- The USDA has updated its plant hardiness zone map for the first time in over a decade.
- The new map shows that half of the country has moved to warmer zones.
- Updated zones may allow gardeners to grow plants they never could before.
The USDA has updated its plant hardiness zone map for the first time in over 10 years. A new map may change the way you garden.
For example, gardening consultant Megan London spoke NPR is now considering growing an array of new treats, including kumquats and tangerines, in his central Arkansas gardens.
Central Arkansas has moved half the zone from zone 7b to zone 8a since the last USDA map update in 2012, according to the new map.
What are plant zones?
The USDA map is the national standard informs gardeners and breeders what types of perennials—plants that come back year after year—are most likely to thrive in specific locations. The map categorizes these locations by zones and semi-zones, – USDA reported on its website.
The United States is divided into 13 growth zones. Each zone represents the average lowest winter temperature that the area typically sees each year USDA.
For example, much of southern Florida is zone 10b (35 to 40 degrees Fahrenheit), while northern Montana has areas of zone 3b (-35 to -30 degrees Fahrenheit).
“This map is all about winter cold,” Chris Daly, director of the PRISM Climate Group at Oregon State University, told Business Insider. “It has nothing to do with planting zones as far as planting date and what can survive the summer.” He helped develop the map with the USDA.
Half of the United States has moved to warmer plant zones
The new map for 2023 pushed half of the country into the warmer half of the zone, while the rest of the country remained in the same zone, – USDA said in a press release.
For example, some areas of Omaha, Nebraska I moved from 5b to 6a. This means that the average winter coldest temperature in this region has increased from a range of -15 to -10 degrees Fahrenheit to -10 to -5 degrees Fahrenheit.
These changes may mean changing what will grow best in your garden. Daly said that when you buy perennials, you often see their zone ranges listed.
However, growers should exercise caution, Jonathan Foster, a horticulture specialist, wrote in an article published by the University of Maine Maine Gardner’s Handbook side.
Restrictions of the new USDA plant zone map
“The map is a guide, not a guarantee,” Foster wrote, and plants may thrive in several zones. However, other factors should be taken into account, such as summer temperature and soil quality. The map does not reflect these nuances.
Foster noticed this too more delicate plantslike poppies, they may not perform as well as hardier trees and shrubs in snowy climates like Maine.
Daly said the map is also limited in detail. Your garden may be sunnier than your neighbors or have shadier areas thanks to trees. “They may have a microclimate that is warmer or colder than what your zone says you have,” Daly said.
What caused half of the United States to move to a warmer zone?
The USDA said the shift to warmer zones “does not necessarily reflect global climate change” because several factors contributed to the changes.
Daly and the USDA used data from 1991 to 2020 and selected the coldest night of the year. “We only have 30 numbers that we average together,” Daly said. Therefore, it is important to analyze data spanning decades, he added.
The temperature on the coldest night can change for a variety of reasons besides climate change. Daly said one year there may be no frost and the next winter could be particularly harsh.
According to the USDA, the new map also relies on information from thousands of other weather stations. Data was pulled from 13,412 weather stations, compared with 7,983 on the 2012 map, according to a USDA press release.
Daly developed Prism, the software that created the map. “It really does a good job of reproducing the effects of features on the Earth’s surface, such as mountains, valleys and coastlines, on climate patterns,” he said. Thanks to this, the map has become more detailed and accurate compared to previous versions.
It is also interactive. “You can enter your zip code or click anywhere on the map and zoom in and pan around to see what your zone is,” Daly said.
How climate change affects plant hardiness zones
Overall, climate change is affecting where plants can grow, Daly said.
“We know for sure that average temperatures are rising because of climate change. There’s no doubt about it,” Daly said. “I think in the long term this should result in a gradual shift of plant hardiness zones north.”