Greetings from southwest montana! I have a small 10 x 20 foot green house in which i grow a few flowers, salad greens tomatoes and the like. I really need to extend my very short growing season so am looking for the most economical way to heat the green house. The greenhouse has insulation in the walls but not the ceiling and is glazed with a single layer of polycarbonate on two sides and 1/3 of the roof. Some options are water barrels, propane, wood stove. What have you all used to heat yours?