Zone Map

How are the zone numbers and colors interpreted? What do they mean?

The Plant Hardiness Zone Map (PHZM) is based on the average annual extreme minimum winter temperature, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest). Each zone is divided into half zones designated as ‘a’ and ‘b’. For example, 7a and 7b are 5-degree F increments representing the colder and warmer halves of zone 7, respectively. These designations serve as convenient labels and shorthand for communicating and comparing the extreme winter temperatures within the United States and Puerto Rico. Zone numbers are typically listed with the descriptions of perennial plants in catalogs and other sales information produced by commercial nurseries, plant suppliers, etc.

The sequence of colors assigned to the zones mimics the chromatic spectrum produced by a prism (i.e., red, orange, yellow, green, blue, indigo, and violet), providing a graphical representation of the plant hardiness zones. The overlay of colored zones on the map provides a convenient tool for understanding and comparing plant cold-hardiness across the United States and can facilitate the selection of appropriate perennial plants based on their observed performance in other regions of the country.

Please visit HERE to view and download full sized national, state and regional maps