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how to plant cosmos seeds in a pot

How to plant and grow cosmos

Even if you only have a tiny garden – or just some pots – you have to grow at least one cosmos. They are the lowest maintenance, floweriest plants in the world.


From Seed

The seeds are long and thin, so can be easily handled, and can be planted into modules or jiffys. Sow under cover March-April at about 3mm deep in good compost. Water in well and place in a greenhouse or warm windowsill to help germination.

In the garden

Plant out after the risk of frost has past, plant out in your garden into soil that has been improved with manure or garden compost. Pinch out the growing tip of each stem when transplanting to encourage stems to branch and produce more cosmos flowers. Plant the cosmos in full sun and water well, then add a mulch to help conserve moisture. Stake and tie plants if necessary during the growing season and don’t forget to water regularly.

For Containers

The shorter varieties e.g. Cosmos Sonata and Antiquity are perfect for pots. Plant the seedlings 10in apart, keep them well watered and they’ll be in flower by early July and, if deadheaded, flower reliably until October.


Cosmos will flower till the first frosts if you regularly dead head and feed the plants. When dead-heading cosmos the trick is to cut the stem right back to the first leaf below, rather than just taking the flower head off (see Sarah’s video for a close-up guide on how to do this).

Cut flowers

Last 7-10 days, no conditioning needed. Pick fresh straight into water, just as the buds are about to bloom, and do not pick in the heat of the day. Deadhead the main blooms that are drooping and going over after about a week. Place out of direct sunlight.

Even if you only have a tiny garden – or just some pots – you have to grow at least one cosmos. They are the lowest maintenance, floweriest plants in the world.

How to sow cosmos seeds

We take you through the easy process of sowing cosmos seeds.

Published: Sunday, 3 March, 2019 at 12:01 pm

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Do To do in March

Do To do in April

Do not To do in May

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Cosmos are half-hardy annuals that grow, flower, set seed and die all in one year, but unlike hardy annuals, they can’t withstand low temperatures.

They’re quick and easy to grow from seed, flowering in as little as 12 weeks, and can be added to borders and pots for a burst of showy colour.

To give your cosmos as long a flowering season as possible, sow the seeds early, indoors, in March or April. Once they’ve sprouted and have two pairs of leaves, transplant into individual 7cm pots and grow on under cover.

In May, harden them off for a week by placing in an open cold frame or on a patio, before planting them out where they are to flower. Feed fortnightly with a liquid tomato fertiliser when the first buds appear.

Learn how to sow cosmos seeds in this quick, step-by-step guide, plus other plants to use this method on, from the experts at BBC Gardeners’ World Magazine.