- The USDA updated its map of plant hardiness zones for the first time in over a decade.
- The new map shows that half the country has shifted to warmer zones.
- The updated zones could allow gardeners to grow plants that they never could before.
The USDA has updated its plant hardiness zone map for the first time in over 10 years. The new map could change how you garden.
Gardening consultant, Megan London, for example, told NPR she’s now considering growing an array of new treats including kumquats and mandarin oranges in her gardens in central Arkansas.
According to the new map, central Arkansas shifted half a zone up from zone 7b to zone 8a since the USDA last updated its map in 2012.
What are plant zones?
The USDA’s map is the national standard that lets gardeners and growers know what types of perennial plants — plants that return year after year — are most likely to thrive in certain locations. The map categorizes these locations by zones and half zones, the USDA said on its website.
The US is divided into 13 growing zones. Each zone represents the average lowest winter temperature an area typically sees every year, per the USDA.
A good chunk of southern Florida, for example, is zone 10b (35 to 40 degrees Fahrenheit) while northern Montana has pockets of zone 3b (-35 to -30 degrees Fahrenheit).
“This is all about winter cold, this map,” Chris Daly, director of the PRISM Climate Group at Oregon State University, told Business Insider. “It has nothing to do with planting zones in terms of when you plant or what can survive the summer.” He helped develop the map along with the USDA.
Half of the US has shifted to warmer plant zones
The new map for 2023 has pushed half the country into a warmer half zone, while the rest of the country has remained in the same zone, the USDA said in a press release.
For example, some areas of Omaha, Nebraska have moved from 5b to 6a. Meaning that the average lowest winter temperature for that region rose from between -15 and -10 degrees Fahrenheit to -10 to -5 degrees Fahrenheit.
These changes could mean a shift in what will grow best in your garden. When you’re purchasing perennials, you’ll often see their zone ranges listed, Daly said.
However, growers should be cautious, Jonathan Foster, a horticulture outreach professional, wrote on The University of Maine’s Maine Gardner Manual site.
Limitations of the USDA’s new plant zone map
“The map is a guideline, not a guarantee,” Foster wrote, and plants can thrive in several zones. However, you’ll want to consider other factors, like summer temperatures and soil quality as well. The map doesn’t reflect these nuances.
Foster also noted that more delicate plants, like poppies, might not fare as well as hardier trees and shrubs in snowy climates like Maine.
The map is also limited in how detailed it can get, Daly said. Your garden might get more sun than your neighbors or have shadier spots thanks to trees. “Those may have microclimates that are warmer or colder than what the zone says that you are,” Daly said.
What caused half the US to shift to a warmer zone?
The USDA said that the shift to warmer zones is “not necessarily reflective of global climate change,” because several factors contributed to the changes.
Daly and the USDA used data from 1991 to 2020 and chose the coldest night of the year. “We only have 30 numbers that we’re averaging together,” Daly said. That’s why it’s important to look at decades of data, he added.
The temperature on the coldest night can shift for a variety of reasons aside from climate change. One year might not have a cold snap, and the next winter’s could be particularly harsh, Daly said.
The new map is also based on information from thousands more weather stations, according to the USDA. It pulled data from 13,412 weather stations compared to 7,983 for the 2012 map, per the USDA’s press release.
Daly developed Prism, the software that created the map. “It’s really good at reproducing the effects of features on the Earth’s surface, such as mountains, valleys, and coastlines, on climatic patterns,” he said. It helped make the map more detailed and accurate than previous versions.
It’s also interactive. “You can enter your zip code or you can click anywhere in the map and zoom and pan on it and see what your zone is,” Daly said.
How climate change is influencing plant hardiness zones
In the grand scheme of things, climate change is influencing where plants can grow, Daly said.
“We know for a fact that average temperatures are rising due to climate change. There’s no doubt about that,” Daly said. “And I think over the long term, this should cause plant hardiness zones to gradually shift northward.”