When seeds germinate the first leaves to appear are the cotyledons or seed leaves. These are usually a pair of oval, fleshy leaves that bear no resemblance to the mature leaves of the plant.The conventional advice is that seedlings should not be pricked out or transplanted until the first true leaves appear, but the gardener must exercise common sense and move them on only when they are large enough to handle.
In the case of large seedlings, such as Courgettes or Marrows, this could be before the true leaves have developed and it is sound advice to sow such subjects individually in small pots.
Removing tiny seedlings from the sowing container into trays of a good universal compost can be a delicate business. The golden rule is never to handle the plants by their stems, which bruise easily, but always by their seed leaves. Some people use a sharpened or tapered piece of wood, such as an ice lolly stick, or a metal device called a widger to separate and ease out the seedlings, taking care not to damage the delicate roots.
Invariably there will be more seedlings to transplant than available trays to accommodate them, so some will have to be sacrificed, given to friends or put into the compost bin. The important point is to give the transplanted seedlings adequate space to become sturdy young plants.
As a rough guide, allow about 50 seedlings to each full size tray.
It is good planning to prepare the planting holes in the trays of well-moistened compost before you actually lift out the seedlings from the sowing container. Simply ease each seedling into position with the roots falling neatly into the hole, then gently firm the compost into contact with the baby plant while still holding it by the seed leaf.
Proprietary composts contain enough plant food to give the pricked-out seedlings a good start in life, but you can, if you wish, start feeding with a dilute liquid fertiliser, such as Maxi-crop, Liquinure or Phostrogen, after a couple of weeks or so.
Half-hardy annuals, half-hardy perennials and some vegetable seeds have to be germinated indoors because they would be damaged by frost, harsh winds or cool growing conditions.They are sown early in the year in a heated greenhouse, propagator, warm room or even, to start off, in the airing cupboard. Most seeds need a minimum temperature of 65F (18C) and will tolerate a drop overnight to about 50F (10C), but there are exceptions and they are dealt with separately.
Once the seedlings emerge they must be given plenty of light, although not direct sunlight, until they are large enough to be pricked off into trays (see pricking out above).
The final operation before planting out is to harden off the young plants. The idea is gradually to acclimatise the seedlings to the harsher conditions of the great outdoors. Allow a minimum of ten days to do this, preferably longer.
Start by putting the trays in a sheltered position outdoors for two hours during daylight and lowering the temperature of the greenhouse or propagator for the rest of the day. Slowly increase the period that the plants are outside so that by the time the frosts are finished, the plants are fully conditioned to being outside. Don't forget that the trays will need watering but should be protected from heavy rain.
When the young plants are transplanted to their flowering positions they may still need some protection against the damaging effects of strong, cold winds.
A very useful aid to successful hardening off is a cold frame. It should be large enough to accommodate all the seed trays, but can be a very simple inexpensive structure. During the day the lights - that's the glass or plastic cover over the walls of the frame - can be opened or removed altogether, but put back into position overnight.