Seedlings that have been raised indoors in generally constant conditions will suffer a major setback if placed directly outside into the garden without some preparation. They need to undergo a gradual acclimation or ``hardening off'' process. This is true of purchased seedlings from the garden center or homegrown ones. Such an acclimation should take place in the heat of summer as well as in the cool weather of early spring. 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 early spring, seedlings need to be acclimated to the harsh outdoors slowly, over a period of days or even a week. Set them outside during the day when the temperatures are above 40F. 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. When nights below 40F are predicted, cover the seedlings with cloches or agricultural fleece.
For serious gardeners with lots of seedlings, a cold frame is a good device for this hardening off process. Place the seedlings in the cold frame. On sunny days, open the cover an inch or so during the day and close it at night. As the days get warmer, open the sash wider, until, finally, you can remove the cold frame entirely when nighttime temperatures do not go below 35F.
Many gardeners do not realize that the hardening off process is important for summer seedlings as well as spring ones. Plants raised indoors under lights, for example, are not going to react well to the heat of the summer sun. Before seedlings are moved into the garden where daytime air temperatures exceed 80F, they should be hardened off. Place them outdoors overnight, and bring them in during the day. After a few nights, leave plants outside during the morning as well, bringing them in only during the heat of the afternoon. After another few days, they are ready to be transplanted outside. Even then, it is wise to shade new transplants during hot afternoons for at least a week to be sure they are not shocked and set back by the hot sun.