It has an inside pond of water that holds heat and releases it at night. It has an undersoil piping setup that takes warm daytime air and pumps it under the soil bed to keep things warmer. Check out their greenhouses that are on their Colorado property. They are often buried in snow, but they grow food all winter.
I have to use supplementary heating because I want to grow tropical plants. It hasn't been very expensive even though I use electric. Many greenhouses can use propane, or even wood heat.
