Welcome!
Wow, I run two greenhouses, one for tropical and the other is our Brugmansia House 12 X 10 feet. We run two heaters set at 65 degrees in the brug house with thermostats, which we can monitor from the kitchen window and have a control to raise or lower the temp. The other is a 8 X 10 where the tropical are housed and that one runs a heater with a thermostat also, set at 70. Both greenhouses have a 55 gallon rain barrel for watering which helps keep the heat when we have a power outage. They keep the heat so other heaters can be put in place. We are zone 8b. You will figure out what is best for you. Our floor is gravel based and we toss a couple tarps over them in the winter months.
