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What is a Hardiness Zone?

What is a Hardiness Zone?

"Hardiness" is the ability of a plant to withstand cold temperatures. In order to provide a tool for identifying the extent of cold temperatures in a particular location, the United States Department of Agriculture (USDA) has issued a map that divides North America into eleven Hardiness Zones. Florida has 4 Hardiness Zones and most of Central Florida is located in Hardiness Zone 9.

How is a Hardiness Zone determined?

The USDA Hardiness Zone Map is based on the average annual minimum temperature readings from thousands of weather stations located around North America. The readings are grouped into bands based on 10 degree gradients. For example Zone 9 includes locations with annual minimum temperatures from 20 to 30 degrees F. The Zones are further segmented into a and b groups. Zone 9a includes sites with 20 to 24.9 degree F. readings and Zone 9b covers 25 to 29.9 degrees F. readings.

How accurate and reliable are the Hardiness Zones?

At best, the Hardiness Zones provide an "average" prediction for the likelihood that a particular plant will thrive, since any location over a period of time will experience temperatures that are lower than the "annual minimum". Hence, Hardiness Zones are a general guide, not an infallible predictor of success.

Another element to consider is the time period used to calculate the average temperatures. The current USDA Hardiness Zones Map (1990) is based on temperature readings from 1974 to 1986. Proposed updates to the 1990 Map use more recent data and reflect a general warming trend - many of the Zones are shown as moving northward.

In addition, the USDA concluded that the most important temperature factor is the ability of a plant to survive over the winter. This ignores the challenges presented during the other seasons. For example, plants can also be effected by excessive heat and humidity during the summer and many plants require a certain number of "chilling hours" during the late winter and early spring in order to flower. As a specific example, much of Florida and California are in Hardiness Zone 9 and influenced by weather from the adjacent large bodies of water (the eastern Pacific and Gulf of Mexico), but the climates could not be more different. Florida's rainy season is in the summer when the hot temperatures facilitated by the bathtub like temperatures of the Gulf create an ideal growing environment for fungus, compared to California when the rains come in the cool winter months off violent Pacific storms. On a typical summer day in Monterey (Hardiness Zone 9b) the temperature will rarely get out of the 70's based on the moderating influence of the Pacific Ocean, while the denizens of Tampa are hidden in their air conditioned homes avoiding the 90 degree hot and humid air. Conversely, Tampa has at least twice as many chilling hours during the winter months as Monterey. Yes, their average minimum winter temperatures are similar, but virtually every other temperature factor between Tampa and Monterey is different.

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