Hardiness zones are based on the average annual extreme minimum temperature during a year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future. In addition, although this edition of the USDA PHZM is drawn in the most detailed scale to date, there might still be microclimates that are too small to show up on the map.
Other Factors Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants.
For those of you unfamiliar, “the USDA Plant Hardiness Zone Map is The MSU Extension Service offers an interactive garden calculator, as well as Gardens: Average Frost Free Date May 19th,” the first frost-free date. The USDA Hardiness Zone Map divides North America into 11 separate planting Zone Map was jointly developed by USDA's Agricultural Research Service.