The US Department of Agriculture publishes a map showing the average annual extreme minimum temperatures in a region.
In the Uinta Basin, we are shown as being in Zone 5, but to be on the safe side, we like to select plants that are hardy in Zone 4 (-30º).
Of course, there are many other variables that affect plant growth, including soils, moisture and the amount of sunlight we receive. This map indicates how winter hardy a plant should be to survive in our area.
The link below leads to a zip code locator for the map.