Slowly increase the amount of time plants are left outdoors and increase the light they receive to the appropriate light levels over a period of 1 to 2 weeks, eventually leaving plants outdoors all night. At the end of this period, plants are fully ready to go into the garden. Remember to protect plants from predicted hard frosts, freezing winds and heavy rains, which can dislodge seedlings. A useful aid in both growing cold-hardy seedlings and hardening off tender plants is a cold frame . A cold frame is an unheated 4-walled structure with a glass or plastic roof.
Materials can be as inexpensive as discarded lumber and an old window sash.
During the day, the glass or plastic top should be raised for air circulation, but at night it is lowered to protect seedlings from frost and freezing winds.