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14 July 2021

How to grow cosmos

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

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

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.

Best Buy cosmos
What it looks like
Variety name
Height x spread
65 x 50cm
Slightly earlier to flower than many of the other varieties on test, this variety was a big hit with everyone at the test site. It impressed us with its vibrant dark-pink flowers with bold yellow centres that covered the plants from mid-July until early September. This variety is shorter than most of those we tried and we found that a bonus as it didn’t need any additional support. Add to this the uniform appearance, and being relatively untroubled by wind and rain means that it’s thoroughly deserving of being a Best Buy. Peak flowering: Jul-Aug
What it looks like
Variety name
Height x spread
130 x 60cm
A Best Buy in two previous trials, this variety put on an excellent show once again in this one. It’s a tall, elegant plant that needs a bit of support, topped with small pink and white flowers. We found some variation in flower colour even on the same plant – from very pale, almost white to a much deeper hue – but all were edged with a darker pink than the main petal colour and we thought this added to the overall floral impact. It was one of the longest flowering in our trial, giving us 11 weeks of blooms, and didn’t suffer at all in high winds or heavy rain. Peak flowering: Jul-Sep
What it looks like
Variety name
Height x spread
85 x 60cm
This relatively new introduction was the earliest and joint-longest flowering of all the varieties in our trial. The first blooms appeared in mid-June and kept on coming until the first week of October. The bright two-tone petals – white edged with pink around a yellow centre– were carried on short plants that needed only a little support and held up brilliantly to adverse weather. Some of the later flowers were paler in colour. Peak flowering: Jul-Sep
What it looks like
Variety name
Height x spread
100 x 45cm
This unusual variety has semi-double blooms with curled, slender, inner petals arranged in a whorl above the outer ones, creating a foamy appearance to a swathe of plants. The white petals, streaked and edged with dark pink-red, deepened in colour as the season progressed, adding to the impact of the display. The first blooms appeared in early July, reaching peak flowering later that month until September. It continued to produce new, but fewer, blooms until early October, standing up to wind and wet weather. Peak flowering: Jul-Sep

How we test cosmos

We selected 19 varieties of Cosmos bipinnatus to grow from seed. We included previous Best Buys and varieties introducedsince 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)