Cosmos in flower

How to sow cosmos seeds

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

A table displaying which months are best to sow, plant and harvest.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
To do
To do

Do not To do in January

Do not To do in February

Do To do in March

Do To do in April

Do not To do in May

Do not To do in June

Do not To do in July

Do not To do in August

Do not To do in September

Do not To do in October

Do not To do in November

Do not To do in December

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.

Discover 10 more half-hardy annuals to grow.

Find out how to sow cosmos seeds, below.

To give your cosmos as long a flowering season as possible, sow the seeds early, indoors, in March or April.

You will need

  • Cosmos seeds
  • Seed tray or small pots
  • Seed compost or multi-purpose compost
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Step 1

Fill a seed tray or small pots with sieve, good-quality seed or multi-purpose compost. Lightly firm down the compost with the back of your hand or a small block of wood.

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Step 2

Sow a small pinch of seeds on the surface of each cell, then cover with a 3mm layer of sieved compost or vermiculite. Place in a tray of water to moisten the compost, drain, then label.

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Step 3

Place in a heated propagator at 18-25ºC, or cover with a clear plastic bag. When the seedlings have two pairs of leaves, transplant into individual 7cm pots and grow on indoors.

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More half-hardy annuals to sow