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Deciphering the USDA Plant Hardiness + Sunset Climate Zone Maps

By Carrie Stephens, College of Western Idaho Horitculture Student


Have you ever planned a vacation with a group of friends? Outside of the occasional miracle, the group can never agree on where to go. Some people will vote for a sunny destination with dry hot days and cool breezy nights, others want to head to the mountains to enjoy the rugged terrain and crisp chill of the air, while others want to head to the tropics to soak in the humidity and abundance of water. What does this have to do with the USDA hardiness zones? Well, like people, plants have climates that suit their “personalities” best. By knowing which plants will thrive in your hardiness zone, you can ensure they will be happy and grow to the best, biggest, happiest, most beautiful versions of themselves. 

Now that we understand that different plants thrive in different zones, let’s explore how the zones were created in the first place. USDA hardiness zones were developed to categorize North America and Southern Canada into distinct temperature regions based on the average annual minimum winter temperature across a 10-year period. The goal of these zones is to provide valuable information as a starting point for farmers and gardeners to determine which plants are most likely to be sustainable and thrive in their specific geographic area. The map is divided into 13 main zones representing a 10°F difference in the average minimum temperature, and 13 sub zones representing a 5°F difference specified by “a” or “b” within the main zone. For example, Zone 1 represents the coldest minimum winter temperatures (typically below -60°) while Zone 13 has the warmest (typically above 60°). The USDA provides interactive online maps where you can input your zip code to determine your hardiness zone here.

Plant Hardiness Map from USDA

It's important to remember that USDA hardiness zones are based solely on long-term temperature trends, and it may not capture short-term fluctuations or localized climate changes. Therefore, it's always a good idea to supplement the information with other sources such as the Sunset Climate Zone Map which primarily focuses on the western United States from the Pacific Coast to the Rocky Mountains and takes into consideration multiple climate factors, including winter lows, summer highs, rainfall patterns, humidity, wind, and elevation. The Sunset Zone system outlined here and developed by Sunset magazine, offers guidance on a broader range of plant characteristics such as heat tolerance, drought resistance, and soil preferences, making it a more comprehensive resource for assessing a plant's suitability for a particular area. You can look up your sunset zone on a map here.

Plants, just like that one friend we all have who is always cold and bundled up under a blanket until barbecue season is in full swing, are grouped by how grumpy they get when the weather is cold. Plants are classified according to their cold hardiness, which is their ability to withstand the lowest winter temperatures typically experienced within a zone without getting damaged or killed. When choosing plants for your garden, it's important to select ones that are suitable for your zone.  It’s recommended to choose perennial plants that are suitable for one zone below yours, for example, if you live in zone 7a you would choose a plant hardy to zone 6a to ensure their success over several years and to ensure they are tough enough to withstand the occasional harsh frosts, freezes and cold snaps.  

As gardeners in the Intermountain West region of the United States, we face unique challenges due to our climate extremes, including hot, dry summers and cold, snowy winters. Selecting plants that are well-adapted to these conditions is crucial for success.  Native plants are excellent choices because they often require less water and are low maintenance once established, making them environmentally friendly options. There are also several species of plants from across the world that have similar climates to ours and will do well here, adding a special uniqueness to suit your personal aesthetic. 

Now that you know the basics of deciphering the hardiness zone for your area, you’re ready to head to your nearest nursery to peruse the shelves of seeds packets where you can find all the plant care information your need, including hardiness zones on the back of the seed package. When you purchase some potted plants on your next nursery visit (because plant math!) you can find information on the plant tags in the pots.  If you’re interested in additional information on the plants you have growing or flowers you’re dreaming about, we recommend jumping on the internet and checking out seed company websites. They have a wealth of additional information, and some will have additional resources such as the history of the plant, farmer information and planting guides for the different seasons.  I hope this helps you better understand and demystify the USDA hardiness zones as you set off onto the wonderful and wild world of gardening.

Grow What You Like to Eat!

Deciphering the USDA Plant Hardiness + Sunset Climate Zone Maps

By Carrie Stephens, College of Western Idaho Horitculture Student


Have you ever planned a vacation with a group of friends? Outside of the occasional miracle, the group can never agree on where to go. Some people will vote for a sunny destination with dry hot days and cool breezy nights, others want to head to the mountains to enjoy the rugged terrain and crisp chill of the air, while others want to head to the tropics to soak in the humidity and abundance of water. What does this have to do with the USDA hardiness zones? Well, like people, plants have climates that suit their “personalities” best. By knowing which plants will thrive in your hardiness zone, you can ensure they will be happy and grow to the best, biggest, happiest, most beautiful versions of themselves. 

Now that we understand that different plants thrive in different zones, let’s explore how the zones were created in the first place. USDA hardiness zones were developed to categorize North America and Southern Canada into distinct temperature regions based on the average annual minimum winter temperature across a 10-year period. The goal of these zones is to provide valuable information as a starting point for farmers and gardeners to determine which plants are most likely to be sustainable and thrive in their specific geographic area. The map is divided into 13 main zones representing a 10°F difference in the average minimum temperature, and 13 sub zones representing a 5°F difference specified by “a” or “b” within the main zone. For example, Zone 1 represents the coldest minimum winter temperatures (typically below -60°) while Zone 13 has the warmest (typically above 60°). The USDA provides interactive online maps where you can input your zip code to determine your hardiness zone here.

Plant Hardiness Map from USDA

It's important to remember that USDA hardiness zones are based solely on long-term temperature trends, and it may not capture short-term fluctuations or localized climate changes. Therefore, it's always a good idea to supplement the information with other sources such as the Sunset Climate Zone Map which primarily focuses on the western United States from the Pacific Coast to the Rocky Mountains and takes into consideration multiple climate factors, including winter lows, summer highs, rainfall patterns, humidity, wind, and elevation. The Sunset Zone system outlined here and developed by Sunset magazine, offers guidance on a broader range of plant characteristics such as heat tolerance, drought resistance, and soil preferences, making it a more comprehensive resource for assessing a plant's suitability for a particular area. You can look up your sunset zone on a map here.

Plants, just like that one friend we all have who is always cold and bundled up under a blanket until barbecue season is in full swing, are grouped by how grumpy they get when the weather is cold. Plants are classified according to their cold hardiness, which is their ability to withstand the lowest winter temperatures typically experienced within a zone without getting damaged or killed. When choosing plants for your garden, it's important to select ones that are suitable for your zone.  It’s recommended to choose perennial plants that are suitable for one zone below yours, for example, if you live in zone 7a you would choose a plant hardy to zone 6a to ensure their success over several years and to ensure they are tough enough to withstand the occasional harsh frosts, freezes and cold snaps.  

As gardeners in the Intermountain West region of the United States, we face unique challenges due to our climate extremes, including hot, dry summers and cold, snowy winters. Selecting plants that are well-adapted to these conditions is crucial for success.  Native plants are excellent choices because they often require less water and are low maintenance once established, making them environmentally friendly options. There are also several species of plants from across the world that have similar climates to ours and will do well here, adding a special uniqueness to suit your personal aesthetic. 

Now that you know the basics of deciphering the hardiness zone for your area, you’re ready to head to your nearest nursery to peruse the shelves of seeds packets where you can find all the plant care information your need, including hardiness zones on the back of the seed package. When you purchase some potted plants on your next nursery visit (because plant math!) you can find information on the plant tags in the pots.  If you’re interested in additional information on the plants you have growing or flowers you’re dreaming about, we recommend jumping on the internet and checking out seed company websites. They have a wealth of additional information, and some will have additional resources such as the history of the plant, farmer information and planting guides for the different seasons.  I hope this helps you better understand and demystify the USDA hardiness zones as you set off onto the wonderful and wild world of gardening.

Grow What You Like to Eat!