flowers with a yellow centre. Its close relative Cosmos sulphureus brings hotter yellows and oranges, while chocolate cosmos, Cosmos atrosanguineus, is a tender perennial with deep maroon flowers and a faint vanilla scent.
The name comes from the Greek kosmos, meaning order or harmony, a lovely description of their evenly spaced petals.
When to Sow Cosmos Seeds in the UK
Cosmos are half-hardy, so they can't go outside until the risk of frost has passed.
- Sow indoors in March or April, around 6 to 8 weeks before your last frost date. In much of southern England the last frost is usually in May, but it can be later further north and in colder gardens.
- Sow outdoors directly where they're to flower from late May into June, once the soil has warmed up.
- Plant out indoor-raised plants after hardening off, once frost has passed. Gardeners' World suggests planting out from late April to early June, depending on your area and the weather.
Don't be tempted to sow too early. Cosmos grow fast, and plants sown in February often become leggy and pot-bound before it's warm enough to plant them out.
How to Grow Cosmos From Seed
- Fill modules or small pots with free-draining seed compost.
- Sow one or two seeds per module, about 0.5 to 1cm deep. The long, thin seeds are easy to handle.
- Water gently and keep them somewhere warm and bright, such as a windowsill or greenhouse.
- Thin to the strongest seedling in each module once they're up.
- Pot on into 9cm pots once seedlings have two true leaves, if their roots fill the modules before planting time.
Caring for Cosmos Seedlings
Healthy cosmos seedlings have a pair of long, narrow seed leaves, followed by the first true leaves, which are already finely divided and feathery.
| Seedling problem | Cause | Fix |
|---|---|---|
| Long, pale, floppy stems | Not enough light or too much warmth | Move to the brightest spot you have and keep them cooler once germinated |
| Seedlings collapsing | Damping off | Use fresh compost and clean pots, and avoid overwatering |
| Yellowing leaves | Roots have run out of space or nutrients | Pot on into a bigger pot |
| Holes and missing seedlings | Slugs and snails | Protect young plants, especially after rain |
A florist's tip: if your cosmos seedlings have stretched, plant them a little deeper when you pot them on. The extra stem below the soil helps them stand up straight.
Hardening off
About 7 to 10 days before planting out, put seedlings outside during the day and bring them in at night, gradually leaving them out longer. This toughens them up for life in the garden.
Planting Out Cosmos
- Choose full sun. Cosmos flower best in a warm, sunny spot.
- Don't feed them too much. Gardeners' World warns that rich soil and too much nitrogen give lots of leaves at the expense of flowers. Ordinary, free-draining garden soil is ideal.
- Space plants about 30 to 45cm apart, or in two rows about 30cm apart in a cutting bed.
- Water in well after planting and protect young plants from slugs for the first couple of weeks.
Pinching, Staking and Deadheading
Pinch out the tips
When young plants have three or four pairs of true leaves, or are about 20cm tall, pinch out the growing tip just above a pair of leaves. This encourages bushier plants with many more flowering stems.
Stake tall varieties
Tall cosmos can reach well over a metre and may flop in wind and rain. Support them with canes and twine, or with horizontal netting about 30cm above the ground in a cutting bed.
Deadhead regularly
Cut faded flowers back to the first leaf or side shoot below. Regular deadheading, or regular picking, keeps cosmos flowering right through to the first frosts. If plants are allowed to set seed, they'll slow down.
The Best Cosmos Varieties
| Variety or series | What it's like | Best for |
|---|---|---|
| 'Purity' | Classic pure white single flowers on tall plants | Cutting and white gardens |
| 'Sea Shells' | Pink, white and crimson flowers with fluted, rolled petals | Unusual texture in bouquets |
| Double Click series | Double, frilly flowers in pinks, crimson and white | Florist-style, peony-like arrangements |
| Cupcake series | Petals fused into a cup or saucer shape | Statement single stems |
| 'Rubenza' | Ruby red flowers that soften to dusky pink | Romantic, moody bouquets |
| Sonata series | Compact plants | Pots and the front of borders |
| 'Xanthos' | Soft primrose yellow, compact | Pots and containers |
| Cosmos sulphureus | Orange and golden yellow flowers | Hot colour schemes and pollinators |
Double cosmos are a lovely, affordable way to get a full, ruffled look in summer when peonies are out of season.
Chocolate Cosmos
Chocolate cosmos (Cosmos atrosanguineus) is different from the annual cosmos above. Gardeners' World describes it as a tender perennial from Mexico, grown from tubers, with dark, cocoa-coloured flowers and a faint vanilla scent. In the UK it needs winter protection: lift the tubers and store them frost-free indoors, or mulch heavily in autumn in milder gardens. It's a favourite with florists for its deep colour, and it looks wonderful in a pot on a sunny patio or deck, where you can enjoy the scent.
Growing Cosmos in Pots
Compact varieties such as the Sonata series and 'Xanthos' are ideal for pots. Use a large container with drainage holes and peat-free multipurpose compost, put it in full sun and water regularly in summer, as pots dry out quickly. Deadhead often to keep the flowers coming.
Common Cosmos Problems
| Problem | Signs | What to do |
|---|---|---|
| Lots of leaves, few flowers | Plants grow huge and leafy | Stop feeding and grow in less rich soil |
| Plants flopping | Tall stems falling over | Stake early or grow with netting |
| Slugs and snails | Seedlings eaten | Protect young plants |
| Aphids | Small insects on shoot tips | Usually tolerable, as ladybirds and other predators soon arrive |
| Late flowering | Few flowers before late summer | Sow at the right time, pinch once and give full sun |
How to Cut Cosmos for the Vase
- Cut in the cool of the morning or evening, and put stems straight into water.
- Pick at different stages. Cut some flowers just as the buds are colouring and opening, and some fully open, for a vase that keeps going.
- Cut long stems back to a side shoot to encourage more.
- Strip the lower leaves and stand them in cool water for a few hours to condition before arranging.
- Change the water every couple of days.
Cosmos look beautiful on their own in a jug, or mixed with zinnias, dahlias, scabious and grasses. Their airy, dancing stems soften any arrangement, from a jam jar on the kitchen table to a big jug of fresh flowers in the hallway.
What Do Cosmos Flowers Mean?
Because their name comes from the Greek word for order and harmony, cosmos are often linked with harmony, peace and balance. In most birth flower calendars, cosmos is also listed as one of the flowers for October, alongside the marigold.
Frequently Asked Questions
When should I sow cosmos seeds in the UK?
Sow cosmos indoors in March or April, around 6 to 8 weeks before your last frost, or sow directly outdoors from late May into June once the soil has warmed up.
Can you sow cosmos directly into the ground?
Yes. Sow cosmos seeds directly where they're to flower from late May into June, once the risk of frost has passed. They'll flower a little later than plants started indoors.
What do cosmos seedlings look like?
Cosmos seedlings have two long, narrow seed leaves, followed by true leaves that are already finely divided and feathery, like miniature versions of the adult foliage.
Why are my cosmos seedlings leggy?
Leggy cosmos seedlings usually need more light or are too warm. Move them to the brightest spot you have, keep them cooler once they've germinated, and plant them a little deeper when you pot them on.
Should I pinch out cosmos?
Yes. Pinching out the growing tip when plants have three or four pairs of leaves encourages bushier plants with more flowers.
Do cosmos come back every year?
Annual cosmos flower for one season and are killed by frost, although they sometimes self-seed. Chocolate cosmos is a tender perennial that can return each year if its tubers are protected over winter.
Why are my cosmos not flowering?
The most common causes are soil that's too rich, too much feeding, too much shade or sowing too late. Cosmos flower best in full sun in ordinary, free-draining soil.
How long do cosmos flower for?
With regular deadheading or picking, cosmos flower from summer until the first frosts.
A Cloud of Summer Flowers
Cosmos are generous, easy and endlessly pretty. Sow them at the right time, give them sun, go easy on the feed, pinch once and keep picking, and they'll reward you with armfuls of flowers all summer long.
For more seasonal growing and flower ideas, explore the flowers and garden section, or if you'd like help designing a home and garden view that show off your flowers, you can book a one-hour online interior design consultation with me.
About the author
Magda Callery is a UK interior designer and florist with more than 15 years' experience designing homes, pubs, restaurants and bars, and her expert commentary has featured in Homes & Gardens, The Mirror and The Spruce. Explore more of the flowers & garden articles.
Continue reading
How to Grow Zinnias: Planting, Seeds and Cut Flowers
A florist's guide to planting zinnias in the UK: when and how to sow zinnia seeds, planting out, spacing, pinching, the best varieties for cutting and how to make cut zinnias last.
November Birth Flower: Chrysanthemum Meaning and Gifts
The November birth flower is the chrysanthemum. A florist explains its meaning, colours and history, the types you'll find at a florist, November birthday bouquet ideas and how to make chrysanthemums last.
Yellow Flowers Meaning: Victorian vs Modern Guide
What do yellow flowers mean? A florist compares the Victorian and modern meanings of more than 20 yellow blooms, plus cultural traditions, the best yellow flowers for every occasion and season, and care tips.