When shopping for plants at your local garden center, you may have noticed a zone number listed on the identification tag. Plant hardiness zones were developed by the U.S. Department of Agriculture (USDA) in 1960 to assist farmers in selecting crop plants that would thrive and over-winter successfully in their local area. Each zone indicates the geographic area where climatic conditions are suitable for the successful growth of particular types of plants. Plant growers soon adopted plant hardiness zones as a handy way of indicating to consumers where a particular plant would thrive.
Plant hardiness zones are differentiated primarily by average low temperatures and are typically depicted on maps as horizontal colored bands crossing the country and encircling the globe. Eleven hardiness zones have been identified from Zone 1 where low temperatures are below -50 degrees F. to sultry zone 11 where temperatures never drop below 40 degrees F. and there is no danger of frost.
Since their debut in 1960, the USDA has updated its plant hardiness zone maps periodically to reflect more accurate data. Each zone represents a 10-degree change in average lowest temperature. Revisions made in 1990, subdivided each zone into A and B zones to indicate more precise 5-degree temperature differences. Long-term changes in regional temperatures have also prompted some revisions. For example, a 2003 revision placed many areas half a zone higher (warmer) than the previous 1990 map, reflecting the trend toward warmer winters.
Zone numbers are less critical when buying plants locally than they are when using gardening books to plan your garden. Gardening books list plants and flowers that grow in many different environments and can have significantly different temperature needs; whereas locally-purchased plants are pre-selected for optimal growth in your local area.