Hardening seedlings
How do you harden seedlings?
Hi Gilkers 
You put them out in the day when it is warm and bring them in at night when it gets cold. You do this gradually to get them used to the change in temperature from the nice cushy environment you started them in to the temperature they have to get used to outside. Slowly over a period of time expose them to slightly colder temperatures leaving them out longer and longer until you plant them out. It will also make them adjust to natural light (if you have used artificial) wind and more irregular watering which they will not have been exposed to in their growing enviroment.
You put them out in the day when it is warm and bring them in at night when it gets cold. You do this gradually to get them used to the change in temperature from the nice cushy environment you started them in to the temperature they have to get used to outside. Slowly over a period of time expose them to slightly colder temperatures leaving them out longer and longer until you plant them out. It will also make them adjust to natural light (if you have used artificial) wind and more irregular watering which they will not have been exposed to in their growing enviroment.