I just can't wrap my head around how a cold frame or hoop house helps you grow things over the winter. I get how it warms it up nicely during the day. Got that. But it won't hold in enough heat at night to keep common cold crops from dying. Temps inside are maybe 1-3 degrees above what they are outside overnight. They may have protection from direct frost, but effectively none against temperatures. I've tried lettuce and broccoli that both died in low 20's temps. I have one friend that has a cold frame, but they bring their plants in the house if its going to be really cold overnight. But I know people say they grow/keep things in there all winter. Over the winter we commonly get overnight lows in the teens and occasionally in the single digits.

If someone could help explain how it keeps plants from dying from extreme cold overnight, I'd love to understand! People say they grow with these methods over the winter and it bugs that I can't seem to understand how it works.

Thanks!

Cold frames often need to be "insulated" with hay or straw piled around them, making berms or stacking hay along cold frame sides and covering the top of the frame with blankets or straw at night to extend the growing season. Some people use heavy black plastic bags filled with leaves to insulate. Some cold frames are even sunk into the ground somewhat. Your zone conditions dictate the type and amount of additional protection needed.

Hardy perennials don't work well in a cold frame, but some annuals can get a faster start in a coldframe.

How big are your cold frames? Size does matter. If you have a very small high tunnel, it won't hold enough heat to be successful. I have 6 high tunnels and grow produce year around. We also use row cover with low tunnels inside our high tunnels.

If you read the winter harvest book, you will be in business.