How to grow cosmos
Find out all you need to know about growing cosmos, from sowing to planting in this easy Grow Guide.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sow | yes | yes | ||||||||||
| Flowers | yes | yes | yes | yes | yes | yes | ||||||
| Cut Back | yes | yes |
This article has been checked for horticultural accuracy by Oliver Parsons.
Cosmos are sun-loving plants with a long flowering season. They fill borders with a profusion of flowers, ranging in colour from white through to pink, red and orange, and they have masses of feathery foliage.
Shorter varieties look good in containers and all cosmos make fantastic cut flowers. Their simple open flowers are guaranteed to brighten up any garden well into autumn - and pollinators like them just as much as gardeners do.
How to grow cosmos
Grow cosmos in moist but free-draining soil in full sun. Deadhead to prolong flowering and cut it back in early winter, after the first frosts. Save the seeds from spent blooms to sow the following season.
More on growing cosmos:
- Cosmos, canna and summer bedding container display
- Eight plants to grow with cosmos
- How to sow cosmos seed
Where to grow cosmos
Cosmos are native to Southern and Central America, so need to be planted in a warm, sunny spot, in free-draining soil.
How to plant cosmos
Sow cosmos seeds in early spring directly into the soil where you want them to grow, or into small pots or modules filled with free-draining seed compost. If growing in pots, pot on seedlings when they are large enough to handle. Plant out in late April, May or early June after the danger of frost has passed.
Watch this video, below, to see Monty Don plant out cosmos for a long summer display.
How to care for cosmos
Once your seedlings have formed 2-3 pairs of leaves, pinch out the growing tips to produce bushier plants with more flowers.
Taller varieties may need staking, otherwise all varieties will flower for a long period if deadheaded regularly. When deadheading, cut the stem back to the first leaf beneath the flower.
The tender perennial chocolate cosmos varieties will need winter protection. Grow them in pots, then when they've finished flowering shelter them over winter in a frost-free place until spring.
In the video below, Monty Don explains how deadheading spent flowers will keep them blooming for as long as possible.
How to propagate cosmos
You can collect seeds from flowering cosmos and save them for sowing next year. Or if you're growing chocolate cosmos, divide the tubers when you lift the plants for storage over winter.
Cosmos problems
Cosmos are pretty much trouble-free to grow, apart from the threat of slugs and snails when the seedlings are young.
Great cosmos varieties to grow
- Cosmos Sea Shells – with pale, fluted petals – looking like a sea shell – it's a great planting partner for dark-flowered dahlias and chrysanthemums
- Cosmos bipinnatus Sonata White – a compact cosmos, good for growing in containers. Masses of pure-white, single, saucer-shaped flowers appear throughout summer
- Cosmos bipinnatus Fizzy Pink – with strong flower colours from white to carmine, this variety is good for cutting or growing in pots
- Cosmos 'Gazebo Red' – a medium-sized, bushy, early-flowering variety of cosmos, with large, velvety red blooms
- Cosmos atrosanguineus – the chocolate cosmos is a tender perennial from Mexico. The dark cocoa-coloured flowers have a faint scent of vanilla and chocolate. It grows from tubers, unlike other cosmos, and should be treated like a dahlia or canna lily, lifting to overwinter indoors or mulched heavily in autumn.
Frequently asked questions
My cosmos are very tall and sturdy, so why do they have no flowers?
If your cosmos are tall, strong and bushy but without blooms, they may have been overfed with nitrogen-based fertiliser or grown in soil that is too fertile. This can cause cosmos to grow foliage at the expense of flowers. Cosmos flower best when planted in full sun and in free-draining, average garden soil. Plants in shade will also struggle to flower. Pinching out cosmos plants should lead to more blooms.
When do I pinch out my cosmos?
The best time to pinch out the tops of cosmos seedlings is when they have grown three sets of true leaves (fern-like leaves). This will encourage plants to branch out and become bushier. It should also lead to more flower production.
My cosmos are leggy. How do I prick them out?
Don’t sow cosmos too early in the year, when sunlight is weak, and make sure they get plenty of light to avoid leggy growth. But if your cosmos seedlings have already become leggy, don’t worry. Just prick out and pot on into a slightly larger container, planting the seedlings a little deeper than they were. If you haven’t already pinched out the tops, do that now to encourage bushier growth.
