My initial thoughts/concerns would be, the generator would ideally be running 24/7 so I assume your average emergency style backup generator wouldn’t be ideal for that. Second would be power requirements. Most modern ASIC miners are using around 3000-3400kwh so I’d look at the type of generator again so that it checks both of those boxes. Noise concern. Again generator running around the clock. Possible to make a sound reducing enclosure of some sort. As far as heating that should be pretty straightforward. They put out significant heat. I would just be sure to make sure it’s protected for water and other elements being in a greenhouse. If you move forward with this I’d be interested to see some updates.