Greenhouses are very popular for extending the growing season, but heating them in cold climates is expensive. In contrast, pit greenhouses — greenhouses built into the earth — use free solar energy from the sun for heating. The temperature of the earth (usually around 55-58 degrees Fahrenheit) helps stabilize the temperature in the greenhouse.
One option for pit greenhouses is to use earthbag walls with about 2’ (60cm) of scoria on the outside for insulation. This design would be more energy efficient than uninsulated walls of earth and should maintain growing temperatures year-round. Watch over the greenhouse and provide adequate ventilation to avoid overheating.