I am (finally) putting in beds in the back yard and am planning to use cut flagstone for the borders. The stones will be about 3" high, 4" deep and of varying lengths. Most of the border will be +/-6" above ground level and will be loose stacked, but there will also be several sections that will be +/-12" above ground level and they will be grouted. In both cases I am planning to set the base course of stone in a shallow trench to help hold everything in place. I am also planning to install some drainage pipe and coarse gravel below several sections of the walls to assist in draining some low spots in the yard. For those sections I intend to place the pipe on the bottom of a deeper trench and cover it with 1/2" to 3/4" gravel and then wrap the whole thing in landscape fabric to keep it from plugging. The soil in the backyard is almost 100% clay.

I expect that soon after the border is built and the beds are filled with soil that the walls will turn into a +/-200' long nursery for fire ants. I currently have raised beds for vegitables and have been successful in keeping the ants away by selectively "shading" the South (and East and West as necessary) sides of the vegitable beds in areas where the ants start a colony. After a week or so of no sun to warm their eggs, they move on (usually to my compost bin, but I can use boiling water on them when they get there). I don't think the shade method will work on the landscape beds and am interested in any suggestions on how to install the borders to make them as uninviting as possible to prospective ant colonies.