How to grow cosmos

If there’s one annual flower that keeps the garden going right up until late autumn it’s the annual Cosmos bipinnatus, a Mexican plant from mountainous areas in the Asteraceae family (daisies, sunflowers and asters).

Cosmos bipinnatus thrives on cooler days and longer nights and will continue to flower into early November if deadheaded, so it’s a useful late bee plant. The feathery foliage is attractive too and it’s easy to grow.

In the wild cosmos tends to be a weedy thing, but plant breeders across the world have worked their magic to bring the gardener lots of flower varieties from heavy-headed pinks, to single flame-oranges and pallid yellows.

As a result of all this breeding 2016 has been declared the Year of the Cosmos by Fleuroselect, an international organisation specialising in ornamentals.

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Where to plant cosmos

Cosmos need good light and reasonable drainage because daisy is a corruption of ‘day’s eye’ so most of them love the sun.

How to grow cosmos

Pink and white cosmos are the hardiest cosmos of all, far more so than the yellow and orange varieties which can struggle in cooler districts and need a good, warm summer to do well.

Growing cosmos from seed

The pinks, whites and clarets, bred from Cosmos bipinnatus, are easy to grow from seed. They take roughly 80 - 90 days to flower and are best sown into trays in late March or April. The large seeds are easy for children to handle.

Grown in the greenhouse, germination is speedy and straightforward in temperatures of 20C (70F). Within 10 days you will have seedlings large enough to transplant.

Lift each seedling carefully with a dibber (or small fork) and handle by the seed leaves, NOT the stem. Transfer to 9cm pots, or 6 x 4 modular trays and grow on, using John Innes no 1.

Keep your potted up seedlings watered and warm.

You can sow the cosmos seeds outside, straight into the soil, from mid-June to mid-July, scattering them thinly.

Growing cosmos from plugs

You can also buy ready-grown plug plants. When the small pot or module is showing roots at the bottom, you can either harden off your plants and then plant them out as small plants, or you can pot them up into slightly larger pots and grow them on for later gapping up.

Plants which have been started indoors should be hardened off, which can take a couple of weeks if the weather is cold. Make sure they are not planted out until the risk of frost has passed.

Garden centres often sell large pots of cosmos in the summer months. Remember that large plants are still vulnerable to slug and snail damage.

Find out how to get the best from your plug plants

How to save cosmos seeds

Stop deadheading some of your plants in early September to allow them to set seeds. These may have hybridised if you are growing several types of Cosmos bipinnatus.

Ripe seeds become brittle and black when ready and the seed heads should feel prickly to the touch.

Collect the seeds on dry day and clean them up. Place them in envelopes in a cold, dry place. An airtight biscuit tin kept in a shed is ideal.

Growing cosmos in pots

Cosmos can be grown in pots, and the smaller varieties are ideal for this, try soft yellow 'Xanthos', pink or white 'Hummingbird' or one of the 'Sonata' range, which comes in a variety of colours. Make sure the pots don't dry out.

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Plant with…

Cosmos slot into herbaceous borders really well because they produce lots of delicate flower, often in soft shades of pink and white, on bushy plants. They make great gap fillers among roses and summer herbaceous, or you could use them in a cutting garden. The shorter ones go well in a container.

Wildlife

Cosmos are typical daisies, with ray petals set round a cluster of nectar rich flowers, so they attract and satisfy lots of insects such as butterflies and bees.

Pests and diseases

Slugs will devour soft young plants, so toughen them up well for a week in a bright exposed place where the air and wind can get to them.

Find out how to control slugs and snails in your garden

Cosmos varieties

Pink and white cosmos

Widely available varieties of Cosmos bipinnatus

‘Purity’ (100cm/39 in)
An old single white, but still effective in September light when it looks gloriously fresh.

‘Sensation Mixed’ (120cm/4ft)
A mixture of single colours in shades of pink-red through to white. It’s tall and stately so not as useful in smaller gardens.

‘Sonata Series Mixed’ (60cm/2ft)
A shorter cosmos, but with equally large flowers, so this is good at the front of the border, or in a container, or grow for cutting. Sarah Raven has an all-pink version ‘Sonata Pink’.

‘Sweet Sixteen’ (90cm/3ft)
This picotee-edged cosmos has pale petals edged in deep-pink and each flower has some extra petals at the heart, giving each flower a frilly look. ‘Sweet Kisses’ seems very similar.

‘Seashells’ (120cm/4ft)
This taller cosmos comes in a mixture of pinks and whites, but the petals are almost cylindrical or fluted.

‘Cupcakes’ (up to 90cm/3ft)
This is very special, with dished flowers consisting of fused petals that look like cupcake cases. The whites turn blush-pink and many of the flowers have extra petals. Bred by Thompson & Morgan.

‘Hummingbird’ (45cm/18in)
A very new Dutch-bred series with fluted petals on larger flowers. There’s a pink and a white and this is far better than ‘Seashells’.

‘Daydream’ (90cm/3ft)
A white cosmos with powder-pink centres - adored by flower arrangers.

‘Double Click’ (100cm/39in)
This Dutch-bred series was the first to contain semi-doubles and doubles and they look very aster-like. However they do get very heavy-headed in summer rain, and can flop. In good summers they add much to the vase and garden and the flowers last for a long time because thy can’t be pollinated.

‘Xanthos’ (60cm/2ft)
Soft-yellow blooms on compact plants, so perfect for the front of a border or a container. Very new and very well-thought of, this May-flowering cosmos bred in Holland has been given a Fleuroselect Gold Medal this year. You can buy seeds or plugs.

'Apollo' (60cm/2ft)
A brand new British-bred variety, these cosmos flowers come in shades of pink, white and purple. 

Orange and red cosmos

These need warmer conditions to thrive. In warm gardens and good summers they will do well. They are also more frost tender, so they will not go on flowering until late.

Cosmos sulphureus ‘Polidor’ (75 cm/32in)
You will need to raise the seeds in a warm propagator in a mixture of compost and vermiculite. Single flowers in shades of orange and yellow follow. There is also a Brightness Mixed’ containing yellows, oranges and reds.

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