A cold frame is kind of like a mini-greenhouse and it can be a fantastic tool to get your plants started early in the spring.
The frame is basically a box, about 2x4 feet and deep enough that your plants can grow comfortably without their leaves hitting the top (so maybe a foot or two?). Then you top with a sheet of glass. The usual approach is to have an old wood-framed window for the lid, in which case you just build the box to match the size. You can either just lift the lid on and off, or add a hinge.
You set your seedling pots inside the frame, and the glass traps the heat inside the box. So when the weather outside is otherwise too chilly for your plants, you have a sheltered and warm spot for them.
In fact, it can trap the heat so effectively, you may need to prop open the lid to let it vent a little bit. Just keep an eye on your plants and see if they start to wilt. Don’t underestimate how much heat these boxes can generate.
You can also use this technique with planted seeds rather than pots; just leave the bottom of the frame open. Just set it up over the garden section you want to protect, and let the seeds grow with the added warmth. When they are large enough, just take the frame away.
Once the sun goes down at night, your frame and its contents will cool down. Cover it up with a blanket or a few bales of straw to help insulate that heat inside. Or just gather up your pots and bring them back into the house if you don’t think they can handle the night chill.
