Heated greenhouses protect plants during cold, freezing nights, and can even extend the growing period yearround. Plants can't handle the cold very well, especially once temperatures drop below freezing. Oftentimes, passive solar or geothermal energy can be harnassed and repurposed to keep a greenhouse warm. Even if this isn't enough to heat your greenhouse on its own, it may make significant cuts to your power and energy spending.
Make sure you're using a double-walled cover when constructing your greenhouse. This provides a layer of air in between the layers, helping insulate your greenhouse even further. In the coldest months, consider using thermal blankets to cover your greenhouse at night.
