The United States Department of Agriculture�s plant hardiness zones were originally developed in 1960 with the assistance of the American Horticultural Society. The original 11 zones became synonymous with �mean minimum temperature� because the zones were stratified by increments of ten degrees Fahrenheit, and were identified with the average lowest winter temperature for a geographic region. In fact, the original basis for the zones was latitude and longitude with local variances applied for wind, snow, winter sunshine, humidity and soil.

The zones were updated in 1990 splitting each of zones 2 through 10 into an upper and lower half, labeled �a� and �b�, which were assigned five degree Fahrenheit increments in the legend. This latest version of the map is based on lowest recorded temperatures in the United States and Canada between 1974 and 1986; and the lowest recorded temperatures in Mexico between 1971 and 1984.

The zone ratings in practice are used to indicate not just mere survival of a plant species, but excellent adaptability of the species to the climate.