If you are an avid gardener or farmer, chances are you have looked at the USDA’s Plant Hardiness Zone Map a time or two. In November, the U.S. Department of Agriculture (USDA) released an updated version of its Plant Hardiness Zone Map (PHZM), and this is the first update of the tool since 2012. According to the USDA, the “Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a given location. The new map—jointly developed by USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group—is more accurate and contains greater detail than prior versions.”

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). Essentially, the designations do not denote the coldest recorded temperature at a specific location, but the average lowest winter temperature for the location over a specified time. This is important due to the fact that low temperature during the winter is a major determining factor for the survival of plants.

The 2023 map is based on 30-year averages of the lowest annual winter temperatures at a given location, the map is then divided into 10-degree Fahrenheit zones and further divided into 5-degree Fahrenheit half-zones (a or b). The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map.

In North Carolina, the majority of the state falls between Zones 6a (in the Mountains) and 9a (the Outer Banks). This means the average lowest temperature in the mountains is -10 to -5 degrees Fahrenheit while the lowest temperature at the Outer Banks is 20 to 25. Here in Richmond County, we fall into zone 8a which represents a lowest annual temperature between 10 to 15 degrees.

Some may have the question, “what changed on the updated map?” The majority of our county was not impacted by the updated map but other areas, such as the coastal and mountain counties, show warmer minimum temperatures compared to 2012 and have shifted half a zone (from 8a to 8b or 6b to 7a).

On a national scale, 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 Fahrenheit without moving to another half zone.

Approximately 80 million American gardeners and growers represent the most frequent users of the USDA Plant Hardiness Zone Map. Folks who want to establish perennial plants, such as shrubs or fruit trees, can utilize the map to ensure they select species

that are adapted to their zone. For example, apple trees typically grown in the mountains (Zone 6) would not grow well if planted in the Outer Banks (Zone 9).

Gardeners and farmers are not the only ones with a need for this hardiness information. For example, the USDA Risk Management Agency (RMA) refers to the map’s plant hardiness zone designations to set certain crop insurance standards. Additionally, scientists incorporate the plant hardiness zones as a data layer in many research models, such as those modeling the spread of invasive weeds and insects.

According to the USDA, these 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).

If you would like to visit the updated hardiness zone map, it can be accessed online at https://planthardiness.ars.usda.gov/. In addition to the map updates, the Plant Hardiness Zone Map website was expanded in 2023 to include a “Tips for Growers” section, which provides information about USDA ARS research programs of interest to gardeners and others who grow and breed plants.

If you have any questions about selecting plants or crops based off of the Plant Hardiness Zone Map please contact the NC Cooperative Extension Office, Richmond County Center at (910) 997-8255; visit our website at Richmond.ces.ncsu.edu and follow us on Facebook for more information.