Ajuga is usually listed as good to zone 3. Don't forget that snow is a good insulator, averaging out to about an R value of 1 per inch, so a foot of snow insulates as well as a 2x4 wall with fiberglass insulation. So if your Ajuga is burried under a couple feet of snow, it'll be good to lower temps.
My orchard floor is mostly red and white clover, but trying to find something lower and thicker for right under the trees.