Some put a small heater on a thermostat in them or you can provide some heat in those when needed just by putting a couple of clip on 60 wt. incandescent lights on inside.

The real problem with them is in the daytime when heat gets trapped in them if they aren't closely monitored and well ventilated. When the sun is out it can quickly get up to 120 degrees in one of them and cook everything.

But also keep in mind that it is a big environmental change for the plants so they will need to be hardened off first. They won't tolerate going 'cold turkey' right from the house into one of those units.

Be sure to include a thermometer inside it.

Dave