Do you use heat mats? Propane? Electric? Cables? Or some combination? My greenhouse is 12x12 with plywood sides. I have a Mr. Buddy propane heater and heat mats but I really don't want huge electric or propane bills. Last year I put cold hardy stuff out April 26 and used 2 small heat mats to sprout peas, lettuce, broccoli, pansies, ect. I only used the propane 2x. It did well but I didn't have a lot out there until the last week of May. By the first week of June everything was in the greenhouse or outside. I want to have more out there this year but I am not sure if the weather will cooperate. Advice?