In the book Winter Harvest Handbook, Eliot Coleman proposes the idea of using double cover over his crops in order to shift 2 USDA zones south. Basically his idea is to use a high tunnel and a floating row cover in combination with no added heat. This allows him to grow cold hardy plants when it is -20 degrees outside. Now I am in Texas so my winter's are not so harsh in the first place, so I could use this idea to a great advantage. But I am wondering, what are some alternatives? Using permaculture techniques how could I create a microclimate which could potentially shift 2 USDA zones south?
An alternative might be Oehler Structures to regulate temperatures. Thermal mass from water or stones can store heat to increase temperatures as well, especially where they get more sun. With my own experiences in Texas, I have found that simply preventing the rare frost from touching the plants is enough for most of them to carry on.
The expense is a matter of how much you can do yourself (digging) and if you can source inexpensive or free materials (dumps, freecycle, defective tempered glass that has odd color swirls in it, etc). I used to have sources while I lived in PA where I could have gotten almost everything I needed to do such a project for free.
As for frost prevention, that comes from observation. I noticed that no matter where I lived in the country, frost never formed more than a few inches into my porch. The same thing is true under shorter trees. I could walk under some without ducking that still managed to be frost free under the ground. The trees however, seem to be more likely to have frost deeper into their radius. Thinking about this, the row covers are more or less the same thing in miniature. The temperature of the air is the same. The water content in the air is the same. So what is different? Two things came to mind. Air flow is left still on a porch and there is an overhang keeping heat from leaching away as quickly. As a bonus, the thermal mass of the porch itself often gathers heat during the day.
I've tested it a few times. If I can stop air from two directions and have a covering of any sort over the top within six feet or so of the ground, frost doesn't seem to bother the plants most of the time. I've gotten a few annuals to extend into an extra year by preventing flowers from going to seed and keeping them frost free.
Using a shelf on the porch to grow some things or having any sort of structure that matches the above, you can have reasonable growth in an area with low frost issues (IE: Texas). Hard frost areas are usually rough for light and heat so that warm weather plants might still freeze or at the very least be non-productive while the sun and heat are in short supply. You might go get one of those inexpensive temporary greenhouses made from clear plastic and some shelves. No risk of frost, concentrated heat, etc. Just some thoughts. Honestly, I have been working out how to best encourage frost in Texas through dips and low points. Cabbage, Kale and others of that family are much better after a frost.