Everyday Math

Math and the Outdoors

What do plant hardiness zone numbers mean?

Plant hardiness zones are another way numbers are used. In this case, they indicate the average annual minimum temperatures for landmasses around the world. For example, one common Plant Hardiness Zone Map is broken down into 20 different zones based on the average annual minimum temperatures. In zone 5a, for example, the average annual minimum temperature ranges from -20 to -15 degrees Fahrenheit (-26.2 to -28.8 degrees Celsius; for example, Des Moines, Iowa). In zone 11, such temperatures are above 40 degrees Fahrenheit (4.5 degrees Celsius; for example, Honolulu, Hawaii). And in zone 1, such temperatures are below -50 degrees Fahrenheit (-45.6 degrees Celsius; for example, Fairbanks, Alaska). There are other maps, too, that break down the zones into even more detail.


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