Cosmos

How to sow half-hardy annuals

Find out how to sow half-hardy annuals, such as cosmos and zinnia, with the help of our step-by-step guide.

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

Plant is not at its best in January

Plant is not at its best in February

Plant is not at its best in March

Plant is not at its best in April

Plant is not at its best in May

Plant is at its best in June

Plant is at its best in July

Plant is at its best in August

Plant is at its best in September

Plant is not at its best in October

Plant is not at its best in November

Plant is not at its best in December

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

Half-hardy annuals are quick to flower and provide a burst of colour in summer borders and pot displays. Many varieties sown indoors by March or April will start to flower as early as June.

Discover 10 half-hardy annuals to grow

To give them as long a flowering season as possible, sow seeds early so that plants have plenty of time to establish before planting out in early summer. They are very sensitive to frost, so must be sown under cover, and only planted out once all risk of frost has passed.

Here’s how to sow half-hardy annuals.

Many varieties sown indoors by March or April will start to flower as early as June.

You will need

  • Seed tray
  • Good quality compost
  • Half-hardy annual seeds
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Total time:

Step 1

Fill a tray with seed compost, then sow a tiny pinch of seeds in each cell. Cover with vermiculite, water and place the tray in a propagator in a warm spot.

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Step 2

Once seeds have sprouted, wait until two pairs of leaves form. Transplant each seedling into a 7cm pot filled with sieved multi-purpose compost.

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Step 3

Water well after transplanting and keep the pots in a warm, bright place. Avoid exposure to bright sunlight, which can scorch the young leaves.

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Step 4

When all risk of frost has passed, move the plants to a cold frame or patio, bringing them in at night. After a week you can plant them outside.

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Sowing half-hardy annuals outside

You can also sow half-hardy annuals directly outside in late spring or early summer once frosts have passed. Try sowing cosmos, zinnia, nasturtiums and French marigolds this way.

Seedlings