What is a Hardiness Zone?

What is a Hardiness Zone? "Hardiness" is the ability of a plant to withstand cold temperatures. A Hardiness Zone Map is a geographic guide to plants that are suitable for your local climate based on the average cold temperatures. If you are buying plants through a mail order catalog or over the Internet or if you are reading a book on gardening, you will likely see references to hardiness zones for each plant. For example, a gardening guide might say that Brugmansia (Angel's Trumpet) grows best in Zones 10b through 11. By referring to the Hardiness Zone Map you could see if you live in either of those zones. If you do, Brugmansia's should do well. If not, you may have to take extra precautions, such as sheltering the plant from colder temperatures or even growing the plant indoors. The primary Hardiness Zone Map used in the United States is issued by the United States Department of Agriculture (USDA). This map divides North America into thirteen hardiness or temperature zones.

What Hardiness Zone do I live in?

If you go to the USDA Hardiness Zone Map you can input your zip code to see your zone (be prepared for some irritating captchas). The USDA site also allows you to download a detailed map of any state. If you live in a state like California where there are micro climates and the potential for multiple hardiness zones per zip code, I would recommend you use the USDA's Interactive Map that will give you a more detailed view of your area. Another good source for detailed maps is plantmaps.com


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.

Note that hardiness zones are based on average minimum temperatures, not the lowest temperatures. While you may live in Zone 10a where the average minimal temperatures range between 30 and 35 degrees, over a ten year period you will likely experience one or more years where the temperature dips into the 20's. So use the hardiness zone map as a guide to suitable plants in your area, but be prepared for occasional cold snaps that may require extraordinary steps to protect your plants.

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 updated in 2012 is based on temperature readings over a 30 year period. We are living in a time of increasing average temperatures and, as a result, hardiness zone maps will vary based on the time period used. As an example, see my page where I show the various hardiness zone maps for Florida compiled over the past 50 years.

In addition, the USDA concluded that the most important temperature factor is the ability of a plant to survive over the winter. However, a gardner or farmer also needs to consider the temperature 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 Zones 9 and 10 and are 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 high temperatures facilitated by the bathtub like heat from 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 Richmond on San Francisco Bay (Hardiness Zone 10a) the temperature will rarely get out of the 70's based on the moderating influence of the Pacific Ocean, while the denizens of Tampa and St. Petersburg (also Zone 10a) 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 Richmond. Yes, their average minimum winter temperatures are similar, but virtually every other temperature factor between Tampa and Richmond is different.

NEXT PAGE (for examples of Hardiness Zone maps)

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