The USDA Unveiled a New Hardiness Zone Map That May Alter Your Garden Plans

The U.S. Plant Hardiness Zone Map got a refresh for the first time in over a decade, providing you with a guide on when and what you should plant in your garden—and things are literally heating up.

The U.S. Department of Agriculture released an updated version of their Plant Hardiness Zone Map last week, which allows gardeners from all over the country to determine which perennials are most likely to succeed in their area. Once you figure out how to navigate the interactive map, it’s a total lifesaver.

“Overall, the 2023 map is about 2.5 degrees warmer than the 2012 map across the conterminous United States,” Christopher Daly, director of the PRISM Climate Group and the map's lead author, told PHYS News. “This translated into about half of the country shifting to a warmer 5-degree half zone, and half remaining in the same half zone. The central plains and Midwest generally warmed the most, with the southwestern U.S. warming very little.”

While this change may seem subtle, it could affect the first average frost date in your area. This date tells you when you may need to bring your more vulnerable plants inside or clear your garden out for the season. It could even expand your planting options beyond your usual perennials.

You’ve likely seen zone numbers in the descriptions of plants in catalogs and at your favorite nursery (or our articles!). These refer to how much cold a perennial can withstand. If you're not sure which zone you live in, you only have to enter your zip code.

The map displays a map based on the average annual extreme minimum winter temperature. Zone 1 is the coldest, and Zone 13 is the warmest, counting in 10 degree intervals. Each Zone is divided into “a” and “b” half Zones. For instance, Zone 2a is five degrees colder than 2b and ten degrees colder than 3a. The map shows the average of the lowest temperatures of the past 30 years—not the lowest ever.

“The addition of many new stations and more sophisticated mapping techniques using the latest PRISM technology led to a more accurate and detailed Plant Hardiness Zone Map but also produced localized changes that are not climate-related,” Daly said.

The USDA webpage featuring the map also includes a series of videos titled “Tips for Growers,” which helps gardeners figure out which plants to plant, how to keep them healthy, and even how to breed them.