How to grow cosmos and best varieties

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.

Make more of your garden - get our free Gardening newsletter for top tips from our experts

Key facts

PLANT TYPE Half-hardy annual

POSITION Full sun

SOIL Moist but well-drained

How to grow cosmos: month by month

Best cosmos varieties

Which members can log in now to see the full results and which are our Best Buy varieties. If you're not a member, join Which? to get instant access.

Full testing results for cosmos

_{USING THE TABLE: 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%.}

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 2020, 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.

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.

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.

Supporting your plant

Some cosmos can grow very tall and need support; tie stems to a cane pushed into the ground. Deadheading will keep plants looking tidy and keep the flowers coming for longer. Snip off spent blooms before they set seed 

Deadheading

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

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.

Read more about grey mould (botrytis)