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 |
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, in contrast with masses of feathery foliage. Shorter varieties also look good in containers and all cosmos make fantastic cut flowers. The 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 well-drained soil in full sun. Deadhead to prolong flowering and lift in autumn, after the first frosts. Save seed from spent blooms to sow the following season.
More on growing cosmos:
- 15 colourful cosmos to grow
- Cosmos, canna and summer bedding container display
- Eight plants to grow with cosmos
Where to grow cosmos
Cosmos are native to Southern and Central America and therefore need to be planted in a warm, sunny spot. Soil should be free-draining.
How to plant cosmos
Sow seed in early spring directly into the soil where you want your cosmos 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 after the danger of frost has passed.
Watch Monty Don plant out cosmos for a late summer display:
How to care for cosmos
Once your seedlings have formed 2-3 pairs of leaves, you can 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 and fed with a liquid fertiliser. When deadheading, cut the stem back to the first leaf beneath the flowerhead.
The perennial chocolate cosmos varieties will need winter protection. Place them in pots until they've finished flowering, then shelter over winter in a frost-free place until spring.
Here, Monty Don explains how deadheading spent flowers will keep them blooming for as long as possible:
How to propagate cosmos
You can collect seed from flowering cosmos and save for next year. Or if you're growing chocolate cosmos, you can divide the tubers when you lift the plants for storage over the winter months.
Growing cosmos: problem solving
Cosmos are pretty much trouble-free to grow, apart from the threat of slugs and snails when seedlings are young and tender.
Great cosmos varieties to grow
- Cosmos 'Sea Shells' – with pale fluted petals, 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 through the 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, lifting to overwinter indoors, or mulched heavily in autumn
