The Plant Hardiness Zones divide the United States and Canada into 11 areas based on a 10 degree Fahrenheit difference in the average annual minimum temperature. (The United States falls within Zones 2 through 10). For example, the lowest average temperature in Zone 2 is -50 to -40 degrees Fahrenheit, while the minimum average temperature in zone 10 is +30 to +40 degrees Fahrenheit.