an 8 year old child was found dead in a cardboard box once. Personally, I believe it was the cardboard box. Call this sarcasm, call it rude, whatever you want. I call it a dry night.

Seriously though, the little kid thing really happened.
http://en.wikipedia.org/wiki/Huang_Na

As for your question, I think that the only bad things that might happen with the cardboard setup is heat. It might get really hot inside the box, too hot for the plants, or even start a fire maybe. Good luck with that.

That wasn't sarcasm. I think it would suck ass if your house burnt down because of cardboard boxes. Maybe keep a fire extinguisher nearby.