Hardening off is the term used to describe the process of acclimating plants that have been grown indoors, or in a greenhouse, to conditions outdoors prior to planting them in the ground or positioning outdoors on a permanent basis. The acclimation period allows the plants to slowly adjust to new fluctuations in light, temperature and humidity in addition to strengthening them to other abiotic factors such as wind and rain. The process will slow down plant growth while the plant moves carbohydrates (food) to the roots and thickens its cuticle. Failure to harden your plants prior to planting is highly likely to result in plant shock due to the sudden change in conditions and this could cause the plants to die after all the hard work of growing them from seeds.
There are several different methods of hardening off your seedlings depending on where you started them. For example, plants which were raised in a heated glasshouse should preferably be transitioned to an unheated glasshouse before being moved outdoors, whereas plants grown by a window in your house or in an unheated glasshouse can start the process directly outside in the garden. The seed starting conditions will also play a role in determining how long the process will take before the plants are ready to live outdoors permanently i.e. the more protected your plants have been, the longer the hardening off process will take.
Very commonly, seedlings will have been started in the house by a bright window. In this instance, you should start the hardening off process by moving your plants to a shady, sheltered place outdoors e.g. beside a South facing wall and leave them out for a couple of hours before bringing them back indoors. Gradually increase the time your plants are outside by 1-2 hours each day making sure you bring them back indoors overnight. After 3 days of doing this, begin to place your seedlings in direct sunlight in the morning, moving them back to shade for the afternoon period. Continue to do this for a week. After this period, your seedlings should be capable of withstanding a full night outdoors (temperatures permitting). Allow the seedling to spend a few nights outside before transplanting.Transplant the seedlings on an overcast day to minimize stress and soak the plants well after planting.
Begin the process by moving plants to a shady, sheltered place outdoors e.g. beside a South facing wall