The last step in the raising of seedlings to be transplanted outside in the garden is preparing them to be outside, and this is known as hardening off. Outside the seedlings must be able to adapt to many new things in their future home. They must adapt to stronger sun light, wind, temperature fluctuations, rain and changing soil moisture conditions. It is kind of like jumping into the lake the first time after ice out, we would like to take our time to adjust to things. If you move them outside to quickly and all at once you will shock the tender plants and this may result in stunted growth or even death.
When we are ready to move plants outside for the season we want to harden them to the conditions or harden them off. We do this by gradually introducing them to the new outdoors climate. One of the best ways is by using a cold frame or hoop house but lets pretend that we don't have those available to us. The next best thing to do would be to move the seedlings outside for a short amount of time and increase the duration of the next few days. The first day out try only 2 hours and then bring them back in the house, then the next day try 4 hours and the next 6 hours, increasing the outdoor time by two hours each day until we are leaving them outside both day and night.
This is easy if you are home all day to tend to these duties, but most of us work outside the home. So here is what I do I start on a Friday afternoon for two hours, Saturday for 4 hours, Sunday for 6 and then Monday they are outside from the time I leave until I get home, about 8-10 hours and then on Tuesday I leave them out for two hours after I get home...you get the picture.
When leaving the tender seedlings outside, protect them from direct winds, and direct sunlight for the first week and be sure that there is good moisture in the soil before leaving them out. Within two weeks of hardening off, the plants are now ready to be transplanted into your garden, that is as long as the snow and frost is over for the year, and up here in Minnesota/North Dakota that can still happen in June. So then we need to provide protection, more on that later.