Thompson & Morgan®: Seed Germination Guide
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).
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.
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