<em>Cosmos</em>: sowing and planting

Description and botany

Cosmos is an annual or perennial plant native to meadows and thickets of the southern United States and Mexico belonging to family Asteraceae like daisies, dahlias and asters.

Genus comprises about 25 species, but only three are cultivated in our gardens. Often regarded as an annual, Cosmos is in fact a frost-tender perennial plant. Cosmos are classified into two main groups: the Cosmos bipinnatus or bipinnate cosmos, the most widespread with large single white, pink or red flowers, and the Cosmos sulphureus with smaller flowers in warm yellow–orange shades.

Both groups have produced numerous cultivars such as ‘Picotée’ or ‘Cosmic Jaune’ and interesting series like ‘Sonata’, a set of dwarf cultivars with very compact growth, ‘Sensation’ which groups giant cultivars, ‘Lady bird’ with its feathery-leaved cultivars or the ‘Double Click’ group which gathers double or semi-double flower varieties.

Occasionally encountered is Cosmos atrosanguineum or “chocolate cosmos”, a tuberous perennial species whose flowers give off a cacao scent.

Cosmos quickly forms a dense clump, erect to spreading with a ramified, highly bushy habit. Depending on variety, height ranges from 30 cm for semi-dwarf cosmos up to 1.80 m for giant cosmos.

Sturdy stems are covered with delicate, finely divided foliage as finely cut as that of fern, dill or fennel. Deciduous leaves measuring 7 to 30 cm are arranged in opposite pairs, single, pinnatisect in Cosmos bipinnatus and Cosmos sulphureus or pinnate in Cosmos atrosanguineus.

Foliage ranges from light to dark green and is often slightly scented when crushed. Cultivars of the ‘Lady bird’ series display feathery foliage.

From May to November, Cosmos provides an abundance of delicate, simply elegant flowers that seem to float above an airy mound of foliage. In Greek, “Kosmos” means adornment, a nod to these timelessly graceful flowers.

Set in numbers at the tips of fine, well-branched stems, the large solitary heads measuring 2 to 9 cm in diameter appear in a broad cup. They consist of rounded or oval petals delicately fringed at the tip, gathered in a single, double or semi-double crown and radiating around a golden-yellow or dark brown-black centre. The translucent petals with a velvety texture are slightly plicate.

Shape of the ligules varies with cultivar, giving flowers looks that range from simple to more fanciful.

Some varieties feature very double pompom-like flowers, while others such as ‘Sea shell’ present astonishing blooms composed of tubular, undulate petals that lend a rather extravagant appearance to the flower.

Cosmos flowers vary by species in romantic pastel tones or brighter, sunnier colours: pure white, pale yellow, pink, purple or crimson in Cosmos bipinnatus and its cultivars, and shades from lemon yellow to scarlet, including orange in Cosmos sulphureus and its cultivars.

Cosmos vary in shapes and colours: Cosmos bipinnatus ‘Rose Bonbon’, Cosmos bipinnatus ‘Picotee’ (©Horticolor), Cosmos atrosanguineus ‘Chocamoca’, Cosmos bipinnatus ‘Xanthos’, Cosmos sulphureus ‘Cosmic Orange’

These corollas are often bicoloured, regularly striped and edged with darker hues.

While Cosmos flowers are usually unscented, those of Cosmos atrosanguineus emit gourmand chocolate–vanilla aromas.

Their flowers make chic, fresh or country-style bouquets and have very good vase life.

Another attraction: Cosmos flowers are edible and have a subtle floral flavour ideal for enhancing salads, decorating cakes and even sorbets and ice cream in summer!

This unusually generous, particularly melliferous flowering attracts many insects and butterflies throughout summer.

This annual is tireless, flowering for more than six months of the year: the light, poetic flowering continues until first frosts.

In the past, flowers of Cosmos sulphureus yielded a pigment used to dye textiles yellow–orange.

Flowers are much appreciated by pollinators and foliage is particularly decorative

Flowers are followed by the formation of bristly, blackish fruits, much appreciated by birds. Cosmos seeds self-sow easily from one year to the next in soil.

Cosmos is a non-hardy perennial plant grown as an annual in our regions. It is planted or sown in sun in any good garden soil not too rich, even poor and dry.

Cosmos is perfect for bringing colour and cheer to garden throughout the growing season with minimal maintenance. Essential in country-style settings, in natural gardens and romantic gardens, it adds lightness and poetry.

It fits everywhere: in summer perennial borders, to flower vegetable plots or a large, slightly wild meadow, dry rockeries, borders and even window boxes or pots on terrace or balcony.

Main species and varieties

Most popular

Our favourites

Other interesting varieties

Cosmos bipinnatus Sonata Light Pink

A particularly compact cosmos that fits anywhere. Adds charm and poetry to borders and even window boxes.

• Flowering time July to November
• Height at maturity 50 cm

See the product sheet

Cosmos sulphureus Mandarin

Its vibrant colour will bring a sunny touch to your garden! Smaller than other cosmos, it does well in tubs, beds and borders

• Flowering time July to December
• Height at maturity 30 cm

See the product sheet

Cosmos Sensation Purity Seeds - Cosmos bipinnatus

A recent variety with large single flowers of very pure white. Perfect for bringing freshness to a bed in summer.

• Flowering time July to November
• Height at maturity 1,20 m

See the product sheet

 

Cosmos atrosanguineus Chocamocha

A true chocolate plant! Its flowers give off, in warm weather, tempting aromas of vanilla cocoa. This tuberous cosmos, not very hardy, is grown like a dahlia. Stunning in containers or beds.

• Flowering time July to November
• Height at maturity 40 cm

See the product sheet

Cosmos bipinnatus Cosmos Double Click Mixed

A mix of large double or semi-double flowers in every colour. An impressive Cosmos suited to the back of beds.

• Flowering time July to November
• Height at maturity 95 cm

See the product sheet

Cosmos Sonata Carmin - Garden Cosmos

The earliest and one of the shortest! Ideal for growing in beds and large tubs.

• Flowering time July to November
• Height at maturity 60 cm

See the product sheet

Cosmos sulphureus Polidor - Sulfur Cosmos

An annual Cosmos variety with semi-double flowers in a striking colour. Early flowering and very long-lasting. Sow in sun to add a warm touch to a bed!

• Flowering time July to November
• Height at maturity 75 cm

See the product sheet

Cosmos Velouette Seeds - Cosmos bipinnatus

This new cosmos offers whimsical, vibrant flowers. In beds or on a bank, they attract attention.

• Flowering time July to November
• Height at maturity 85 cm

See the product sheet

Cosmos Sensation Picotee - Cosmos bipinnatus seeds

An annual variety with giant, delicately coloured flowers. Perfect in beds or borders.

• Flowering time August to October
• Height at maturity 90 cm

See the product sheet

Cosmos Sensation Radiance - Cosmos bipinnatus seeds

Loved for large single flowers and long, abundant flowering. Essential in romantic beds!

• Flowering time August to October
• Height at maturity 90 cm

See the product sheet

Cosmos Pied Piper Red Seeds - Cosmos bipinnatus

A surprising variety for the shape of its flowers and its unique colour: crimson velvet red. A touch of originality for beds and bouquets.

• Flowering time July to November
• Height at maturity 75 cm

See the product sheet

Cosmos Xanthos Seeds - Cosmos bipinnatus

A new compact cosmos, with flowers of a delightful yellow. Best reserved for borders and poetic window boxes!

• Flowering time July to November
• Height at maturity 60 cm

See the product sheet

When and how to plant cosmos

Where to plant Cosmos?

From its South American origins, Cosmos has retained a strong sensitivity to cold. It is a frost‑tender perennial that only withstands light frosts. As soon as first frosts arrive, Cosmos dies. That is why in our climates it is often grown as an annual in the garden.

One exception, however: the chocolate Cosmos, which is grown like a Dahlia — lift the tubercles before frosts and store them in a cool place.

Cosmos’s fragility is only apparent! This non‑hardy annual is very easy to grow across France and copes with drought once established. To flower well, Cosmos needs a sunny position.

Undemanding regarding soil, it is an all‑rounder that will adapt to any well‑drained soil that stays cool during summer, preferring poor, sandy or even stony soils. It will even flower in dry soil and will self‑sow easily there without becoming invasive.

Cosmos atrosanguineus prefers a lighter, more humus‑bearing and cool soil.

The taller varieties, with their slender silhouettes, may bend under severe weather. It is best to shelter them from prevailing winds or stake them if exposed to strong winds. Grouping plants can help them support one another. Smaller Cosmos tolerate windy spots better.

Versatile, Cosmos is perfectly suited to small gardens as well as large flowering meadows.

Cosmos’s endless flowering, from May to the first hard frosts, makes it an excellent choice to quickly adorn very sunny flowerbeds, borders or even pots and containers. Use it freely to fill empty spaces within a still‑sparse group of perennials.

Semi‑dwarf varieties, particularly compact, slip easily into borders, rockeries, among perennials and even into window boxes.

The largest cultivars create vibrant backdrops for flowerbeds, are planted in the second or third row of a border, dress a wall or form a small temporary hedgerow with a wild appearance.

Cosmos also has a place in the vegetable garden as it is suitable for cut flowers.

When to plant Cosmos?

Plant Cosmos in spring from April to June, once risk of frost is finally past and soil is warmed, for flowering from June to October.

How to plant Cosmos from plug plants?

In ground

Upon receipt, pot up and keep our annual Cosmos plug plants under cover at temperatures above 14°C for a few weeks. As soon as risk of frost is past, plant them out in the ground or move pots into the sun.

Plant roughly 4 to 9 plants per m2 spaced 40 to 50 cm in all directions for tall Cosmos, 20 to 30 cm for low varieties, if you want an effect of density and profusion.

Soil should be moderately fertile but well‑drained. See our advice sheet to plant annuals in plug plants.

• Loosen soil to depth of a spade
• Dig a hole three times wider than the plug plant
• Make a bed of gravel beneath each plug in the planting hole
• Plant in slightly sandy garden soil
• Place the plug in the centre of the hole, collar level with the soil
• Cover with soil
• Firm down with foot
• Water and mulch
• Stake tall varieties if necessary

In pots

Small, compact Cosmos varieties are perfectly suited to pot culture. Plant one or two plants per 50 cm diameter pot depending on variety.

• Spread a good drainage layer (gravel or clay pebbles) at bottom of container
• Plant in a mix of garden soil and sand
• Place in sun
• Water very regularly for extended flowering

→ Find out more in our article Growing Cosmos in a pot.

The famous Cosmos Chocolat accompanied by Buenos Aires Verbena – photo Ibulb

Sow Cosmos seeds

Allow between 80 and 90 days between sowing and flowering for Cosmos. Cosmos seeds sown at the very start of spring flower until late in season.

Sowing under cover

• In March–April, sow 1 or 2 Cosmos seeds in a tray at a depth of 3 mm in a light compost
• Cover seeds lightly
• Place at 18–25 °C
• Keep soil moist until germination
• Keep only strongest seedlings and prick out into buckets
• When risk of frost has passed, acclimatise young plants gradually to outdoor conditions
• Plant out after frosts, spacing young plants about 30–45 cm apart or in a planter
• Pinch out tip of each stem to encourage branching

Direct sowing

Sow Cosmos seeds in May after last frosts in ordinary garden soil mixed with compost. Note, seeds are quite fine: sow by scattering for natural look. Discover all our tips for how to sow annual seeds directly in the ground or in trays.

• Fork over soil well
• Add river sand to lighten if necessary
• Sow by scattering
• Rake seeds in
• Water regularly until germination
• When seedlings reach 10 cm, thin out to leave only one seedling every 20–45 cm, depending on variety

Maintenance, pruning and care

Cosmos is a short-lived perennial that is really very easy to grow and undemanding. Once well established in full sun and well-drained soil, it grows on its own.

To prolong flowering, remove faded flowers regularly. Pinching the stems also encourages the plant to ramify and to produce more flowers.

Although it tolerates short periods of drought fairly well, it appreciates soil that remains cool: as soon as dry weather sets in, water regularly.

You can mulch around the base to keep it sufficiently cool in summer.

It requires no fertiliser.

In situations too exposed to wind, tall Cosmos may need to be staked to prevent them from being blown over.

Pull up clumps at the start of winter.

In cold regions, before frosts, lift the tubercles of Cosmos atrosanguineus or “chocolate cosmos” and overwinter them in pots in a cool frost-free place.

Potential diseases and pests

When seedlings emerge, it is wise to protect young shoots of cosmos from slugs and snails: discover our 7 ways to tackle slugs effectively and naturally and how to make a slug trap.

Young plants can be infested with aphids: spray with water containing 5% black soap.

Cosmos is also susceptible to powdery mildew. If affected, follow our advice for dealing with powdery mildew.

More worryingly, Cosmos may be prone to botrytis disease (grey mould) which attacks roots. Avoid soils that are too heavy and compact as they retain moisture.

Multiplication

You can harvest seeds before they disperse and follow our advice to sow cosmos seeds properly. However, cosmos usually self-seed from year to year. These spontaneous sowings are not always true to type. ‘Cosmos chocolat’ can, however, be propagated by dividing tubercles in autumn.

Companion planting with cosmos

With its airy, elegant silhouette and simple flowers, Cosmos is ideal in a naturalistic garden, in cottage gardens, in country gardens for composing wild, romantic-looking scenes with ease. Cosmos is essential in a pink garden or in a white garden.

An example of pairing: Cosmos Sonata white, Giant dahlia ‘Café au Lait’, Cactus dahlia ‘My Love’, Cynara cardunculus, Salvia farinacea ‘Victoria’

Capable of filling a border on its own, Cosmos is also easy to pair with many summer flowers equally undemanding.

Cosmos brings density to borders and lightness to heavier-flowering perennials.

To create a sunny naturalistic meadow atmosphere, make a cheerful mix of cosmos, yarrow, poppies, large poppies, annual grasses such as foxtail barley, centaureas, delphiniums, Nigellas, flax and scabious.

This plant adds density to borders and lightness to heavier summer perennials such as sunflowers, heleniums and cannas.

Another example of pairing: Cosmos bipinnatus ‘Purity’, Salvia farinacea ‘Blue Plus’, Zinnia elegans ‘Purple Prince’, Dahlia pompom ‘Red Fox’, White mustard (©MAP Arnaud Descat – Jardin de Saint-Jean-de-Beauregard)

Cosmos’s long flowering period allows it to accompany many flowers from spring to autumn, such as Alchemilla mollis, June or September roses and clumps of Perovskia.

To enliven a border that is a little too tame and flat, pair Cosmos with tall, airy plants such as fraxinellas, Argentinian verbena, Gauras, Crocosmias, and other sun-loving daisies such as Echinaceas.

An idea for pairing: Euphorbia (E. wallichii, E. martinii or variety of your choice), Astrantia (‘Ruby Wedding’, ‘Ruby Star’, ‘Lars’…), Cosmos atrosanguineus, Achillea millefolium ‘Terracotta’

Play shape contrasts with grasses such as Stipa tenuifolia, Pennisetum.

In a sunny border with bright colours, mix small cosmos with annual rudbeckias. All cosmos have a fairly bare base and are best planted behind lower-growing plants such as dianthus, hardy geraniums or sages.

In a summer pink border, pair a few cosmos plants with dwarf dahlias, summer-flowering bushes such as buddleias and annual lavateras. Soften with a large clump of airy Artemisia: festival of blooms guaranteed all summer!

You can pair it with other annuals, such as Coreopsis, Cleomes and Bidens for a colourful display.

→ Discover 6 more beautiful pairing ideas with Cosmos in our advice sheet!