Thanks for the responses! I've got some more thinking to do. Good thing it's winter or I'd already be out there mucking around with stuff before I have a good understanding of the big picture.
The idea of water being a better heat buffer than granite is extremely thought provoking. I had also though about burying the heat retainers to slow down heat loss through radiation. My husband is already starting to raise an eyebrow at me at the thought of digging up rocks just to bury them again, but he'll get over it!
I think I need to rein it in a bit and work on some simpler things before I try to grow lemons. I have a bad habit of rushing into things too quickly. We've gotten to the point where we've got dairy, eggs, chicken, beef, goat meat and pork provided for ourselves from the sweat of our own brow using very sustainable and mostly organic methods (except for "milk stand grains" - still working on that). I struggle with veggies, but I think hugulkultur/permaculture is the answer there. That will happen this spring. So exciting!
Thanks to everyone for your passion and thoughtfulness!
-April