USDA Hardiness Zone Map 2023: What the New Data Means for Your Garden
The 2023 map is based on data from over 13,000 weather stations and shows an average national warming of 2.5 degrees Fahrenheit. Learn how these shifts affect what you can grow and where.
Map Updates and Data Methodology
The USDA Plant Hardiness Zone Map received a significant update in November 2023, replacing the previous version from 2012. The new map relies on a much larger dataset: it uses information from 13,412 weather stations, which is roughly a 70% increase from the approximately 7,983 stations used in 2012. Much of this data was collected by volunteers through the Community Collaborative Rain, Hail & Snow Network (CoCoRaHS).
The 2023 map calculates zones based on 30-year averages of the lowest annual winter temperatures from 1991 to 2020, aligning with a standard "climate normal." The previous map used data from 1976 to 2005. To create the map, data is interpolated between stations based on elevation and proximity to water, resulting in tens of millions of roughly half-mile-by-half-mile grid cells.
Temperature Shifts and Zone Changes
Nationally, the 2023 map is about 2.5 degrees Fahrenheit warmer on average than the 2012 version. Roughly half of the U.S. grid cells shifted to a warmer half-zone, representing a 5°F increase in average annual minimum temperature, while the other half remained unchanged.
Regional shifts vary. In Illinois, the state now has six zones with 12 sub-zones. The boundary between zones 5b and 6a moved 60 miles north, from Springfield to Peoria. Zone 7a moved 50 miles north from Cairo to the St. Louis Metro East area, and Zone 7b appeared in southern Illinois for the first time. In Washington, Spokane shifted from Zone 5b to Zone 7a—a two-zone increase—in the lifetime of local horticultural experts.
Implications for Planting and Gardening
Warmer zones extend the growing season by moving last spring frost dates earlier and first fall frost dates later. This benefits long-ripening crops like tomatoes. Borderline plants that were previously marginal for a zone are now more likely to survive, and plants rated one zone warmer than the previous map's zone are now triable.
In Zones 5 and 6, plants like hydrangeas, hardy figs, Japanese maples, butterfly bush, lavender, crepe myrtles, and globe artichokes (with mulch) show improved reliability. In Zone 7, fig trees, camellias, and gardenias (in sheltered spots) can overwinter without protection. In zones that have warmed significantly, crops such as sweet potatoes, okra, yard-long beans, and lemongrass become more viable.
However, warming winters may also introduce invasive species from the south, increase pest pressure on crops, and create ecological competition.
Limitations and Caveats
Hardiness zones reflect only average annual extreme minimum temperatures. They do not account for summer heat, humidity, rainfall, soil type, or growing season length. Zone averages do not guarantee survival against extreme weather events; for example, the 2021 Texas freeze killed plants in Zones 8–9.
Local microclimates can differ significantly from the map. South-facing walls or urban areas can be 5–10°F warmer (effectively a higher zone), while low-lying "frost pockets" can be 3–5°F colder. Additionally, a specific zone number implies different conditions depending on geography (e.g., Zone 7a in Seattle vs. Charlotte).
Points of Disagreement and Nuance
There is mixed perspective on the attribution of these changes. The USDA is reticent to attribute the shifts directly to climate change and downplays the connection, despite scientific consensus. Conversely, climatologists explicitly link the changes to human-caused global warming, noting that the warming winters are a direct result.
Experts describe the shift as "jarring" and "unnerving," confirming widespread warming that is "undeniable." However, some, such as Chris Daly, describe the average change (a quarter-zone) as "not earth shaking" and "fairly small," suggesting the shifts are manageable. The map is just one tool and does not account for increasing heat waves, droughts, or extreme rainfall, which are not fixed by zone updates.
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