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

Description and botany

The 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.

The genus comprises about 25 species, but only three are grown in our gardens. Often regarded as annual, Cosmos is actually a frost-tender perennial plant. Cosmos are grouped into two main varieties: the Cosmos bipinnatus or bipinnate cosmos, the most common, with large single white, pink or red flowers, and the Cosmos sulphureus with smaller flowers in warm yellow–orange tones.

Both have given rise to numerous cultivars such as ‘Picotée’ or ‘Cosmic Jaune’ and to interesting series like ‘Sonata’, a group of dwarf cultivars with very compact vegetation, ‘Sensation’ which gathers giant cultivars, ‘Lady bird’ with cultivars having feathery foliage, and the ‘Double Click’ group which includes varieties with double or semi-double flowers.

Occasionally encountered is Cosmos atrosanguineus or “chocolate cosmos”, a tuberous perennial species whose flowers give off a cocoa-like scent.

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

Stiff stems are clothed with an elegant, finely cut light foliage as delicate as that of a fern, dill or fennel. Deciduous leaves measuring 7 to 30 cm are arranged in opposite pairs, simple, pinnatisect in C. bipinnatus and C. sulphureus, or pinnate in Cosmos atrosanguineus.

Ranging from light to dark green, the foliage is often slightly fragrant when crushed. Cultivars of the ‘Lady bird’ series show particularly feathery foliage.

From May to November, Cosmos produces an abundance of simple, wonderfully refined flowers that seem to float above an airy clump of foliage. In Greek, “Kosmos” means ornament, a nod to these timelessly graceful flowers.

Set in groups at the tips of finely ramified stems, the large solitary heads measuring 2 to 9 cm in diameter appear in wide cups. They are made up of rounded or oval petals delicately fringed at the tip, gathered into a simple, double or semi-double corolla and radiating around a golden-yellow or dark-brown centre. The translucent petals with a velvety texture are slightly plicate.

The shape of the ligules varies with cultivar, giving flowers a look ranging from simple to more fanciful.

Some varieties are characterised by very double, pompon-like flowers, while others such as Cosmos ‘Sea shell’ of the Shell type present surprising flowers made of tubed and undulate petals that lend a rather extravagant appearance.

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

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

These corollas are often bicoloured, frequently striped and edged with a darker shade.

While Cosmos flowers are usually unperfumed, those of Cosmos atrosanguineus give off gourmand notes of vanilla-chocolate.

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

Another attraction: Cosmos flowers are edible, with a subtle floral flavour ideal for garnishing salads, decorating cakes and summer sorbets or ice creams!

This exceptionally generous, particularly melliferous flowering attracts many insects and butterflies all summer long.

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

In the past, flowers of Cosmos sulphureus were used to produce a pigment for dyeing textiles yellow–orange.

Flowers are much loved by pollinators and foliage is especially decorative

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

Cosmos is a non-hardy perennial plant that is cultivated 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 season with minimal maintenance. Essential in country-style settings, in natural gardens and romantic gardens, it adds lightness and poetry.

It slips everywhere into summer perennial borders, to flower vegetable patches 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. It will bring much charm and poetry to borders and even planters.

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

See the product sheet

Cosmos sulphureus Mandarin

Its brilliant colour will bring a sunny touch to your garden! Smaller than other cosmos, it suits containers, 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

It’s a true chocolate plant! Its flowers give off rich vanilla-cocoa aromas in warm weather. This tuberous but not very hardy cosmos is grown like a dahlia. Magnificent in pots 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 well 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

Earliest and one of the shortest! Ideal for growing in beds and large containers.

• 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. Its early flowering is very long. 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 produces whimsical, vibrant flowers. In beds or on a slope, they catch every eye.

• 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

We love its large single flowers and long, abundant flowering. Essential in a romantic bed!

• 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 its flower shape and unique colour: crimson velvet red. A touch of originality for your 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 deliciously yellow flowers. Best reserved for borders and poetic planters!

• 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. Cosmos dies at first frosts. That is why in our climates it is often grown as an annual in the garden.

One exception: Chocolate Cosmos is treated like a Dahlia: lift tubercles before frosts and store in a cool place.

Fragility of Cosmos is only apparent! This non‑hardy annual is very easy to grow throughout France and withstands drought once established. For good flowering, Cosmos needs a sunny position.

Undemanding about soil, it is an all‑rounder that will suit any well‑drained soil that remains cool during summer, preferring poor, sandy or even stony soils. It will even flower in dry soil and will self‑seed easily without becoming invasive.

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

Tall varieties, with their delicate habit, may flop in bad weather. Better to shelter them from prevailing winds or stake them if site is exposed. Grouping plants can help them support each other. Smaller Cosmos suit windy spots better.

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

Endless flowering of Cosmos, from May to first frosts, makes it an ideal choice to quickly adorn very sunny beds, borders or pots and urns. Use freely to fill gaps in a patch of perennials that is still sparse.

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

Tallest cultivars form vibrant backdrops for borders, are planted in second or third rank of a border, dress a wall or a small temporary hedge with a wild appearance.

Cosmos also has a place in the vegetable garden and 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 open ground

On receipt, pot up and keep our annual Cosmos plug plants under cover at a temperature above 14°C for a few weeks. As soon as risk of frost is past, plant out in open ground or place pots in sun.

Plant about 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 a dense, abundant effect.

Soil should be moderately fertile but well drained. Use our advice sheet to plant annuals from plug plants correctly.

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

In pots

Small, compact Cosmos varieties are well suited to container growing. Plant one or two young plants per pot of 50 cm diameter depending on variety.

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

→ Find out more in our article Grow a cosmos in a pot.

Famous Chocolate Cosmos with Buenos Aires Verbena – photo Ibulb

Sow Cosmos seeds

Expect between 80 and 90 days from sowing to flowering for Cosmos. Cosmos seeds sown at very start of spring continue flowering until late in season.

Sowing under cover

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

Sowing directly in place

Sow Cosmos seeds in May after last frosts in ordinary garden soil mixed with potting compost. Be careful, seeds are quite fine: sow broadcast for a natural look. Discover all our tips to sow annual seeds directly in ground or in trays.

• Fork over soil well
• Add river sand to lighten soil if needed
• Sow broadcast
• Cover seeds using a rake
• Water regularly until seedlings emerge
• When seedlings reach 10 cm, thin out to leave only one plant every 20–45 cm depending on variety

Maintenance, pruning and care

Le Cosmos is a short-lived perennial that is very easy to grow and undemanding. Once well established in full sun and in well-drained soil, it grows by itself.

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

Although it tolerates short dry spells quite well, Cosmos prefers soils that remain cool: as soon as dry weather sets in, water regularly.

You can mulch around the base to ensure sufficient coolness in summer.

Requires no fertiliser.

In sites too exposed to wind, tall Cosmos may need staking to prevent them from being blown over.

Pull up clumps at onset of winter.

In cold regions, before frosts, lift tubercles of Cosmos atrosanguineus or “chocolate cosmos” and overwinter them potted 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 effective, natural ways to combat slugs and how to make a slug trap.

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

Cosmos is also susceptible to powdery mildew. If affected, follow our advice to control powdery mildew.

More troublesome, cosmos can be prone to botrytis disease (grey mould) which attacks roots. Avoid soils that are too heavy and compact and that retain moisture.

Propagation

You can collect seeds before they disperse and follow our advice to sow cosmos seeds successfully. However, cosmos usually self-seed from one year to the next. These spontaneous seedlings are not always true to type. “Cosmos chocolat” can be propagated by division of tubercles in autumn.

Pairing cosmos in the garden

With its airy, elegant silhouette and simple flowers, the Cosmos is ideal in a naturalistic-inspired garden, in cottage gardens, in country gardens to compose wild and romantic scenes, with complete simplicity. It is a must in a pink garden or in a white garden.

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

Able to occupy a whole bed on its own, Cosmos is also easy to combine with many other summer flowers that are just as undemanding.

Cosmos adds density to borders and lightness to the heavier flower displays of perennials.

To create a sunny naturalistic meadow, make a joyful mix of cosmos, Achillea millefolium, poppies, large poppies, annual grasses such as foxtail barley, centaureas, delphiniums or even Nigellas, flaxes and Scabiosas.

This plant brings a lot of density to beds and lightness to the heavier summer flowering of some perennials such as sunflowers, Helenium or cannas.

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

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

To wake up a border that is a bit too tame and flat, pair Cosmos with tall, airy plants such as Dictamnus, Buenos Aires verbena, Gauras, Crocosmias, and other sun-loving daisies such as Echinaceas.

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

Play contrasts of shape with grasses such as Stipa tenuifolia and 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 some cosmos with dwarf dahlias, with summer-flowering shrubs such as buddleias and annual lavateras. Temper the display with a large airy clump of Artemisia: guaranteed festival of flowering all summer!

You can also combine it with other annuals, such as Coreopsis, cleomes and bidens for a colour-packed display.

→ Discover 6 other great pairing ideas with Cosmos in our advice sheet!