Avid gardeners refer to the United States Department of Agriculture (USDA) Hardiness Zone Map of their region when purchasing plants. These zones cover a 10-degree Fahrenheit range of average annual, minimum temperatures (cold hardiness). The American Horticulture Society (AHA), recognizing that extreme heat also damages plants, developed the AHS Heat Zone Map. Sunset Magazine recognized that adjoining zones merge near their boundaries and developed a climate zone map that takes in factors other than cold and heat.
USDA Hardiness Zones
When referring to the USDA Hardiness Zone map, note that the numbers on the map correlate to the coldest winter temperatures in that region. For instance, Des Moines, Iowa ranks number 5 on the map. This region's coldest winter temperatures range from -20 to -15 degrees Fahrenheit. When purchasing plants, the label usually tells in which cold zone the plant can survive. According to the Azalea Society of America, azaleas are hardy in Zones 6 to 8 with minimum winter temperatures -10 to +20 Fahrenheit. Since Des Moines' low winter temperatures dip to -20 to-15 degrees Fahrenheit, azaleas are not hardy in Zone 5.
American Horticulture Society Heat Zones
Use the American Horticulture Society Heat Zone map the same way you use the USDA map. The AHS map displays 12 heat zones. These zones represent the average number of days each year that a given region experiences heat days (temperatures over 86 degrees Fahrenheit). Four numbers correlate to each plant. For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 7 and AHS Zone 7, tulips can remain in your garden year-round. An ageratum may be 10-11, 12-1 meaning that it thrives in the hottest regions of the United States in summer and the warmest U.S. zones in winter. An English wallflower may be 5-8, 6-1 meaning that it thrives in extreme cold conditions in winter and can tolerate moderate heat during the summer.
Sunset Climate Zones
According to Sunset Magazine, "A plant's performance is governed by the total climate: length of growing season, timing and amount of rainfall, winter lows, summer highs, wind and humidity." Therefore, Sunset developed climate zone maps that take all of these factors into consideration. Gardeners in the western United States are familiar with Sunset's Western Garden Book. As of the seventh printing, Sunset updated climate zones for all states and included new maps for Alaska, Hawaii and southwestern Canada. The Western Garden Book includes comprehensive information on each plant and its cultivars.