Answer: Freezing temperatures rupture the cells within the plant tissues. Some plants are not as susceptible to this as others. Cole crops, for instance are not as affected as thin leafed lettuces. When you place plants in a cold frame, the structure itself physically keeps the elements off the plants but the structure also helps trap heat radiated by the soil which can warm the air within the structure a few degrees, which keeps the plants slightly warmer. I plant cold hardy veggies in my cold frame and am able to harvest bok choy, carrots, kale and romaine lettuce in my Seattle area garden. Best wishes with your coldframe.
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