USDA Hardiness Zones

You’ve probably seen plant labels, seed packets, online information, and comments in gardening books about hardiness zones, but what are these zones, and how can they help you in the garden? ​

Plants cannot migrate to warmer climates each winter or hide under an umbrella in the peak of summer. They are stuck where they are. Plants must get the sun exposure, nutrients, appropriate soil pH, and rain they need where they are.

Planted in the right location, they can grow and thrive. Life can become much more difficult if there are not enough favorable conditions. And the ambient temperature is one of those critical components of plant health. Choosing plants suited to your local climate can make all the difference in whether or not they thrive or die. The USDA Plant Hardiness Zone map can help.

History of the Hardiness Zone map

To help farmers and gardeners select the best plants for a particular geographic area, the U.S. Department of Agriculture created the Plant Hardiness Zone map in 1960. More accurate measurements and expanding heat islands (cities, roads, and urban development) have led to revisions in that map in 1965, 1990, 2012, and 2023.

How the Hardiness Zone map works

The Hardiness Zone map collects average annual minimum winter temperatures from weather stations nationwide to create specific zones. These zones are bands that are 10°F apart. Average winter minimum temperatures are critical to a plant’s ability to survive. Each zone is assigned a number between 0 and 12 and subdivided into “a” or “b” groups. Knowing your Hardiness Zone can help you select plants best suited to your region.

Limitations of the USDA Hardiness Zone map

As useful as the Hardiness Zones map is, it has limitations. Simply reading a label that says a plant is “hardy to Zone [insert your number]” is not enough to make an informed decision. Each location is also a microclimate, with unique temperatures, weather patterns, and countless other variables. The Hardiness Zone map is simply a starting point. For example, the Hardiness Zone map does not consider summer temperatures, snow cover, frost dates, soil moisture, or the likelihood of a severe cold snap into account.

For example, Seattle, Washington, and Yuba City, California, are rated zone 9b because they have similar winters. However, their summers are extremely different. You must include these differences when selecting plant species and varieties if you want them to survive.

Hardiness Zone temperature ranges (USDA)

How to use the USDA Hardiness Zone map

The 2023 Hardiness Zone map is available online. All you have to do is enter your zip code. Under the search field, you will see your zone. You can also look at a static state map that has more detail. 

But you need to know your yard. Your microclimate may be warmer or colder than local weather reports suggest. Those differences will dictate which plants you select, where you put them, and how much irrigation they need.

Try hanging a thermometer in your garden to fine-tune the information you get from the USDA Hardiness Zone map.