Hardening off seedlings is a process by which gardeners slowly introduce their tender, indoor-grown seedlings to the harsher conditions of living outdoors. This can be done either by simulating those conditions indoors or by bringing the seedlings outdoors for increasingly long periods of time before transplanting.
It’s important to begin the process by initially placing seedlings into a spot protected from wind and direct sunlight. After two or three days in a shaded location, you can place the seedlings in a location that receives morning sun. Over time they can be gradually exposed to more direct sunlight.
While the seedlings are being exposed to increasing amounts of sunlight the time of outdoor exposure can also be increased, eventually having the plants left outdoors overnight.
The process actually involves toughening them up by thickening the cuticle on the leaves so they lose less water when exposed to the elements.
Without this thickening of the cuticle, tender plants may get burned by the sun or wind, or suffer from cold shock. And while they may (or may not) recover from these effects their growth will be set back a few weeks while they recover.
Hardening off itself should take at least one week and possibly up to two.
Once seedlings have been properly hardened off, it’s still best to ease their way into the garden by transplanting early in the morning, late in the afternoon, or on a cloudy day.