I refer to the USDA Hardiness Zone Map to locate the information I need to know before designing a garden. These zones refer to the coldest temperature that area experiences in any given year. A plant, tree, and/or shrub needs to be resilient to these low temperatures to be considered a perennial (a plant that returns year after year) for that area. There are 11 zones that divide up North America. Each zone is 10 degrees difference from zone before. This map was updated in 2012 to reflect a more comprehensive collection of data collected over 30 years. New techniques of mapping were able to take in account the elevation of a location, proximity to water, topography such as canyons and ridge tops, amongst other factors that you too can take into account as you scan your own landscape. For example, being in a canyon can mean that the cold air hangs around longer. Where as the ridge top just above has a warmer micro climate.
Teton County, Wy is predominantly a zone 3. Teton County, Id is a bit warmer and is predominantly a zone 4-5. The topography of Jackson being a "hole", a fur trapper's term for an area that is enclosed on all side by mountains, holds our colder temperatures. We also have the influence of the Snake River and the myriad of creeks that flow through our valley. Teton Valley also known as "Pierre's Hole", is much more open allowing the colder air to move out. The Teton River's source bubbles up from the ground and is fed by a few warm springs and thus flows at a warmer temperature than the Snake River which flows from Jackson Lake and makes it brisk winter journey through Grand Teton National Park. The two valleys have similar summer temperatures but remember the zone map refers to the lowest temperatures.
