There are a number of vegetables that are not frost hardy and require a longer growing season than would be possible if their seeds were not sown until conditions outside were suitable for germination. For this reason, their seeds should be sown indoors in a greenhouse or conservatory, on a windowsill or, in some cases, in a cold frame.

Containers

Seeds can be sown in seed trays or pans; these should have a system of drainage holes in the base to prevent the seedlings from becoming waterlogged. This will also enable them to be used on water or capillary matting. Alternatively, a multi-cell (or modular) system may be used. This enables seedlings to be grown in single 'cells', which ensures that roots do not become entangled, reducing root disturbance when seedlings are planted out. Many types of module are available, including plastic or polystyrene cellular trays and 'rootrainers', which have a number of hinged segments, allowing the plant to be removed without damaging the root system.

Filling the Trays

Fill the tray or pot with standard seed compost to the rim, and then tap it sharply on the table or bench to settle the compost. Firm the surface gently using the bottom of another tray or pot. However, take care not to over-firm the compost, as if it becomes too compacted, it may not drain sufficiently and will make it more difficult for seedling roots to penetrate.

After filling, water the tray well by standing it in a few centimetres of water and leave until the surface is wet. Then, remove and allow it to drain for at least half an hour.

Sowing the Seeds

Once the tray has drained, sow the seeds thinly and evenly over the surface, or one per cell in plug trays, cell inserts or individual pots. Leave them in a well-lit, draught-free place to grow fast and evenly. Check them every day to ensure that the compost does not dry out. If it does, stand the tray in lukewarm water until the surface starts to glisten, or large seeds may be watered using a fine rose or mist sprayer.

When germination is well under way, remove the glass or plastic cover so that air can circulate around the seedlings, and move the tray to a slightly cooler location. Keep the developing seedlings moist at all times, although you should wait until the compost surface starts to dry slightly to avoid over-watering. No feeding is needed at this stage.

Pricking Out

Once the seedlings in trays have developed their first set of true leaves, they should be transplanted to other trays or pots to give them more space to grow. Seedlings will become weak and spindly if deprived of sufficient light or space, and transplanting them to another container will enable them to continue to develop until they are ready for planting out in the garden.

Fill the new containers with a loam-based potting compost and firm gently to remove any air pockets. Tap the tray of seedlings against a bench or table top to loosen the compost slightly, then remove it intact from the container. Carefully separate the seedlings, holding them gently by their seed leaves to avoid bruising the foliage, stems, or growing tips. Loosen them from the soil with a widger or other small implement, taking great care not to damage the roots.

Then, carefully lift each plant from the soil, keeping plenty of the moist seed compost around the roots to ensure that there will be little or no hindrance to growth when the seedlings are replanted. Discard any weak, unhealthy-looking seedlings. If, despite sowing thinly, they are too crowded to be lifted singly, lift them in clumps and then gently tease them apart with as little damage to the roots as possible.

Use a dibber to make holes in the compost 4-5 cm (1 1/2-2in) apart, roughly to the same depth as the seedlings were planted in their old pot or seed tray. However, it may be necessary to plant seedlings such as tomatoes a little deeper than they were originally, as they tend to have a long stem below the first leaves (the seed leaves or 'cotyledons') and may need a little more support. Place a seedling into each hole. Make sure that all the roots are covered with compost, then gently firm in each seedling using fingers or a dibber, and tap the container to settle the compost.

When each container is full, water it from a can fitted with a fine rose to settle the compost around the roots. Cover the plants with clear plastic for a few days to increase the humidity around the seedlings whilst they re-establish, but make sure that the plastic does not touch the leaves as this may encourage rotting. Seeds that have been sown singly in packs or space-sown will not normally need pricking out and may be hardened off before planting out.

Hardening Off

Before planting seedlings outside, they must be acclimatised to lower temperatures by hardening them off in a cloche or cold frame over 10-14 days. The amount of ventilation that young plants receive should be increased gradually every day, and then also at night until they are hardy enough to be planted out.