USDA Plant Hardiness Zone Map gets upgrade; data helps gardeners decide what can be grown in the garden.
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| Image source: USDA |
Gardeners are waking up to the news that they may have shifted into a new growing zone, thanks to updated temperature data released by the U.S. Department of Agriculture on Nov. 15.
The 2023 version of the USDA Plant Hardiness Zone Map features the average annual extreme minimum temperature data of the United States from 1991-2000. On average, there was an increase of 2.5 degrees Fahrenheit overall across the country, according to Dr. Christopher Daly, who led the plant hardiness zone map effort in 2012 and 2023.
Changes across the U.S. were quite variable since the 2012 version, which reflected temperature data from 1975-2005. “Half the country went up a half zone and the rest stayed the same,” Daly said during the Plant Hardiness Zone Map virtual press conference. Daly is a professor and director of the PRISM Group, Oregon State University.
With the new map, gardeners located along the zone borderlines now have the validation of a long-suspected zone creep in their gardens. Todd Rounsaville, of the USDA Floral and Nursery Plants Research division, noted that the changes were minimal, and that gardeners can still push the limits in gardens with microsites – such as growing a plant in front of a building which absorbs heat and influences how the plant grows.
The 2023 version is more accurate than the prior 2012 map, thanks to a 68% increase in weather station reporting and use of Geographic Information System data, which factors the effects of cities, coastlines and mountains.
Gardeners use the USDA Plant Hardiness Zone Map to help determine the likelihood of a plant’s ability to survive in the garden. Other factors also play a role in a plant’s survival, but using the growing zones as a guide helps gardeners quickly decide if a plant – shrub, tree or perennial – will survive the winter and return the next year.
During the Q&A portion of the press conference, the experts agreed that in the long term, the average temperatures are rising and climate change is an influence, but the data used in the zone maps represents 30-year averages of extreme cold temperatures. Climate changes are usually based on overall average temperatures in 50-100 years, so the data in the USDA zone map cannot be used to determine global warming.
The 1991-2020 period for the USDA map also aligns with the period currently in use by climatologists to describe baseline climate “normals” in the U.S.
Digging in to the findings
- The Midwest and Central Plains generally experienced the largest change – as much as a 5-degree Fahrenheit difference. Situated in a “tension zone,” this geographic region did not experience the same cold temperature outbreaks during the updated time frame as it did in the 1980s.
- There were fewer changes noted in the Southwest but more changes noted in the upland areas of Alaska, which experts attributed to more station data to contribute to the current finding (13,412 stations versus 7,983 for the 2012 version).
- Zones 12 and 13 were retained for the 2023 version for Hawaii and Puerto Rico, which contain regions with average annual extreme minimum temperatures above 50 degrees and 60 degrees Fahrenheit, respectively. The USDA site states that this data can be helpful for gardeners who keep tropical and semi-tropical plants as patio plants: “The two new zones will provide a way to share information about differences in cold sensitivity of tropical ornamental plants and could help gardeners decide when to bring tropical plants indoors from a deck or patio as the temperature cools.”
Check your zone
To locate your growing zone quickly, visit https://planthardiness.ars.usda.gov/ and enter your ZIP code into the top search box that loads with the U.S. map.
You can also visit the Map Downloads page to view state and regional maps.
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