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?

Public Domain

Being unable to head to warmer weather each winter, hide under an umbrella in the peak of summer, or go in search of more food or better conditions, plants are stuck where they are. Wherever they are, plants have basic needs that must be met:

• enough sun exposure and rain
• appropriate temperatures
• adequate nutrients
• favorable soil pH
• good air movement
• not too many pests and diseases

If they are planted in the right location, they can grow and thrive. Remove any of those favorable conditions and life can get downright difficult. One of the most important factors to plant health is temperature. Choosing plants suited to your local climate can make all the difference in whether or not they thrive or die. The USDA Hardiness Zone map can help.


History of the Hardiness Zone map

In an effort to help farmers and gardeners select the best plants for a particular geographic area, the U.S. Department of Agriculture came up with the Hardiness Zone map in 1960. This map was revised in 1965, 1990, and 2012. These revisions are due to more accurate measurements and expanding heat islands (cities, roads, and urban development). The most recent version is based on temperatures collected from 1976 through 2005. In this version, two new zones were added.

How the Hardiness Zone map works

The Hardiness Zone map collects average annual minimum winter temperatures from weather stations across a region to designate a zone. These zones are divided into bands that are 10°F apart. Average winter minimum temperatures are used because they are critical to a plant’s ability to survive. Each zone is given a number between 0 and 12. Each number is then subdivided into “a” or “b”, depending on those average minimum winter temperatures. 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 its own microclimate, with unique temperatures, weather patterns, and countless other variables. The Hardiness Zone map is simply a starting point. The Hardiness Zone map does not take summer temperatures, snow cover, number of frost dates, soil moisture, or the likelihood of a severe cold snap into account. For example, Seattle, Washington, and parts of Dallas, Texas, are both rated as zone 8b, because they have very similar winters, but their summers are extremely different. These differences must be taken into account to ensure plant survival.

Hardiness Zone temperature ranges (USDA) Public Domain

How to use the USDA Hardiness Zone map

The 2012 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. I live in Hardiness Zone 9b, which means the local average winter minimum temperature is 25 to 30°F. But, because I know my yard, I know that those temperatures are not entirely accurate. For more reasons than I know, my yard has a microclimate that is always a little warmer than local weather reports suggest. This means I can provide plants with slightly warmer winter temperatures. It also means that I must provide more irrigation and better sunburn protection through the summer.


Try hanging a thermometer somewhere in your garden to fine tune the information you get from the USDA Hardiness Zone map. And check out your Sunset Zone, as well.