Billvon,
Your plants' temperature is set by the environment. If it's cold, they cool. If it's warm, they don't. This has nothing to do with plants radiating out to space. Put on a cold cover set to -5C, they will freeze.
I don't know how you think less negative is positive.
Let's say your plants are constantly heated underneath to 5C. Without a cover, you may get a gradient from 5C to -5C on a cold night. With a cover, you'll get 5C to 4C. There no extra temperature due to the cover at the source. All you're doing is trapping warmed air from convecting up.
I just linked you to official data showing barely any increase in upwelling radiation from the presence of clouds. You can see different amount of radiation going to space and it makes no practical difference to surface upwelling radiation.
"So there are no clouds in Antarctica? "
There's an inversion. It gets hotter as you move up the atmosphere. That's where the clouds are.
"you could not use a room temperature laser"
The container may seem to be room-temperature, but the thermochemical process driving it is certainly not.