USDA Plant Hardiness
Source: Plant Hardiness Zones | USDA
Because air temperature can impact the temperature of soil, there is are certain ideal requirements for plant growth. These ideal air temperatures, based on the average annual minimum winter temperature, and optimum soil temperatures are used to help create the USDA Plant Hardiness Zones. The map is a standard by which gardeners and growers can determine which plants are most likely to thrive at a location. These zones are updated every few years based on newly observed climatic data.