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How to read the USDA Frost Zone Map

Date Posted: April 5, 2016

Seeds of the Month Club by Mike the Gardener How to read the USDA Frost Zone Map
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You may have been gardening your whole life and never knew that your corner of the country is in a zone. A plant hardiness zone that is. If you live in the United States of America like I do (and you may having something equivalent outside of the USA), then you will come to know, as your vegetable gardening experience continues to grow, that the country is broken into plant hardiness zones.

Plant hardiness zone designations represent the average annual extreme minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time.

These zones appear on a map of the US that is distributed by the United States Department of Agriculture. The last time a zone map was released was back 1990 and the zones were created using temperature data from a 13 year period (1974 to 1986).

The USDA believes they have improved upon the original map with new data. There are now 13 zones as opposed to 11, and the new zones are now divided into “A” and “B” zones using 5 degree Fahrenheit differentials.

According to the USDA, “Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period (1976 - 2005).”

Large posters of the new map will no longer be available for purchase. The USDA website at has an interactive map that users can click on down to their exact location and give them detailed plant starting dates.

For example, on the old map, New Jersey, where I am from, was split into zones 6 & 7. Now there is 6a, 6b, 7a and 7b with all 4 zones being spread throughout the entire state. My zone went from 7 to 7a (ironically the same exit off the turnpike {inside joke for New Jersians}), however the temps and cold frost dates are virtually unchanged for my area.

So what does this mean for your growing adventures? Well, the more detailed version will allow you to better estimate when to start your plants indoors and of course move them outdoors to avoid any chance of frost. While this new map is more accurate, according to the USDA, you should always take precautions with your plants outdoors as you never know when you will get that one last blast of frost.

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Mike the gardener

About the Author

Mike Podlesny is the author of the book Vegetable Gardening for the Average Person as well as the creator of the Seeds of the Month Club where members receive non gmo, heirloom variety seeds every month. You can listen to Mike each week on the Vegetable Gardening Podcast where he interviews gardening industry experts. Don`t forget to link up with Mike on Twitter, Instagram, Pinterest and Facebook.


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If you want to learn more about USDA Zone Map, check out these titles below:


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