As soon as seedlings not already in cells or plug trays are large enough to handle they should be transplanted to other seed trays, cells or small pots to give them more space to grow. This is generally referred to as ‘pricking out’. The fresh trays, cells or small pots should be filled, as before, using general purpose compost and a pot/cell size or spacing should be chosen that is appropriate to the vigour of the seedlings concerned. The right time for pricking out is usually when seedlings have developed their first set of true leaves but some plants, such as petunias, which have very small seedlings may need to develop more than one set before they are large enough to manage. Seedlings should never be left beyond the point at which they can first be handled, however, as their rapidly growing root systems very quickly become difficult to disentangle and relocate without major damage.
Lift the seedlings carefully from their tray or pot using a suitable implement such as a pencil or an old fork from the kitchen. 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. A few, such as lobelia, can be left in small clumps. Always hold them by their leaves, never by their stems as, if you crush their stems, they will die.
Drop the roots of each seedling into a prepared hole in the fresh compost, firming it in gently afterwards. In trays, space the holes about 3cm or a little over an inch apart. To ensure it is well enough held by the compost that it doesn’t flop over when watered, it is normally best to plant each seedling a little deeper than it was previously. This is particularly true of seedlings, such as tomatoes, which tend to have a long stem below the first leaves (the seed leaves or ‘cotyledons’) but in all cases the lowest leaves should still be above the compost surface. A normal seed tray should hold about 24-40 seedlings spaced evenly apart and, when pricking out is complete, water from below or gently from above with a fine rose. Never use very cold water on seedlings at any time; allow each can to stand for a while or add a little warm water to raise the temperature.
If a mixed variety has been grown don’t just prick out the largest seedlings and discard the rest. In such mixtures not all of the colours always germinate at the same time or have the same vigour. Unless all the seedlings are given an equal chance it is possible to end up with only one or two of the colours.
For growing on, return the pricked out seedlings to a position where temperature and light are similar to previously. Keep moist but, as fresh general purpose composts contain a certain amount of fertiliser, no feeding should be necessary until they are well established. After that, for optimum growth they will need regular feeding with a balanced liquid fertiliser at recommended rates. If they become excessively large or crowded before they can be planted out don’t be afraid to ‘pot them on’ again into larger cells or pots. Most seedlings developing from seeds sown individually in plug trays or cells will also need potting on at some stage when they outgrow their accommodation.
Before they can be planted outside your seedlings will need to get used to the somewhat harsher conditions in the garden. Plants in the protected environment of a windowsill or greenhouse are rather soft and may suffer a check to their growth if placed directly outside. To accustom them to life in the open air, put them outside in a warm and sheltered spot during the day and bring them in again at night. Do this for about two weeks before the last frost date in your area. This process is called ‘hardening off’. An alternative to moving seedlings in and out is to transfer them to a cold frame where they can be covered over at night. Where large numbers of seedlings are raised it may, anyway, be necessary to relocate some of them to frames as they outgrow available space.
This guide applies to most seedlings e.g. Begonia SeedNext page