saw this in the forum at Daves Gardens. Put the water bottles on the floor and top with a shelf and then the plants. Larger greenhouses can use larger buckets instead of bottles. Noted that milk jugs tend to scrunch down where juice bottles do not.

They also suggested you can get containers from local schools to use for this and I'm thinking would be great as planters because they wild be food safe. Might smell like pickles!!

It's called Passive Solar Heating. I use 2ltr soda bottles painted black and filled with water. I keep my extra, extra tropicals on top of them as I've found it will keep the soil about 5 degrees warming than the rest of the plants not on them.

They collect heat in the day time and release it at night. Even in direct sun in the Winter they don't get as warm/hot as you think they would, but the do work for me!

The water trick really works. I just set the pots right on top directly. It warms the soil and keeps the roots warm. I think my dwarf bamboo is needing some warmth in this way.