Patty, right below the Frugal Gardening forum heading is the Garden Experiments forum. You should post this over there, those guys are up for anything. They've tried some wild experiments and then they report how it went. It's very interesting reading.

But to answer your question, yes, I think it would work and I would probably put a large clear plastic bag over the soda bottles if the weather as very cold. The advantage of the soda bottles is that they are significantly larger than the cells in the wall-o-waters and would stay warmer longer. They're not as tall, so if your plant had outgrown the tops of the bottles, the clear plastic bag would help keep the cold out. Cheryl