Cosmos are half-hardy annuals that produce feathery leaves and beautiful, daisy-like flowers throughout the summer and into autumn. Delivering vibrant colour and texture until the first frosts, they’re a magnet for pollinators like bees and butterflies. Here’s a step-by-step guide to growing cosmos from seed. 

Browse our full range of cosmos seeds for inspiration.

When to sow cosmos seeds

Cosmos ‘Tango’ is perfect for attracting pollinators
Image: Cosmos ‘Tango’ seeds from Suttons

Cosmos seeds are easy to grow. They can be started off in seed trays indoors, or scattered directly outdoors on the soil where you want them to bloom.

If you’re germinating your cosmos seeds indoors, start them in late-March or April. This will give them plenty of time to establish strong roots before it’s time to plant them out. They will start flowering from late June.

If you prefer to direct sow your flower seeds outdoors, wait until the last frost has passed and the soil has warmed up. This is usually sometime in May, depending on where you live. Direct sown seeds will start flowering slightly later than those started inside. But remember, cosmos seeds need full sun to germinate and flower, so be sure to choose a warm, bright and sunny spot.

How to sow cosmos seeds indoors

‘Candyfloss Mix’ is a beautiful blend of pink, white, and deep-red blooms
Image: Cosmos ‘Candyfloss Mix’ seeds from Suttons

Here are some tips on sowing cosmos seeds:

1. Fill trays: Use peat-free seed compost and fill your trays or pots to about 1cm below the rim. Gently tamp the soil down.
2. Sow seeds: Hold the seeds in your palm, and sow each one about 0.5cm deep. Alternatively, take a small pinch and scatter them evenly over the tray of soil. Gently cover with compost.
3. Water lightly: Use a watering can with a fine rose.
4. Provide warmth: Place the trays in a warm, bright spot (18-21°C). A bright windowsill or propagator works well. Cover with a clear lid to retain moisture and speed up germination, usually about 7-14 days.
5. Prick out seedlings: Move the seedlings to their own pots, once they’re big enough to handle, to avoid overcrowding.
6. Harden off: Prepare your seedlings to be transplanted into their final positions by placing them outside for a few hours each day. Get them used to it gradually, over a period of 7-10 days.
7. Plant out: After the last frost (usually late May or early June), transplant your seedlings into the garden. Space them 45-60cm apart to allow for growth.

How to direct sow cosmos seeds outdoors

‘White Knight’ can produce single and double flowers on the same plant
Image: Cosmos ‘White Knight’ seeds from Suttons

Cosmos seeds are suitable for direct sowing outdoors, and provide an easy way to fill a large border. In fact, they look wonderful when planted en masse.

To direct sow, dig the soil over lightly, removing any weeds or debris, then rake it into a fine tilth. Make sure the area is well-draining, as cosmos dislikes heavy, wet soil. You can either broadcast-sow the seeds, or create rows about 15-20cm apart. Cover the seeds lightly with soil and water gently.

Quick tips on caring for cosmos

The petals of this pretty cosmos are completely fused
Image: Cosmos ‘Fondant Fancy’ seeds from Suttons

• Cosmos loves full sun, and won’t flower in shade.
• It tolerates poor soil but prefers well-draining conditions.
• Avoid over-fertilising your cosmos, as this can lead to fewer flowers.
• Water regularly during dry spells, but avoid overwatering. Cosmos is drought-tolerant once established.
• Remove faded flowers to encourage more blooms.
• Cosmos is generally disease-resistant, but make sure there is good air circulation to prevent fungal issues.
• You may need to support taller varieties with stakes to prevent them from flopping in windy conditions.

Five cosmos varieties to try at home

This award-winning variety has vibrant, show-stopping petals
Image: Cosmos ‘Apricotta’ from Suttons

If you’re looking for inspiration, here are some of our horticultural team’s favourite cosmos varieties to try in your own garden:

1. Cosmos ‘Tango’ – fiery orange blossoms that are a beacon for butterflies.
2. Cosmos ‘Fondant Fancy’ – dainty, dark-pink saucer-shaped flowers that look fantastic in drifts.
3. Cosmos ‘Apricot Lemonade’ – beautiful petals that open in soft apricot and fade to cream with a mauve ring around the centre.
4. Cosmos ‘Cupcakes Blush’ – a cup of pale pink fused petals that hold a flounce of smaller ruffled petals in the centre.
5. Cosmos ‘Apricotta’ – a tall, back of the border variety in a striking blend of apricot and raspberry.

We hope this has given you plenty of useful information on growing cosmos from seed. Find out the best flower seeds to direct sow for quick and easy colour, and read our expert advice on growing flower seeds for practical tips.

Lead image: Cosmos ‘Cupcakes Sakura’ seeds from Suttons