Millions of American households have a garden of some sort, whether they grow vegetables, fruits flowers or other plants. Gardening has always been a popular hobby, but more Americans turned to tending plants during the height of the COVID-19 pandemic for both stress relief and to grow their own food so they could make less trips to the store. For many people, it’s a seasonal ritual that’s therapeutic and rewarding.
But a shift is occurring in the gardening world. Now, due to rising temperature data, half the country find themselves in a different “plant hardiness zone”—the zones that indicate what plants work well in an area and when to plant them. Gardeners rely on knowing their hardiness zone to determine what to plant and when, but they haven’t been updated since 2012.
The U.S. Department of Agriculture updated its Plant Hardiness Zone Map in late 2023, months before people in most of the country start planning their planting. We saw the 10 hottest summers ever recorded in 174 years of climate data between 2014 and 2023, but hardiness zones are actually determined by the coldest winter temperatures each year. Winters are warming at an even faster pace than summers, according to nonpartisan research and communications group Climate Central, but that may or may not be the entire reason behind the zone changes.
The USDA acknowledges that some of the zone shifts could be due to climate change but cautions against using them as hard evidence for it since factors such as improved data collection also contribute to changes in the map.
“Temperature updates to plant hardiness zones are not necessarily reflective of global climate change because of the highly variable nature of the extreme minimum temperature of the year, as well as the use of increasingly sophisticated mapping methods and the inclusion of data from more weather stations,” the USDA wrote in November 2023. “Consequently, map developers involved in the project cautioned against attributing temperature updates made to some zones as reliable and accurate indicators of global climate change (which is usually based on trends in overall average temperatures recorded over long time periods).”
At the same time, Chris Daly, director of the PRISM Climate Group at Oregon State University that developed the map with the USDA, told NPR, “Over the long run, we will expect to see a slow shifting northward of zones as climate change takes hold.”
As an example of zone shifting, Dallas, Texas, was classified as Zone 8a in 2012, when data showed the coldest winter temperature in the city was between 10 and 15 degrees Fahrenheit on average. In 2023, with data showing the coldest winter temps falling between 15 and 20 degrees Fahrenheit, it’s been shifted to Zone 8b.
Some zone shifts resulted in moving to an entirely new zone number, such as Seattle shifting from Zone 8b to Zone 9a. The overall trend was for zones to be pushed northward, but not all areas saw a shift. NPR has a helpful tool here in which you can enter your zip code, see what zone your city was previously in, what zone it’s in now, and the temperature changes that caused the shift.
The bottom line is if you have a gardening book with a hardiness zones map printed before 2024, it’s time for an updated map. Or check online to see what zone you fall in now to give your garden the best chance of thriving this year.