Ken, the basic premise of 'passive solar'; you let the sun heat up a heat sink that has a lot of density. Like water. Bricks work well too, and a greenhouse built with a dark brick wall on the north side stays very warm because the wall is a big, dense heat sink. The bigger the greenhouse, the bigger heat sink you would need, of course.
Say on your 41deg. sunny day your greenhouse gets up to what? 75 or 80deg. or so for several hours while the sun shines on it? Everything in the greenhouse will be that warm by the time the sun goes. Less dense materials like the soil in the pots will cool off faster, but the water cools very slowly and also supplies humidity if you leave the top off the water container.
It's true, jugs are not as effective as a larger mass of water like a plastic storage bin, but if you group them together, they maintain a goodly amount of heat. I use big 2gal. water jugs that we buy our hurricane supply of water in. We drink the expensive water after hurricane season, then fill the jugs with well water and put in used coffee grounds to make the water a dark color. Then I set the jugs in the full sun all day if the night is forecast to be cold. They get very warm, and 2 or 3 jugs on the bottom shelf of my little greenhouse shelf units keep them above 60deg. all night. Not to mention the weight keeps the shelf units from blowing over, which is an added benefit.
