Greenhouse Heater

It would be to risky, given the heat would tend to rise to the top of greenhouse directly above the heater,also the temperature control on parrafin heaters are not exactly the most accurate, I think you could end up with a frame instead of a greenhouse unless you are very careful.

I would imagine it would get too hot for your plants, as well - those plastic greenhouses are very small, aren't they? As Mac says, there isn't a lot of temperature control on a paraffin heater.