A planting zone in the United States usually refers to one of the USDA horticultural planting zones which, according to the USDA, "divides the United States and southern Canada into 11 areas of 10 degree F differences in the average annual minimum temperature." These zones are also known as hardiness zones, with the definition of hardiness as the coldest average temperatures a plant can tolerate. For example, a plant suitable for zone 8 should survive the coldest average temperature of zone 8.
The first hardiness zone maps were developed in the 1920s by Henry Skinner, Director of the U.S. National Arboreteum. The idea was to establish hardiness zones and match plant hardiness to the zones so gardeners as well as professional growers would know the coldest zones in which a plant can survive.
The maps and zonal information were improved as more climatic data allowed the maps to be more accurate. In 1990, zone 11 was added so Mexico and Canada could be included in the map, and the previous 10 zones were divided into two sections each, known as A and B zones.
Other zone maps are now available through the American Horticultural Society that include average last and first frost date zones and a plant heat zone map that refers to the hottest temperatures in which certain plants can survive.
Consulting the planting zone maps and designations means that you won't waste time and money trying to plant things that cannot grow in your area. Plant nurseries and other plant retailers usually know what plants to sell to customers and will help you can accurately choose plants that will grow well where they are planted.
Landscaping and gardening that involves ornamental plants has become very popular as a hobby and leisure activity. This is different from years ago when gardening involved raising food crops as a means of survival and only a small portion of the landscape involved ornamental or flowering plants, usually given to the homeowner from a friend or relative. In addition, invasive plant species are now growing outside of their native territory because they can survive according to the designated zones. Many times they are planted where they have no natural means of control or natural enemies; they escape into the countryside and crowd out native species.
Just because a plant is designated to grow in a particular zone and is planted in that zone does not guarantee survival. Soil pH, available moisture, light levels and soil structure are other factors that affect gardening success. Designated planting zones are sometimes affected by microclimates--small areas such as dense urban landscapes that may stay warmer or cooler than the general zone. For example, citrus plants are often seen growing in areas well north of their appropriate planting zone.
Zonal maps are updated every few years due to changes in the earth's climate. For example, many of the changes that occurred on zonal maps between 1960 and 1990 were specifically related to global warming.