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Planting Zones Explained

How To Find Your Hardiness Zone And Why It’s Important To Your Garden’s Success

Even if you’re fairly new to gardening, you’ve probably heard the term hardiness zone. It’s mentioned frequently in growing guides and seed packets, and most nursery plant tags are labeled with a corresponding number. You might be wondering: what is a planting zone? What is my growing zone? Why does it even matter?

Hardiness zones–also called planting zones or growing zones–are geographic locations defined by that region’s average lowest winter temperatures. Hardiness zones are an important reference point for gardening because plants thrive in different climates. Knowing which plants are best suited to a particular region will improve your success with that crop. 

Keep reading to learn more about planting zones, how they are used to plan out gardens, and what pieces of the picture the USDA hardiness system lacks. 

What is plant hardiness?

Plant hardiness is essentially the ability of a plant to survive in a particular climate. While hardiness is typically used to refer to a plant's ability to withstand winter lows, sometimes summer temperatures and precipitation are more of a stressor that freezing temperatures.

Knowing your area’s climate is important for choosing perennial plants that grow year-round. While most annual plants will survive nearly everywhere for the summer growing season, knowing your growing zone will help you determine when you can start annual seeds and transplant seedlings outdoors to maximize your harvests. Based on your hardiness zone, you can predict how well your plants will do. 

Hardiness zone by zip code

The quickest way to find your hardiness zone is to use your zip code as a reference. 

USDA map

The growing zones most commonly used in the United States were devised by the United States Department of Agriculture, available here. Simply put your zip code into the search bar, and the website will automatically calculate your hardiness zone for you.


If you’re not familiar with the growing zones concept, you might still be wondering what the map even means. In CNET’s excellent article on planting zones, the author writes that

The USDA Plant Hardiness Zone Map divides the United States and Canada into 13 zones, based on the average annual minimum winter temperature. Each zone is, on average, 10 degrees warmer or colder in the winter than the zone next to it, with Zone 1 being the coldest and Zone 13 the hottest (Price).

The coldest region, zone 1, is located in interior Alaska, while the warmest region, zone 13, describes the Hawaiian islands and Puerto Rico. As a general rule, lower numbers correspond to average lower temperatures. 

Planting zones are divided in increments of 10, but within each zone is another designation, a and b. A corresponds to the warmer five degrees of that zone, while B describes the colder five-degree range.  

Although updated about every five years, the USDA map is far from perfect. The hardiness zone map accounts for average low and high temperatures but doesn’t fully predict changes due to global warming. Other environmental factors, including precipitation, elevation, freeze-thaw cycles, and daylight hours are not accounted for.

Hardiness zone by climate zones

The north-south pattern that roughly dictates growing zones is a useful starting point but may better describe gardening zones in the east rather than the west. Western climates are harder to define by latitude since elevation, natural features like mountains and lakes or rivers, and arid conditions affect temperatures, moisture, and other factors not so easily defined. 

Sunset Climate Zones

For this reason, the University of California and Sunset Magazine devised a new system called Climate Zones (“Gardening Zones: A Guide to Western Climates”). 

Sunset climate zones take into account all the factors that the hardiness map does not. Divided into 15 climates, the Sunset Climate Zone system ranks mountain states like Montana, Wyoming, and Idaho as having the coldest winters, while California and Nevada’s high desert areas have the hottest summers. This new system also differentiates between maritime climates like the Pacific Northwest and arid climates like areas of Arizona and New Mexico. 

Conclusion

Neither hardiness zones nor climate zones are meant to be the rule–these are just tools to help gardeners in some areas of North America raise successful gardens. Neither is a perfect system, not always accounting for microclimates created by natural and manmade features. However, both of these resources are a great starting point to discover which plants will thrive in your region. 

Experience is always the best teacher, so don’t be afraid to try something a little out of your (hardiness) zone! There are plenty of tools, like cold frames and frost cloth, that can serve as a cold-weather buffer. You can also use shade cloths to protect sensitive plants from the scorching heat. Take note of what does well and what does not, and let your research, along with these resources, inform your crop planning decisions. 

Resources

USDA Plant Hardiness Zone Map, https://planthardiness.ars.usda.gov/. Accessed 23 June 2022.

Price, Molly. “Understanding Your USDA Hardiness Zone and Why It'll Keep Your Plants Alive.” CNET, 29 April 2022, https://www.cnet.com/home/yard-and-outdoors/understanding-your-usda-hardiness-zone-and-why-itll-keep-your-plants-alive/. Accessed 23 June 2022.

“Gardening Zones: A Guide to Western Climates.” Sunset Magazine, 19 August 2004, https://www.sunset.com/garden/climate-zones/sunsets-garden-climate-zones. Accessed 28 June 2022.

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Even if you’re fairly new to gardening, you’ve probably heard the term hardiness zone. It’s mentioned frequently in growing guides and seed packets, and most nursery plant tags are labeled with a corresponding number. Keep reading to learn more about planting zones, how they are used to plan out gardens, and what pieces of the picture the USDA hardiness system lacks.