Many hardiness zone maps are available. Some cover small areas such as individual states, while other encompass entire countries. Unfortunately, many do not agree in their numbering schemes, and therefore zone numbers can be related only to the map used in assigning them. The first plant hardiness zone map for the United States and Canada was developed by the USDA in 1960. Zones were numbered 1 to 10 with Zone 1, the coldest, located in sections of northern Canada, and Zone 10, the warmest, at the southern tips of Texas and Florida.
Lower temperatures occur in the 'a' zone.
The new Plant Hardiness Zone Map was released by the USDA in 1990. As in the old map, each zone is divided into 'a' and 'b' regions based on five-degree differences. It is important to remember that the temperatures shown on the Plant Hardiness Zone Map are developed by taking an average of temperatures that fall above and below the final listed temperature, and that no year will exactly fit an average. Because of this limitation, the map cannot and should not be expected to predict record low temperatures as experienced in 1996. Finally, hardiness zone boundaries should not be recognized as absolute.