Bill, when I clicked on the link for ground covers it had zones listed. Are those the zones that the plants are hardy to?

Theoretically, yes..... The zone map that those are base on doesn't take into account "micro climates" that exist everywhere. When there is a multiple range (i.e. 3-9) it generally indicates that there different varieties of that plant that will survive within that zone range.

Food for thought....
Take your typical zone 3 plant..... Achillea?
It will survive 35 degrees below zero, and make a flower next Spring!
How long would you survive at -35 F.????
brrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr.........

I WOULD NOT!!!