I am planning on building a greenhouse for tropical fruit plans. My plan is to use cattle panels for its construction and use swivels on the sides to open it during the hot summers. My region posses a really unpredictable weather..however only 2-3 months a year is when I will see the most coldest temperatures. my plan is to use natural sources of heat to keep my greenhouse hot and humid. Things like a compost pile next to the greenhouse, black water jugs and perhaps a propane gas portable heater. I would like to avoid using the later. During my research I came across a underground oven that many parts of Mexico people use to cook meats. I was just wondering if have anyone thought about this, and how save do you think it could be. Would it be warm enough to maintain the greenhouse warm enough for the plants? what are your thoughts?