We have had rare lows of -22F, but most years the worst we get is about 20F. I keep the greenhouse heated to 60F. I tried a propane heater but we are at 6500 ft elevation and the un-vented units all have safety devices that turn them off when oxygen is low. Ours turned off in less than 5 minutes and had to be manually restarted. Now we just use an electric heater. It hasn't been too expensive. What we need to do is put in a small vented propane heater which would be much cheaper.
