Seedlings that have been raised in protected conditions, like in greenhouses or indoors, need to be gradually exposed to outdoor conditions for one to two weeks before transplanting. This process is known as “hardening off.”
To harden off seedlings, first move them outside to a shady, wind-sheltered area for a few hours a day, bringing them inside at night. After a few days of that treatment, place them in the sun for a few hours in the morning or late afternoon and gradually increase the sun exposure. Reduce the frequency of watering, but do not allow the plants to wilt. Do not place seedlings outside on really windy days or when the temperatures are below 45 deg F.
After a couple of weeks, once the seedlings are used to full sun and can stay outside overnight, they are hardened off and are ready to transplant into the garden.