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.

Do it:

Mar, Apr

Takes just:

15 minutes

At its best:

Jun, Jul, Aug, Sep

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.

You will need

  • Seed tray
  • Good quality compost
  • Half-hardy annual seeds

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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.

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

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

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

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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Seedlings

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.

Discover more ideas and inspiration

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