HOW DO YOU HARDEN OFF SEEDLINGS?

So, you have just spent 6 to 8 weeks germinating and growing on your seedlings inside and now is the time to release them out into the great outdoors. In many respects this is perhaps the riskiest time for your seedlings because being grown under protection typically produces soft foliage easily damaged by direct sunlight and drying winds.

Half-hardy annuals, half-hardy perennials and some vegetable seeds will all have to be germinated under protection during the early spring period. This will ensure that they have reached a decent enough size to get the most out of our short growing season - without the risk of being damaged by frosts or cold winds. .

They will need to be sown early in the year - anytime from February/March onwards - in a heated greenhouse, propagator, warm room or even an airing cupboard for some species! Most seeds will need a minimum temperature of 65F (18C) and will tolerate a drop overnight to about 50F (10C), but there are exceptions and will need to be dealt with accordingly.

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.

The final operation before planting out is to harden off the young plants. The idea is gradually to acclimatize 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 permanent 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 and 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 make sure that they are put back into position overnight.

For related articles click onto the following links:
HOW DO YOU HARDEN OFF SEEDLINGS?
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