What is a Hardiness Zone?

Have you ever wondered why certain plants thrive in your garden while others struggle? Well, that's where hardiness zones come into play!

Hardiness zones (sometimes called growing zones, gardening zones or planting zones) are like neighborhoods for plants. They're geographical regions based on an area's average annual minimum temperatures, and they help determine which plants are likely to flourish in a particular area.

Back in the early 1960s, the clever folks at the United States Department of Agriculture (USDA) created the USDA Hardiness Zone Map which, with revisions based on climate change and modern measuring techniques, is the primary map used today by gardeners and farmers throughout the United States. The USDA map wasn't the first though. The Arnold Arboretum in Boston Massachusetts developed similar gardening zone maps starting as early as 1927; and there are zone maps for many other countries around the world as well.

Okay, back to the map...

Picture the United States divided into 13 unique zones, ranging from the coldest Zone 1 to the warmest Zone 13. Each zone even has sub-zones, labeled a or b, to provide even more specific information. For example, Zone 4a is slightly cooler than Zone 4b; and Zone 5a is slightly cooler than Zone 5b; and so on as shown below. You can learn more about the latest map, and get data about your own location by visiting the USDA Plant Hardiness Zone Map Website.


usda hardiness zone


USDA Hardiness Zone Average Extreme Minimum Temperatures Typical Last Frost Date Typical First Frost Date
Zone 1 -60 to -50 F May 22 - June 4 Aug 25 - Aug 31
Zone 2 -50 to -40 F May 15 - May 22 Sept 1 - Sept 8
Zone 3 -40 to -30 F May 1 - May 16 Sept 8 - Sept 15
Zone 4 -30 to -20 F Apr 24 - May 12 Sept 21 - Oct 7
Zone 5 -20 to -10 F Apr 7 - Apr 30 Oct 13 - Oct 21
Zone 6 -10 to 0 F Apr 1 - Apr 21 Oct 17 - Oct 31
Zone 7 0 to 10 F Mar 22 - Apr 3 Oct 29 - Nov 15
Zone 8 10 to 20 F Mar 13 - Mar 28 Nov 7 - Nov 28
Zone 9 20 to 30 F Feb 6 - Feb 28 Nov 25 - Dec 13
Zone 10 30 to 40 F Typically No Frost in Region
Zone 11 40 to 50 F Typically No Frost in Region
Zone 12 50 to 60 F Typically No Frost in Region
Zone 13 60 to 70 F Typically No Frost in Region


So, how do they figure out these zones? Well, it's all about the average minimum winter temperature recorded in a particular area over about 30 years. Using that data, the USDA map provides a general idea of the average low temperatures in each region of the country, which is a great indicator for determining which plants will be likely to survive winter in any particular area.

Let's dive into a few important things to know about hardiness zones:

Temperature Range: Hardiness zones are all about that average annual minimum temperature. By knowing the temperature range of your zone, you can determine which plants can brave the winter in your neck of the woods.

Frost-Free Period: Hardiness zones can also be used to tell you how long your growing season might be. The frost-free period is the typical number of days between the last frost in spring and the first frost in fall. It helps you figure out if you have enough time to grow certain plants.

Plant Selection: Just like people, plants have different preferences. Some like it hot, while others prefer a cooler climate. By knowing your hardiness zone, you can choose plants that are perfectly suited to your local climate, increasing their chances of thriving.

Plant Hardiness: We call a plant "hardy" if it can handle the cold like a champ. That's why plants often come with a recommended hardiness zone. It tells you which zones they're most likely to survive and thrive in, keeping your garden happy and healthy.

Climate Variations: Keep in mind that hardiness zones are a helpful guide, but the USDA map doesn't take into account other climate factors like humidity, rainfall, or wind patterns. Plus, little pockets of microclimates within a zone, like urban heat islands or breezy coastal areas, can influence plant growth too.

To find out your specific hardiness zone, you can check out the USDA Hardiness Zone Map or similar regional maps for your part of the world. Knowing your zone is like having a secret weapon when planning and selecting plants for your garden. It increases the odds of successful gardening and ensures your beloved plants have the best shot at survival. So, go ahead and embrace your hardiness zone—it's your plant's personal comfort zone!