Now that the front garden plot has been worked up with first the tractor then finely tilled with the walk behind rototiller, the row of tires has been placed one swath wide from the northern edge of the plot.
The vertical growing potatoes can have extra tires stacked on top, but only one or two more. If the stack gets higher than that the potato plants get choked out and the end result would be a pile of tires filled with dirt but no tubers (I learned the hard way).
There are several advantages to growing potatoes in tires, it makes weeding easier (this is very important to me). Also the plants and tubers are neatly contained so if someone wanted to help with weeding they would be less confused about the difference between spud and weed. The tires make a good barrier for those ground dwelling rodents that are attracted to the below ground goodies. And the best advantage is that for harvest, all one has to do is flip over a tire or two and the spuds are right there in the open instead of digging with a shovel and missing some good ones.