So what did I do this time? I bought a thermostatically controlled outlet from Amazon. It's an outlet that turns on when the temperature gets to 35* and turns off when it warms up to 45*.
I plugged the greenhouse heating until into this outlet which was then plugged into an extension cord.
Prior to having this, I had to watch the weather forecast and then go outside at night and plug the thing in. Then I had to remember to unplug it in the morning before leaving for work. This automates the process and gives me one less thing to worry about.
So how does it work? Since I got it, we've only had one night that it got down into the 30's. That was last night. Sure enough, when I checked the thermometers this morning, it was 37* outside and 50* in the greenhouse. The heating unit (aka lights) were still on. So I have to say that it works well.
I'm sure someone is wondering, "It's supposed to go off at 45*. Why was it still on if the thermometer in the greenhouse said 50*?" Good question. I think it's due to location. The plug is at one end of the greenhouse and the thermometer is at the other. The heating unit (aka lights) are somewhat but not exactly near the center. The temperature isn't uniform in there. What can you do... As long as my plants don't freeze, I'm not going to worry about it.