How to grow and care for cosmos flowers

Cosmos are among the most generous and easygoing flowers in the summer garden, raising tall, airy stems of daisy-like blooms from seed in a single season with almost no attention. They flower for months, right up to the first frost, they make superb cut flowers, and they draw bees and butterflies in numbers. Best of all, they flower most freely in poor soil, so they reward neglect rather than fuss. This guide covers how to grow and care for cosmos, from sowing seed to keeping the flowers coming.

Cosmos belong to the daisy family and come in annual and tender perennial forms, so a quick look at the plant and its main types helps before sowing.

What is a cosmos flower

Cosmos is a genus in the daisy family (Asteraceae), native to Mexico, Central America and northern South America, and often called Mexican aster. The plants carry open, daisy-like flowers on slender stems above feathery, finely divided foliage, in a wide range of colours. The name comes from the Greek word kosmos, meaning order or harmony, given by Spanish priests in Mexico for the neat, balanced arrangement of the petals.

Most cosmos are grown as annuals, fast from seed to flower, though one species is a tender perennial, and all are prized for how little they ask of the gardener.

Types of cosmos

Three species cover almost all the cosmos grown in gardens, and they differ in colour, height and life cycle.

Garden cosmos (Cosmos bipinnatus): the classic tall cosmos, reaching 90 cm to 1.8 m, with feathery leaves and large flowers in pink, white and crimson. It is the best type for cutting, and includes cultivars such as the frilled 'Double Click', the cupped 'Cupcakes' and the dwarf 'Sonata' series.

Garden cosmos (Cosmos bipinnatus).png

Sulphur cosmos (Cosmos sulphureus): shorter and bushier, with hot flowers in yellow, orange and red, and good drought tolerance.

Sulphur cosmos (Cosmos sulphureus).png

Chocolate cosmos (Cosmos atrosanguineus): a deep maroon, chocolate-scented flower that is a tender perennial grown from a tuber, rather than an annual from seed.

Chocolate cosmos (Cosmos atrosanguineus).png

How to grow cosmos from seed

Cosmos are simple to grow from seed and dislike root disturbance, so direct sowing where they are to flower is the easiest and most reliable method. Sow the seeds about 0.5 to 1 cm deep once the soil has warmed, and they usually germinate within one to two weeks. From sowing to the first flowers takes roughly two months.

For an earlier start in cooler areas, sow indoors four to six weeks before the last frost in individual pots, harden the seedlings off and plant them out once the danger of frost has passed. Space plants around 30 cm apart, or a little more for the tallest varieties.

When to plant cosmos seeds

Cosmos are frost-tender, so sow outdoors only after the last frost has passed and the soil has warmed, usually in mid to late spring. There is no advantage to sowing early into cold soil. Sowing a second batch a few weeks after the first extends the display later into the season.

Sun, soil and why poor soil means more flowers

Cosmos need full sun, at least six hours a day, to flower well; in shade they grow leggy and bloom sparsely. Their soil preference is the key thing to get right, because it runs opposite to most plants. Cosmos flower most freely in poor to average, well-drained soil, and rich soil or feeding drives tall, leafy, floppy growth with few flowers. For the best display, do not fertilise them, and a little compost at sowing is all they need. Once established they are notably drought-tolerant.

Watering, support and pinching

Cosmos need only moderate watering once established and cope well with dry spells, though young plants and those in containers should not be left to dry out completely. The tall varieties can reach well over a metre and tend to flop, so give them the shelter of other plants or a few discreet stakes or supports. Pinching out the growing tips of young plants when they are around 30 cm tall encourages them to branch and produce more, bushier flowering stems.

Deadheading and cosmos as cut flowers

Deadheading is the single most useful job with cosmos. Removing the faded flowers before they set seed pushes the plant to keep producing new buds rather than seed, so cut each spent bloom back to just above the next set of leaves and more flowers soon follow. This also makes cosmos a classic cut-and-come-again cut flower, and the more you pick, the more they bloom, with tall types giving the longest stems for the vase.

Are cosmos annual or perennial

Most cosmos, including garden and sulphur cosmos, are annuals that complete their life in one season and die at the first frost. They self-seed very freely, though, so a patch often returns on its own the following year. The exception is chocolate cosmos, a tender perennial grown from a tuber, which can be lifted and stored frost-free over winter, or protected in place in mild areas, to grow again the next season.

Cosmos for pollinators

The open, nectar-rich flowers draw bees, butterflies and other pollinators in numbers, which makes cosmos a valuable addition to a pollinator-friendly garden. Left to set seed at the end of the season, the seedheads also feed birds such as goldfinches. Cosmos combine beautifully in a cutting or pollinator bed with other easy annuals such as zinnias and marigolds.

Common problems

Cosmos are largely trouble-free, but a few issues can appear. Aphids sometimes cluster on soft new growth and can be hosed off or left to ladybirds and other predators. Slugs and snails can damage young seedlings, so protect them early on. In humid or crowded conditions cosmos may develop powdery mildew or grey mould on the foliage, which good spacing, full sun and airflow help prevent. Removing affected leaves and avoiding overhead watering keep these fungal problems in check.

Are cosmos toxic to cats and dogs

Cosmos are non-toxic. According to the ASPCA, they are safe for cats, dogs and horses, which makes them a sound choice for a pet-friendly garden and a good companion for children's gardening projects.

Frequently asked questions

When should I plant cosmos seeds? After the last frost, once the soil has warmed, usually in mid to late spring. Sow directly where they are to flower, since cosmos dislike being transplanted.

Do cosmos come back every year? Most are annuals that die at the first frost, but they self-seed freely and often return on their own. Chocolate cosmos is a tender perennial grown from a tuber.

Why are my cosmos all leaves and no flowers? Almost always because the soil is too rich or the plants are being fed. Cosmos flower best in poor soil, so stop feeding and grow them lean in full sun.

How tall do cosmos grow? Garden cosmos can reach 90 cm to 1.8 m and may need support, while dwarf varieties such as 'Sonata' stay around 30 to 60 cm and suit containers.

Are cosmos good cut flowers? Yes. They are excellent cut-and-come-again flowers, and regular cutting or deadheading keeps them blooming until frost.

Important notes

Every garden is different, and conditions vary with your climate, soil and sun. The guidance above is a general starting point rather than a strict rule.

Cosmos self-seed readily and in some regions can spread beyond where they are wanted, so remove seedheads if you want to limit them. If plants show persistent problems that don't respond to better spacing and drier conditions, consult a horticulturist or your local nursery, and try non-chemical methods first.