The best cosmos varieties to grow
Cosmos are as brilliant in the garden as they are in the vase. Plant expert Graham Rice chooses his favourite cosmos varieties to grow, and gives advice on sowing and growing them. Photographs Jason Ingram
For too long, annuals have been looked down upon by some gardeners as not in the same league as perennials. “Oh, it’s just an annual,” people say dismissively. Well, a recent trial of almost 90 annual cosmos at the RHS Garden Wisley made it clear what valuable plants they are.
There are about 30 species of cosmos, annuals and perennials, growing wild across subtropical America, and in Mexico in particular; two species extend their range into southern USA. The familiar garden cosmos, Cosmos bipinnatus, grows naturally in rough meadows and scrub in Mexico, but has also established itself farther afield, including much of southern and eastern USA as well as South Africa and Australia.
The only other cosmos species widely grown is the perennial Cosmos sulphureus, with orange or yellow flowers, usually grown as an annual, and the popular and tender tuberous perennial Cosmos atrosanguineus, with its chocolate-coloured and chocolate-scented daisy-like flowers.
C. bipinnatus is a vigorous annual reaching from 60cm to 3m or more, with stout upright stems carrying large, attractive, fresh-green foliage repeatedly divided into thread-like segments. From midsummer into autumn, large golden- centred daisies open in a wide range of pink and rich-red shades with white. 2017 saw the introduction of the first effective yellow-flowered variants.
One of the most intriguing features of C. bipinnatus is its range of flower forms from fully double, to flowers with quilled petals and various semi-double forms as well as singles. The petals may be in pure colours or edged or streaked in darker shades or feature rings around the golden eye, creating some striking bi-colours. These features make them unusually interesting both in
the garden and in a vase, but there’s an unexpected factor to keep in mind.
Cosmos seed is not a high-value crop, so time is rarely spent ensuring top quality in plants grown from seed of older cultivars that do not attract high prices. This was borne out in the Wisley display, where old cultivars, such as ‘Sensation Mixed’, ‘Purity’, ‘Sea Shells’, ‘Dazzler’, ‘Daydream’ and ‘Versailles Tetra’, flowered late and sparsely, often on uncomfortably tall plants. I would not recommend them and instead would look at new and recent introductions where a slightly higher price translates into better quality. The one group where this approach fails is the double-flowered Double Click Series, where not one cultivar consistently produces double flowers.
The British-bred Apollo Series in carmine, white and pink is the best choice for containers. ‘Sea Shells’, with each petal rolled into a tube, is striking and especially good in vases, but on the Wisley evidence it deteriorated badly; try the shorter Hummingbird Series instead.
Recent introductions also bring us new colours and patterns. Single-flowered, single-coloured ‘Rubenza’ in deep red, and ‘Xanthos’ and ‘Lemonade’ in lemon yellow, stand out. Of the striped types, ‘Velouette’ in white and dark red is dramatic. ‘Capriola’ is my favourite, with white petals edged in pink and prolific flowers on 1.2m plants. Along with others in softer colours, it is ideal among shrub roses or perennials; taller types are good at the back of beds or as a temporary screen.
So don’t dismiss these easy, productive flowers just because they are annuals; they really are worth growing. The best varities are usually only available as seeds.
When to plant cosmos seeds
Sow cosmos seeds in pots in April, in a greenhouse or on a sunny windowsill. Transplant the young seedlings to individual 7cm pots and then plant outside in late May or June. Cosmos can be treated like hardy annuals and sown directly outside where they are to flower, but sowing outside should be delayed until the middle of May.
Once the seedlings have formed two or three pairs of leaves, pinch out the growing tips to produce more bushy plants that will go on to produce more flowers.
How to grow cosmos
Grow cosmos in a warm, sunny and sheltered spot, in well drained soil.
Before planting out, acclimatise plants to the outdoors by hardening them off first. Space the plants 30-60cm apart, depending on their eventual height. Support may well be necessary for these plants: smaller cultivars can be supported with brushwood, while taller cultivars need the more solid support of canes and twine. Mildew may prove a problem if the plants are not thinned out or if they are grown in very sheltered situations; free air movement is the best preventative measure in this case.
Caring for cosmos
Regular deadheading is crucial to maintaining a continuing display of flowers into September and even October. Both Cosmos bipinnatus and Cosmos sulphureus are good as cut flowers; pick them when the petals are starting to open (but are not fully open) and change the water every day. Feed cosmos growing in pots with a liquid feed every two weeks.
The best cosmos varieties to grow in 2022
Cosmos bipinnatus 'Antiquity'
A shorter, newcomer cosmos variety. The unusual and beautiful single flowers fade from a beautiful rich red to paler ‘antique’ tones as they age, creating a two-tone effect and with a variety of flower colours at any one time on the plant. As Cosmos bipinnatus 'Antiquity' it's a shorter variety, it doesn't need staking and is ideal for both borders and containers. Deadhead when the flowers begin to look ragged, or cut regularly for cut flowers – the blooms should last for 10 days in a vase. A beautiful and easy flower to grow from seed.
Cosmos bipinnatus 'Cupcakes White'
For something a bit different, Cosmos bipinnatus 'Cupcakes White' has unusual, cupped flowers with pure white petals that are fused into large bowl-shaped flowers that are reminiscent of paper cupcake cases. Some of the flowers may also be pink-tinted or may have split or semi-double flowers. 'Cupcakes White' was Voted 'People’s Favourite' by the visitors to the cosmos trial at Wisley. Grow Cosmos bipinnatus 'Cupcakes White' in a border or in a container. ‘Cupcakes Mixed’ has a balance of white and pink shades.
Cosmos bipinnatus 'Candy Stripe'
Tall, rather leafy cosmos plants with fresh-green foliage carry single, long-stemmed white flowers, each vividly edged with bright carmine. Some flowers are almost fully white, the variability in the markings adds to the appeal.
Cosmos bipinnatus 'Double Click Rose Bonbon'
Double-flowered cosmos forms can be disappointing. This is the pick from Wisley because although there were very few doubles, the singles and fluted flowers matched perfectly in colour to create a fascinating, rosy-pink blend.
Cosmos bipinnatus 'Rubenza'
Deep-red, scarlet or deep-pink single cosmos flowers on tall but prolific plants. Rich colouring and good with silver foliage, the flowers age harmoniously but eventually need regular deadheading.
Cosmos bipinnatus 'Xanthos'
Very pretty cosmos that is a Fleuroselect Gold Medal winner. The small, mostly single, primrose-yellow flowers have a neat, white ring around the golden eye; a few flowers have extra petals and there are a few white flowers.
Cosmos bipinnatus 'Cosimo Red-White'
Like a shorter version of ‘Candy Stripe’, the striking plants are best early and mid-season, while the compact foliage allows the flowers to bloom unhidden. Cruder in colour than the softer, but taller, Cosmos ‘Capriola’.
Cosmos bipinnatus 'Apollo Carmine'
One of the best cosmos in the Wisley display, short plants carry rich, carmine single flowers that hold their colour well and fade harmoniously into pinkish tones. Good flower shape and size, ideal in large containers.
Cosmos bipinnatus 'Apollo Pink'
Nicely rippled, rose-pink single cosmos flowers face upward from uniformly neat plants, the colour hardly fading before the petals start to drop. An improvement on the similar, but now unacceptably variable, ‘Sonata Pink’.
Cosmos bipinnatus 'Apollo White'
Large, flat, upward-facing, pure-white flowers on plants of uniform height, Cosmos ‘Apollo White’ flowers consistently from June until well into the autumn. The Apollo Series is the new standard in short cosmos.
Cosmos sulphureus 'Brightness Red'
Semi-double flowers in vivid reddish orange, with a few orange-flowered plants mixed in. Flowers are sparse at first but prolific later on, and open at various heights on the plant, not only at the top. Seeds are currently hard to track down – it is mostly available as part of a mix.
Cosmos bipinnatus 'Capriola'
My favourite, the white flowers are exquisite with their unique pattern: each petal on this cosmos is boldly tipped and edged in magenta pink with a similar ring around the eye. Sadly, it can be variable, and there are some ‘off’ types.
Cosmos bipinnatus 'Veloutte'
Perhaps the most dramatic cultivar, white flowers are tipped and streaked in reddish carmine, though some flowers are almost entirely white and some completely carmine. Eventually this cosmos fades to parched pink, when deadheading is necessary.
Cosmos bipinnatus 'Rosetta'
A new medium-height cultivar with large, mainly semi-double flowers striped in lavender pink and blushed white. Very pretty, early, long-flowering and prolific cosmos, but, strangely, its flowers all face in different directions.
Cosmos bipinnatus 'Versailles Dark Rose'
The best of the tallest types of cosmos; the large, single, rich-rose flowers have a dark-red ring around the golden eye. Too tall and leafy for many gardens, but good at the back of the border and for cutting. It is currently hard to track down seed.
Cosmos sulphureus 'Carioca'
Wonderful, glowing-orange single cosmos flowers on plants with appealingly open growth, slow to start but prolific later. Lovely interplanted among dark-leaved cannas or dahlias for contrast in habit and colour. It is currently hard to track down seed.
You can find more information on hardiness ratings here.
Want to grow more flowers from seed? Discover the best sunflowers to grow from seed.
Graham Rice is a garden writer and plantsman, who is editor-in-chief of the RHS Encyclopedia of Perennials. He writes for a wide range of newspapers and magazines including The Garden and The Plant Review and is a member of the RHS Herbaceous Plant Committee and Floral Trials Committee.
Jason Ingram is an award winning garden photographer based in Bristol, UK. He travels widely shooting for magazines, book publishers and advertising agencies. He also works with top international garden designers and Landscape Architects on private projects worldwide.
Summer sale! 3 issues for £5
Try 3 issues of Gardens Illustrated today for just £5.
Subscribe today and enjoy beautiful places, top planting ideas and expert advice for your garden.
*Offer ends 1st September 2022. This offer is only available to UK residents paying by Direct Debit.
Container Gardening Special Edition
The Gardens Illustrated Guide to Container Gardening.
In this special edition, discover colourful flower combinations and seasonal planting schemes for pots designed by leading plantspeople, and essential know-how for container gardening success.
Just £9.99 inc UK p&p