Hardening Off in a Cold Frame
If you start your own seeds inside, a cold frame is an indispensable tool in getting your plants off to the best start. A cold frame is, simply, a box with a clear top that you can open and close. You can construct one out of wood and old windows, or hay bales and plexi-glass. The important part is that the lid be moveable so that you can regulate the temperature inside the box.
The cold frame helps acclimate your plants, which are going from a controlled environment in your home or, if you're very lucky, greenhouse, to the harsh, unpredictable conditions in a Michigan garden. Moving the plants directly out would shock their systems, and at worst it could kill the plants you've worked so hard to grow. Here is an easy process to harden off your plants in a cold frame. Two weeks before you plan to plant them in the garden, move your plants into the cold frame.
Week One: Keep the lid about halfway open during the day, and close it at night.
Week Two: Keep the lid fully open during the day, and open halfway at night.
At the end of week two, your plants will be acclimated to the outdoors, and you can plant them in the garden. There are two caveats to keep in mind. First, don't forget to open your cold frame during the day. A cold frame is basically a mini greenhouse, trapping the warm air inside. On sunny days, it won't take long for the cold frame to get very, very warm inside if the lid is left closed, and your little plants will bake in no time. All of your work will have gone to waste in a matter of hours. Second, make sure to check your plants for water. They will dry out a lot faster outside, between the sun and the wind, than they did inside. This is an easy technique, and guarantees happy plants, making your garden successful. ITGO