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08 Jan 2024

How to grow cosmos and best varieties

Cosmos are easy to grow from seed for colourful flowers all summer. Discover our best cosmos varieties and tips for how to grow them.

Ceri ThomasEditor, Which? Gardening

In this article

Key facts
How to grow cosmos: month by month
Best cosmos varieties
Full testing results for cosmos
Sowing
Caring for your plants
Common growing problems
How we test cosmos

Bearing masses of daisy-like flowers atop feathery foliage, cosmos are a wonderful addition to summer borders, where they should give you weeks of trouble-free colour. They’re easy to grow from seed and make good cut flowers.

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Key facts

PLANT TYPE Half-hardy annual

POSITION Full sun

SOIL Moist but well-drained

How to grow cosmos: month by month

January	February	March	April	May	June
			SOW	SOW/PLANT	PLANT
July	August	September	October	November	December
FLOWERING	FLOWERING	FLOWERING

Best cosmos varieties

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Full testing results for cosmos

Variety name	Overall rating	Height x spread (cm)	Germination	Flowering duration	Impact of display	Weather tolerance
'Apollo Carmine'
'Apollo White'
'Apricot Lemonade'
'Candy Stripe'
'Capriola'
'Cupcakes Blush'
'Daydream'

_{The more stars the better. Overall rating: Score ignores price and is based on: flowering duration 50%; impact of display 40%; germination 5%; weather tolerance 5%.}

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Sowing

Sow under cover in April in modules of a Best Buy compost for seed sowing, covering the seeds with a thin layer of vermiculite or sieved compost. Keep moist with a constant temperature of around 20°C. Pot on once large enough to handle. Cosmos are fast growing, so don’t sow too early or they could outgrow their pots before the weather is warm enough for planting out.

Identify your plants with Best Buy plastic-free labels

Caring for your plants

Planting

Harden off and plant outside in a warm sunny spot once all risk of frost has passed in mid to late May.

Help your plants with a recommended soil improver

Supporting your plant

Some cosmos can grow very tall and need support; tie stems to a cane pushed into the ground.

Deadheading

Deadheading will keep plants looking tidy and keep the flowers coming for longer. Snip off spent blooms before they set seed.

Discover our Best Buy secateurs

Common growing problems

Grey mould

Cosmos are generally trouble free, but they can be affected by grey moulds (botrytis) and mildew. Make sure you leave enough space between plants to allow good air flow and keep mould at bay.

How we test cosmos

The Which? Gardening magazine researchers selected 19 varieties of Cosmos bipinnatus to grow from seed. We included previous Best Buys and varieties introduced since our last trial.

In late April, we sowed seed of each variety in module trays in the polytunnel at our trial site in Cambridgeshire.

The seedlings were pricked out into 7cm pots in mid-May and planted out at the end of the month after hardening off.

We planted our cosmos in blocks and used sprinkler irrigation to help them establish. We used minimal support in the form of string and canes between the plots to separate the varieties from one another.

We assessed our plants throughout summer for how well they flowered, whether the blooms were true to their descriptions and how well they stood up to bad weather.

We deadheaded all of our plants two or three times a week to keep the flowers coming.