The USDA Hardiness Zone Map is primarily based on the average annual minimum winter temperature in different regions of the United States. It uses temperature data to categorize various geographic areas into distinct hardiness zones, which represent the range of temperatures that plants can be expected to withstand during the winter months.
The specific criteria used for defining these zones are as follows:
- The USDA map uses the lowest average annual minimum winter temperature as its primary criterion. It looks at the coldest temperatures typically experienced in a region over a 30-year period. The map uses temperature data from weather stations across the country to calculate these averages.
- The zones are defined in 10-degree Fahrenheit (approximately 5.6-degree Celsius) increments, meaning each zone represents a 10°F difference in the average annual minimum winter temperature.
- The USDA then assigns a numerical zone designation to each region based on the temperature data. For example, Zone 1 represents areas where the lowest average annual minimum temperature can fall below -50°F (-45.6°C), while Zone 13 represents areas where the lowest temperature typically stays above 60°F (15.6°C).
It’s important to note that the USDA Hardiness Zone Map provides valuable information for gardeners, farmers, and horticulturists to select plants and crops that are well-suited to the local climate conditions, particularly in terms of cold hardiness. However, it does not take into account other climate factors such as humidity, rainfall, or summer temperatures, which can also impact a plant’s overall suitability for a particular region. As such, while the USDA Hardiness Zone Map is a useful tool, it’s just one of many factors to consider when planning and designing gardens or agricultural endeavors.