Hardening off your seedlings is a very important process. Without this acclimation period, your hard work sowing the seed and nurturing the seedlings indoors could be wasted.
Hardening off you seedlings involved placing them outside for increasing durations of time in order to acclimate the plants to increased light intensity, temperature, and wind. The light intensity and temperature of your seed starting area is probably much lower than natural conditions outdoors. If you plant a seedling directly outside without hardening off, the plant may undergo some shock which will either inhibit growth for a while, or kill the plant completely. It is also important to protect the seedlings from freezing night temperatures that may occur prior to your transplant date.
These trays of vegetable seedlings can be easily transported inside and outside as needed.
Start hardening off your seedlings by bringing them outside and putting them in a shaded area for a few hours. Keep an eye on watering (the higher temps may dry out your plants much faster than when indoors). The next day increase the duration and light intensity. After 4 or 5 days of increasing both the time outside and the amount of sunshine, your plants should be ready to transplant.
These healthy tomato seedlings have been acclimated to full sun conditions and are ready for transplanting.