The greenhouse is a great place to overwinter many of my plants, like these ferns. As long as the temperature inside the greenhouse stays above 28 degrees Fahrenheit, the plants don't freeze. If the temperature does drop below that point, I have a wood burning stove that I can use to warm the place. But I use the stove as a last resort, and I've implemented some other techniques to retain heat naturally. The idea is to absorb warmth from sunlight during the day and release it at night. The color black does this, and so does clay, brick, and rock. The greenhouse floor is gravel, which was absorbing some heat already. But, I wanted better results, so I started paving it over with salvaged bricks to add some extra absorption power. Then, I painted the back pegboard wall black, and I moved my collection of clay pots inside the greenhouse. So far so good! I'm always on the lookout for other energy saving techniques I can try--every little bit helps, and even though it's not a sauna in there during the winter, I rarely have a plant freeze to death!
My greenhouse gets a lot of use, mainly due to our crazy Ozark weather. It serves as a shelter for cuttings and seedlings, as a space to over-winter tender plants, and even has a small seating area for use as a sun room.