Seeds of non-hardy or half-hardy plants have to be sown indoors with gentle heat in pots, seed trays or cell trays filled with good compost.
This technique is used for tender plants that can take several weeks from sowing a seed to flowering, but can’t be planted outside until frosts and cold weather have finished. These half-hardy annuals and half-hardy perennials include aster, begonia, geranium (pelargonium), petunia, busy Lizzie, French marigold and dahlia to name but a few.
Some vegetables are best grown in this way and include tomatoes, aubergines, peppers and cucumbers as well as runner beans, sweet corn, courgettes and marrows.
Many other vegetables can also be started into growth in this way if you want to start them earlier in the year and so produce an earlier crop.
Getting the best results
- Use a clean plastic seed tray or cell tray for small seeds or individual pots for large seeds.
- Fill the container with a suitable seed or multipurpose compost. For best results use purpose seeds & cutting compost.
- Firm lightly with a piece of board and water the compost with tepid water.
- Sow the seed thinly and check with the seed packet to see if they should be covered with more compost or vermiculite. Cover thinly if recommended.
- Cover with a clear plastic bag and place in a heated greenhouse or on a shaded windowsill where the temperature can be kept at a consistent 14-18°C (60-70°F); some seeds may need a higher temperature - always check on the seed packet.
- Better, more consistent results can usually be achieved by using a heated propagator.
- As soon as the seedlings pop through the compost, remove the bag or propagator lid and keep in a light position. If grown on a windowsill, turn the tray or pot regularly to prevent the seedlings from becoming drawn towards the light and thin.
- When the first set of true leaves have emerged, transplant the seedlings individually into small pots or into trays filled with rich compost.
- Give them enough room to grow and spread. Gradually harden off the plants, acclimatising them to the outdoor conditions, ready for planting out when frosts and cold weather have finished.
Seeds can be sown in this way early in the year (January to February), providing you can give them the right germination temperature and good light to prevent the seedlings from becoming drawn and spindly. It is also important to have a good growing on temperature - for most plants, this is a constant minimum of 7-10°C (45-50°F). If you can’t provide these conditions, delay sowing until spring.