(Credit: Shutterstock)

For decades, the USDA plant hardiness zone map has been the "gold standard" for gardeners and other growers wanting to know if a perennial plant (herbaceous or woody) will survive the cold temperatures of a typical winter in their area. Additionally, scientists incorporate the plant hardiness zones as a data layer in many research models, such as those predicting the spread of exotic weeds and insects. Recently, the USDA unveiled a revised version of the map, updating the 2012 edition of the document.

The USDA Hardiness Zone Map provides gardeners with a good reference to determine average winter low temperatures in their area. (Credit: Shutterstock)

The new map was jointly developed by the USDA's Agricultural Research Service and PRISM, a highly sophisticated climate mapping technology developed at Oregon State University. The revised version of the map is more accurate and contains greater detail than prior versions. As a U.S. Government publication, the USDA Plant Hardiness Zone Map as a graphic is not copyrighted and is in the public domain. Map graphics may be freely reproduced and redistributed.

Available online at https://planthardiness.ars.usda.gov/, the 2023 map is based on 30-year averages of the lowest annual winter temperatures at specific locations. It is divided into 10-degree Fahrenheit zones and further divided into 5-degree Fahrenheit half-zones. The 2023 map incorporates data from 13,412 weather stations compared to the 7,983 that were used for the 2012 map.

Plant hardiness zone designations represent what is known as the "average annual extreme minimum temperature" at a given location during a particular time period. In the case of the 2023 map just released, that period was 30 years. 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.

Hardiness zones represent expected annual low temperatures based on 30 years of data from 13,625 reporting stations. (Credit: USDA)

In other words, the hardiness designations do not reflect the coldest temperature ever recorded at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low winter temperature greatly influences the ability of a perennial plant to survive in a particular area.

Like the 2012 map, the 2023 version has 13 hardiness zones across the United States and its territories. Each zone is broken into half zones, designated as "a" and "b." For example, zone 6 (which includes most of Missouri) is divided into half-zones 6a and 6b. 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.

The 2023 version of the USDA Hardiness Zone Map places most of Missouri in zone 6 which has an average winter low temperature from 0 to -10 degrees F. (Credit: USDA)

Once a draft of the map was completed, it was reviewed by a team of climatologists, agricultural meteorologists, and horticultural experts. If the zone for an area appeared anomalous to these expert reviewers, experts doublechecked the draft maps for errors or biases.

These national differences in zonal boundaries are mostly a result of incorporating temperature data from a more recent time period. However, 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. Additionally, some changes in zonal boundaries are also the result of using 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. The latter usually is based on trends in overall average temperatures recorded over long time periods.

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 research programs of interest to gardeners and others who grow and develop new varieties of plants.

Credit: Adapted from an article by the USDA.