Hi Jai,
Very glad to have you joining this conversation. I do indeed use heat sources in my basement indoor garden. I have three 2' x 4' shelves devoted to germinating seeds at the present time, and each of them is covered by a large heating mat with a thermostat set at 80 F. Their sensors are buried in one of the pots on the shelves. They are not on a timer, so they are always energized, even at night. Their indicator lights indicate that the heating mats are usually on when it is dark, and spend a fair amount of time on during the light periods.
My growing shelves are lit by 32 watt 4-foot T8 fluorescent lights, and there are a bunch of them (3 or 4 2-bulb shoplight fixtures per shelf), so during the 16-hour light periods they actually put out a significant amount of heat. The fluorescent lighting puts out much more heat than the heating pads. This is a picture of a growing shelf from a previous year, and the shelving units from a previous time when we were using them upstairs. (You can click on these pictures to see larger versions.)
I have since added several more shelving units to my indoor gardening project. The utility room is connected to the rest of the furnished basement via double doors, which I keep open to get air exchange with the heated part of the basement. It probably never drops below 60F in the utility room even when the temperature is below zero outside with a North wind. (The utility room is on the north side of the house, but it is almost completely underground.)
I do have a cooking thermometer that I use to probe the growing medium temperatures, to satisfy my curiosity. Zinnias actually do well in hot weather, but can do OK in cooler temperatures as well. However, Powdery Mildew is more likely in cooler temperatures.
I would be glad to comment on any plans you might have for growing zinnias indoors.
ZM
