Probably the simplest, safest thing to do is get a 1500 watt, oil-filled electric heater with a thermostat (about $50). Just bear in mind that this will draw enough energy that you can't have much else plugged in to the same circuit without tripping a breaker.) I set mine for about 37-38F, which is not too hard on the electric bill, but warm enough to keep sensitive plants safe. However, you should make sure that you have sealed your greenhouse well so that you don't lose a lot of heat to air leaks. And, if you do not have double wall glazing, you should apply some type of thermal barrier to the interior of the greenhouse, such as Tempshield insulation, or bubblewrap; this will pay for itself in terms of heat savings.

I agree.