Planting Hardiness Zones

Each plant requires a certain temperature range for its growth and survival. A plant's hardiness refers to the minimum temperature at which the plant will survive. In 1960, The United States Department of Agriculture published its first Plant Hardiness Zone map. In 2010, the USDA and American Horticultural Society were preparing an updated map to reflect warming temperature data.

Features

• The USDA Plant Hardiness Zone map of 1990 divides the United States, Canada and Mexico into 11 color-coded zones based on average annual minimum temperature. Zone designations range from minus 50 degrees F in Zone 1 to 40 degrees F in Zone 11. According to Colorado State University Cooperative Extension, the updated map will add four new zones, and it will remove the "a" and "b" divisions within Zones 2 through 10.

Function

• Growers consult a hardiness zone map to identify the most suitable plants for their areas. According to the University of Illinois Extension Service, the USDA map is consulted more than any other map for this purpose.

Considerations

• An urban or high elevation area may have a zone designation that differs from its surrounding area. According to the University of Illinois Extension, each garden or neighborhood may contain microclimates that fall under various hardiness zones. Choose plants for your zone according to actual temperature, not wind chill temperature.