What Are Plant Hardiness Zones?

Plant Hardiness Zones are areas on a map that tell you which plants will do best where you live. They are based on the average lowest temperatures of each region of the US. (Find your Plant Hardiness Zone on the official map here.) Hardiness zones are sometimes referred to as “growing zones” or “gardener’s zones” and are often confused with last frost dates. Paying attention to Zones will help to increase your planting probabilities, so that your efforts will be most successful. Read more.

If you live in Maine, the hardiness zone range is 3-6. So, as much as you would love to have orange trees (Zone 8) in your yard, their odds of surviving winter and bearing fruit are slim. They grow naturally as far north as North Carolina. Ure pears (similar to Barlett pears), on the other hand, will do nicely in Maine. They are Zone 4.

Look For Zone Numbers On Plant Tags Before Buying

When buying a plant (or seeds) look for the Plant Hardiness Zone number on its tag. This number, from 1-13, refers to the lowest temperatures that each plant can withstand without withering away—the lower the number, the lower the temperature it can endure.

For instance, Zone 3 is 10 degrees colder than Zone 4. Sometimes letters “a” and “b” are added to be more specific. Zone 4a has an average lowest temperature of -25 to -30 degrees Fahrenheit, whereas 4b’s lowest temperature is -20 to -25 degrees. Each city within a state can vary.

This information is calculated and updated periodically by the United States Department of Agriculture (USDA), based on a 30 year average of the single lowest temperature recorded.

Plant Hardiness Zones for the USA
Planting according to your area’s plant hardiness zone will help you be a better gardener.

Do Plant Hardiness Zones Change? 

Plant Hardiness Zones get adjusted on occasion. One of the first major changes was in 1990, when scientists at the USDA began looking at climate change data (What Is The Difference Between Weather And Climate?). A shift of the Hardiness Zone seemed to be moving north at a rate of 13 miles per decade. The latest Hardiness Zone map was adjusted in 2012, which introduced two new zones: 12 and 13. Prior to this, the range was only 1-11. This year marks ten years since the last official update, which means there is a chance that Hardiness Zones may change again very soon.

If the USDA changes the Hardiness Zones in your region, don’t worry! Whatever you have planted should continue to grow. However, if you start sowing new seeds into your garden, it’s wise to keep apprised to any changes to Plant Hardiness Zone maps.

History Of Plant Hardiness Zones

During the Great Depression, when people were growing their own food out of dire necessity, researchers began cultivating visual aids to educate people on what crops would yield the best results in the US. Maps were made separating the country into temperature zones. Prior to this, the Farmers’ Almanac was the only way gardeners could get information on the upcoming season in terms of planting.

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Larry Fleury

Larry Fleury is a writer and outdoor photographer who has a background in atmospheric science, marketing, astrophotography, creative writing, and all things outdoors. His photography has been featured by The Weather Channel, Midwest Living Magazine, and National Geographic Your Shot. Larry lives on the edge of the Ozark Mountain Range in Southeast Kansas, where he spends his free time fishing, camping, hunting, hiking, storm chasing, and playing guitar on the porch.

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