The U.S. Department of Agriculture on Nov. 15 released its new Plant Hardiness Zone Map, the national standard by which gardeners can determine which plants are most likely to survive the coldest winter temperatures at a certain location.

The USDA describes the latest map, jointly developed by Oregon State University’s PRISM Climate Group and the USDA’s Agricultural Research Service, as the most accurate and detailed it has ever released.

PRISM, part of the OSU College of Engineering, stands for Parameter-elevation Regressions on Independent Slopes Model. The previous version of the plant hardiness map, also based on PRISM data, was released in January 2012. The new plant hardiness map incorporates data from 13,412 weather stations, compared to the 7,983 that were used for the 2012 edition.

Furthermore, the new map’s rendering for Alaska is now at a much more detailed resolution (down from a 6 ¼ -square-mile area of detail to a ¼ square mile).

"These updates reflect our ongoing commitment to ensuring the Plant Hardiness Zone Map remains a premier source of information that gardeners, growers and researchers alike can use, whether they’re located in the continental United States, Alaska, Hawaii or Puerto Rico,” said ARS Administrator Simon Liu.

Viewable in a Geographic Information System-based interactive format, the map is based on 30-year averages (1991 to 2020) for the lowest annual winter temperatures within specified locations. The 2012 edition was based on averages from 1976 to 2005.

Low temperature during the winter is a crucial factor in the survival of plants at specific locations. The latest edition of PRISM’s signature product, the 30-year climate normals, was released in December 2021. PRISM releases new climate normals every 10 years, taking away one decade’s worth of data and adding another.

“The addition of many new stations and more sophisticated mapping techniques using the latest PRISM technology led to a more accurate and detailed Plant Hardiness Zone Map but also produced localized changes that are not climate related,” said Christopher Daly, director of the PRISM Climate Group and the map’s lead author. 

Divided into zones

The plant hardiness map is divided into a total of 13 zones, each zone representing a 10-degrees-Fahrenheit range of temperatures. Each zone is further divided into two half zones, with each of those representing a 5-degree range.

“Overall, the 2023 map is about 2.5 degrees warmer than the 2012 map across the conterminous United States,” Daly said. “This translated into about half of the country shifting to a warmer 5-degree half zone, and half remaining in the same half zone. The central plains and Midwest generally warmed the most, with the southwestern U.S. warming very little.”

Accompanying the new map is a “Tips for Growers” feature that provides information about Agricultural Research Service programs likely to be of interest to gardeners and others who grow and breed plants.

The approximately 80 million American gardeners and growers are the most frequent map users, according to the USDA. In addition, the USDA Risk Management Agency uses the map in setting certain crop insurance standards, and scientists incorporate the plant hardiness zones into research models, such as those looking at the spread of exotic weeds and insects.

Plant hardiness zone designations represent what’s known as the “average annual extreme minimum temperature” at a given location during a particular time period (30 years, in this instance). Put another way, the designations do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations.

Warming up

When compared to the 2012 map, the 2023 version reveals that about half of the country shifted to the next warmer half zone, and the other half of the country remained in the same half zone. That shift to the next warmer half zone means those areas warmed somewhere in the range of 0-5 degrees Fahrenheit; however, some locations experienced warming in the range of 0-5 degrees without moving to another half zone.

These national differences in zonal boundaries are mostly a result of incorporating temperature data from a more recent time period. The 2023 map includes data measured at weather stations from 1991 to 2020.

Notably, the 2023 map for Alaska is “warmer” than the 2012 version. That’s mainly because the new map uses more data representing the state’s mountain regions where, during winter, warm air overlies cold air that settles into low-elevation valleys, creating warmer temperatures.

The annual extreme minimum temperature represents the coldest night of the year, which can be highly variable from year to year, depending on local weather patterns. Some changes in zonal boundaries are also the result of using increasingly sophisticated mapping methods and the inclusion of data from more weather stations. 

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. 

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).

A global leader

OSU’s College of Engineering is a global leader in artificial intelligence, robotics, advanced manufacturing, clean water and energy, materials science, computing, resilient infrastructure and health-related engineering.

Among the nation’s largest and most productive engineering programs, the college awards more bachelor’s degrees in computer science than any other institution in the United States. The college ranks second nationally among land grant universities, and fifth among the nation’s 94 public R1 universities, for percentage of tenured or tenure-track engineering faculty who are women.

The ARS is the USDA’s chief research arm. Government officials say each dollar spent on U.S. agricultural research results in $20 in economic impact.

Although a paper version of the 2023 plant hardiness map will not be available for purchase from the government, anyone may download the new map free of charge and print copies as needed.

To view the map, click here.

Source: Oregon State University, USDA ARS