How to grow cosmos

Cosmos flower non-stop in summer and autumn, providing food for wildlife. Hazel Sillver looks at the history of these easy annuals and perennials, as well as the best ones to grow and how

Cosmos bipinnatus ‘Fizzy Pink’'

Sabina Ruber
  • Common names: cosmos, cosmea, Mexican aster
  • Botanical name: Cosmos
  • Family: Daisy (Asteraceae)
  • Type: Half-hardy annuals and tuberous perennials
  • Flowering time: Summer and autumn
  • Sowing and planting season: Spring and early summer
  • Height: 30cm-1.2m (1-4ft)
  • Spread: 30cm-60cm (1-2ft)
  • Aspect: Full sun
  • Hardiness: H3
  • Difficulty: Easy to average

Possessing incredible flower power, annual cosmos produce a glut of bloom from June until the first frosts, making them a joy to grow. Resembling large daisies, they are friendly looking plants that children warm to, and they open in shades of white, yellow, orange, pink, and red, above gorgeous apple-green ferny foliage. In recent years, they have enjoyed a surge in popularity because they are easy to grow as cut flowers. Like sweet peas, they need to be harvested regularly to maintain their flower production, and they last well in the vase.

Another reason for their vogue is the flowers’ high nectar content. ‘Cosmos have been long-overlooked as plants for insects,’ writes bee expert Dave Goulson in his book Gardening for Bumblebees. ‘They appear on few lists of pollinator-friendly plants, but I find them to be well worth growing, drawing in a broad range of short-tongued bumblebees and hoverflies.’ He is referring largely to the annual Cosmos bipinnatus, which is easy to grow from seed and increasingly available as a pollinator-friendly bedding plant at nurseries and garden centres. During summer and autumn, it hums with the sound of foraging bees.

Most of the 35 species of cosmos hail from Mexico (including C. bipinnatus), but the natural range of the genus extends up into North America and down into South America. C. bipinnatus has been introduced into and naturalized across parts of Europe, Asia, Australia, and Africa. In the 16th century, Spanish missionary priests, who had been working in Mexico, brought the flower back to Europe. They named it for its perfect circle of petals around a central disc of stamens, which – to them – represented the perfect haloed structure of the universe. The Greek word kosmos means ‘orderly arrangement or good order’, and the philosopher Pythagoras applied it to the order of the universe. Although modern science has established that the universe has no centre, in the 16th century, it was believed that the centre of all things was either the earth (following Anaximander’s theory) or the sun (in line with Aristarchus’s model). Geographers such as Petrus Apianus drew the universe as a series of geocentric circular layers, while mathematician Nicolaus Copernicus produced a similar heliocentric image. Both are reminiscent of a C. bipinnatus flower, although the latter – having the sun (rather than the earth) at its centre – appears to have been the inspiration for the naming of the plant, since its centre is yellow like the sun.

C. bipinnatus has been used as a traditional herbal remedy in Mexico and Africa, and research shows that the plant has significant antibacterial and antioxidant properties, giving it potential as a modern medicine. It also has a solid future as an eco-friendly ingredient in cow feed, since it reduces their methane output by 26%.

Which cosmos to grow

Cosmos bipinnatus 'Purity'

The cosmos that people are most familiar with is the half-hardy annual Cosmos bipinnatus, which is available in a range of white, peach, pink, and red varieties. Of the taller forms that reach 90 centimetres or more, the most beautiful is ‘Purity’. Its saucer-shaped white flowers with golden button centres can reach 8 centimetres across and are produced in succession, above lovely, ferny green foliage, sometimes from June into November; a generous vaseful of nothing else is a delight and will look good for over a week. Other long-legged gems include ‘Dazzler’ (dark raspberry), ‘Velouette’ (carmine and white), and ‘Apricotta’ (which won a Fleuroselect Award last year for its pink and peach blooms). The Sea Shells mix has interesting fluted petals, and ‘Double Click Cranberries’ is a fabulous wine-coloured double form. There are also more compact hybrids, reaching around 60 centimetres, making them ideal for pots. Two of the best are ‘Rubenza’ (a velvet red that fades to rose) and ‘Xanthos’ (a charming pale yellow).

C. sulphureus is another half-hardy annual cosmos that is well worth growing for its fiery blooms that are reminiscent of geum flowers. ‘Cosmic Red’ is scarlet, and ‘Sunset Orange’ is the colour of fire. Both are short plants, while Bright Lights is a mix of yellow, pumpkin, and red flowers atop 90-centimetre stems.

Perennial cosmos don’t have much in common with the annuals, since they grow from tubers. The most well-known is the chocolate cosmos (C. atrosanguineus), with its velvet maroon daisies that look sophisticated in the border and pair well with grasses. Depending on your nose, the flowers either smell of chocolate or raw meat. The species reaches around 60 centimetres and is widely available, but there are now superior forms on the market that are worth hunting down. They include Eclipse, which has 5-centimetre rich-burgundy flowers on tall stems, and Spellbound, which produces fragrant crimson-red blooms on 90-centimere stems. The compact forms Chocamocha and Dark Secret reach around 50 centimetres. There is also a chocolate cosmos called ‘Black Magic’ that can be raised from seed, but its flowers vary in colour and shape.

Another perennial cosmos worth seeking out is C. peucedanifolius, which resembles a sugar-pink annual C. bipinnatus. It grows from tubers, but – like Black Magic – can also be grown from seed.

How to sow cosmos

Being half-hardy annuals, cosmos are best sown under cover in early spring. Sow in March in module trays or coir jiffy pellets, only lightly covering the seed with compost. They enjoy a temperature of 18-23°C to germinate, but can be in slightly cooler conditions after that. Once they start to grow strongly, pinch out the tips to create a bushier plant. When the weather warms in late spring, plant the seedlings outside. Alternatively, cosmos can be sown direct outside in May or June, in well-drained soil in full sun; however, the plants tend to be healthier if sown in early spring.

The few perennial cosmos that can be grown from seed (including Black Magic) can be sown in a similar way: either under cover in early spring or direct outside once the weather warms in late spring or early summer.

How to grow cosmos

Garden centres increasingly sell annual cosmos as wildlife-friendly bedding plants, if you don’t want to sow your own. Plant them out in May or June, in a sunny border or in well-drained containers of peat-free compost.

If you are growing a tall variety of annual cosmos (such as ‘Purity’ or ‘Dazzler’), it is well worth erecting a support structure around the plants, once they start to bulk up. This will prevent them being flattened by wind, rain, footballs, and cats, once they have reached their full height. It can be made from bamboo or hazel sticks and a taut sheet of pea netting that the plants will gradually grow up through and conceal.

Treat the tuberous perennial cosmos as you would a dahlia: plant outside after the risk of frost has passed; then, towards the end of autumn, give the tubers some protection from the winter weather. In milder regions, such as southern England, this can be a bucketful of mulch over the soil; in colder areas, they ought to be lifted and stored indoors in a dry, frost-free place.

Deadhead cosmos to ensure continuous bloom throughout summer and into autumn. They have a cut-and-come-again habit, so harvesting them regularly for the vase will help to maintain flower production.

Cosmos pests and diseases

Cosmos occasionally suffer from fungal diseases, including powdery mildew and Fusarium wilt. Prevent mildew with air flow and moisture. Plant the seedlings in well-drained, retentive soil and water regularly in hot, dry weather. Choose a site with good air flow and ensure you don’t crowd them too close together. If you spot a powdery coating on the plants, remove the affected areas and spray the foliage with comfrey tea. If the plant has wilted and has yellowed leaves with a reddish core in its stems, it may have Fusarium wilt, in which case the plants are best discarded entirely.

Slugs and snails like to munch the young foliage of cosmos, so protect young seedlings when they are first planted out.