I read article 547 but didn't see any mention of GFCI or GFP. I am wondering what part of the code, if any, deals with a residential greenhouse. There is some concern about installing GFCI protection where plug and cord connected heating equipment is used, that the GFCI could trip, causing loss of valuable plants.
There is no specific directions given for greenhouses- only a requirement for GFI protection in "wet" locations.
I admit to being a little puzled...having worked in greenhouses, and not recalling very much in the way of electrical appliances.
Lighting would generally be above any misters or spray equipment, and would not need GFI protection.
Even greenhouses in cooler climes typically have little in the way of heating equipment. It is amazing what a little sun will do.
That said, if you must have heat, and believe a GFI is necessary- go buy a $20 electronic oven thermometer. Those typically 'bottom out' at 32, and sound an alarm when they do. Want something fancier? Make your own with a small transformer, thermostat, and telephone bell.