Fall seedlings that have been raised indoors in generally constant conditions will suffer a major setback if placed directly outside into the summer garden without some preparation. They need to undergo a gradual acclimation or ``hardening off'' process just as the spring seedlings do. This is true of purchased seedlings from the garden center or homegrown ones. Radical changes in air temperatures, whether cold or hot, can set transplants back if they have not been given a chance to adjust beforehand.
In the heat of the summer, seedlings need to be acclimated to the outdoors slowly, over a period of days or even a week. Set them outside during the day, especially if it is a bit cloudy. Try not to set them in the direct sun, and shelter them from too much wind. Put them on a porch, under shrubs, or in a similarly protected place. Move them out a bit more each day for greater exposure to sun and breeze. Each night, bring the plants indoors. After 3 to 5 days of this acclimation or hardening off, they are ready to be transplanted outside.