The term, hardening off, refers to the process of gradually exposing seedlings that have been raised in protective covering system to the climatic condition they will be grown in.
Depending on vegetable plant variety, 4-6 weeks after germination, the seedlings should well grown and ready to plant out into the garden, before this can be done, they must first be hardened off.
This is done to reduce stress and subsequent growth check when seedlings are transplanted to the main garden.
This simple, but important, process should start a week or two before you want to transplant them outside. Seedling that are due to be transplanted, but, for some reason can't be, should be hardened off anyway. The longer young seedlings remain in a controlled environment, the harder it is on them to adapt to the outside conditions.
Although you can't pick and choose the weather conditions, the hardening off process is much easier when the temperatures are consistent.
When starting from a heated glasshouse or hotbed, to outside conditions that are significantly cooler, make the first transition to a cold frame for the first day or two. You can then introduce them gradually to the outside conditions as for other seedlings.
Set the seedling containers outdoors in a lightly shaded, sheltered spot. Begin with just a few hours outside each day, and then gradually, over 3-4 days increase the time until the seedlings are there for more than half the day.
Continue Increasing the period outside until the seedlings are in their sheltered shady spot for a full 24 hours. Keep a close eye on them and if they are showing signs of wilt or stress, make these transitions more slowly. In the early period of hardening off, if there is the possibility of late frosts, bring the plants inside at night.
By now the seedlings should be almost fully hardened, so keeping them well watered and still protected from the wind they can be brought out into the full sunlight. During the first day or so in full sun, if they start to wilt during the hottest part of the day, leave them there but give them some temporary shade.
During this process ensure the seedlings have adequate water so they are not unduly stressed from lack of it.