Understanding Gardening Zones

Written by April Reinhardt (last updated February 8, 2023)

The next time you pick up a garden catalog, pay attention to the hardiness zone number assigned to each plant. Those numbers correspond to specific regions in which the plant will thrive. For instance, the tulips I want to plant are recommended for zones 3 through 8. That means that if I live in any zone outside of that range, I'm taking a chance that the tulips will not grow optimally, will sprout but not produce blooms, or will not sprout at all.

First developed and published by the United States Department of Agriculture in 1960, the hardiness zone map indicates by geographical region minimum and maximum temperatures for that zone. The USDA averages the lowest temperature for a zone over several winters, and concludes an average annual minimum temperature. As an example, if five winters in succession in a region reach a minimum temperature of -8, -5, -12, -3, and -7 (all in Celsius), then the mean coldest temperature is 7, placing the region in zone 7b. In the United States, that zone encompasses parts of Arkansas and Georgia.

In 2006, the Arbor Day Foundation revised the USDA hardiness zone map to reflect the changes in warming in North America, based on fifteen years' data provided by the National Oceanic and Atmospheric Administration. While some zones have not changed, others have warmed two full zones. One of the main differences between the two zone maps is that the Arbor Day map does not include any half-zone delineations and, instead, uses ten zones, as compared to the eleven zones of the USDA map.

An organization named Sunset developed climate zone maps of their own that consider total humidity, rainfall, and summer highs and lows—instead of just the winter lows the USDA map uses. Sunset subscribes to the theory that a plant's performance is controlled by many factors, and their climate zones take into account all of those factors. The USDA zone chart basically lets you know where a plant will survive a winter, while the Sunset chart tells you where the plant will grow year 'round.

Finally, the American Horticultural Society published a Heat Zone Map in 1998. Using data from the National Climatic Data Center, the Society created a map divided into twelve zones. Each zone specifies an average annual number of heat days. Heat days are days with an average temperature over 86 degrees Celsius, and that is the temperature at which plants begin to experience damage from heat.

So, what does all of this information mean for the average gardener? Simply put, if you want to grow a wide variety of plants, cross reference all of the information from all zone maps, and then choose plants that will thrive within your environment. Keep in mind that other factors within your own environment will also determine your success as a gardener, especially if you use a greenhouse or enclosed patio to grow plants.

Author Bio

April Reinhardt

An admin­istrator for a mutual fund man­age­ment firm, April deals with the writ­ten word daily. She loves to write and plans to author a memoir in the near future. April attend­ed More­head State Uni­ver­sity to pursue a BA degree in Ele­men­tary Edu­ca­tion. ...

MORE FROM APRIL

Organizing the Kitchen Pantry

Organizing your kitchen pantry is relatively easy, if you envision it as a micro version of a grocery store. Your grocer ...

Discover More

Making Furniture Dusting Easier

Furniture dusting is no longer a chore, now that there are better products available for dusting. You don't even have to ...

Discover More

Polishing Silver

Used to make flatware, jewelry, and dishes, silver is a versatile metal. While not as soft as gold, silver is soft and ...

Discover More
More Gardening Tips

Setting Up Gardening Zones

One of the best things you can do for your lawn is to plan what you plant. Make a map of your lawn and the different ...

Discover More

Creating an Adventure Garden

If you are looking for a themed garden that everyone can enjoy, you really can't go wrong with an adventure garden. These ...

Discover More

Combining Colors in the Garden

Perhaps one of the most common dreams for all gardeners is have a beautiful garden that is full of colors. However, there ...

Discover More
Subscribe

FREE SERVICE: Receive an e-mail several times each week with a featured gardening tip. Enter your address and click "Subscribe."

Comments

If you would like to add an image to your comment (not an avatar, but an image to help in making the point of your comment), include the characters [{fig}] (all 7 characters, in the sequence shown) in your comment text. You’ll be prompted to upload your image when you submit the comment. Maximum image size is 6Mpixels. Images larger than 600px wide or 1000px tall will be reduced. Up to three images may be included in a comment. All images are subject to review. Commenting privileges may be curtailed if inappropriate images are posted.

What is 3 - 0?

There are currently no comments for this tip. (Be the first to leave your comment—just use the simple form above!)


Subscribe

FREE SERVICE: Receive an e-mail several times each week with a featured gardening tip. Enter your address and click "Subscribe."

(Your e-mail address is not shared with anyone, ever.)