If you grow a garden, then you probably know about 'hardening off.' In this process, you take young plants that have only been in the warmth of a greenhouse or in protective indoor shelter under grow lights and bring them outdoors for a few hours each day. You're supposed to do this for increasing lengths of time. The newly matured plants will become strong enough to withstand the elements on their own. Once your plants are fully hardened off, they're ready to be planted outside in your garden beds.