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New ‘Plant Hardiness Zone’ map released by the USDA

The U.S. Department of Agriculture (USDA) has introduced a new version of its Plant Hardiness Zone Map (PHZM), marking a significant update to this essential resource for gardeners and agricultural researchers.

This is the first major revision since 2012, offering a more detailed and accurate guide for plant cultivation across the United States.

Updates provide better accuracy

The 2023 PHZM, developed jointly by USDA’s Agricultural Research Service (ARS) and Oregon State University’s PRISM Climate Group, represents a notable improvement over previous versions. This enhanced map provides a refined tool for determining the most suitable plants for specific locations, catering to the needs of gardeners and growers.

In tandem with the map updates, the PHZM website has expanded, now featuring a “Tips for Growers” section. This addition offers insights into USDA ARS research programs, benefiting gardeners and plant breeders with valuable information.

2023 Plant Hardiness map innovations

The latest map is based on 30-year temperature averages, categorized into 10-degree Fahrenheit zones and subdivided into 5-degree Fahrenheit half-zones. These divisions are crucial for understanding the nuances of local climate conditions.

Like its 2012 predecessor, the 2023 map maintains a user-friendly, Geographic Information System (GIS)-based interactive format. However, it surpasses the previous version by incorporating data from 13,412 weather stations, significantly more than the 7,983 stations used in 2012.

A noteworthy improvement in the new map is its detailed rendering for Alaska. The area of detail for this region has been refined from a 6 ¼-square-mile area to a more precise ¼ square mile.

ARS Administrator Dr. Simon Liu comments, “These updates reflect our ongoing commitment to ensuring the Plant Hardiness Zone Map remains a premier source of information for users across the U.S., including in Alaska, Hawaii, and Puerto Rico.”

Understanding plant hardiness zones

The PHZM is not only a resource for approximately 80 million American gardeners and growers. It also serves various other sectors. For instance, the USDA Risk Management Agency utilizes the map for setting crop insurance standards. In addition, scientists incorporate the zone designations in numerous research models.

The map’s zones represent the “average annual extreme minimum temperature” at a location over a 30-year period. This measure is crucial for determining the survivability of plants in different regions, based on their winter temperature tolerance.

The 2023 map’s zone structure

Maintaining the 13-zone structure of the 2012 map, each zone in the 2023 version is further divided into ‘A’ and ‘B’ half zones. A comparison between the two maps reveals significant shifts in some areas to warmer half zones, indicative of a 0-5 degrees Fahrenheit warming, though not uniformly across the country.

The 2023 map includes temperature data from 1991 to 2020, contributing to its updated zone boundaries. Notably, the map shows a “warmer” Alaska, primarily due to the inclusion of data from the state’s mountainous regions.

Temperature variability and climate change

The extreme minimum temperature, representing the coldest night of the year, varies significantly and is influenced by local weather patterns. Map developers caution against using these temperature updates as indicators of global climate change, as they are based on highly variable and specific data points.

In a shift towards digital accessibility, a paper version of the 2023 map will not be available for purchase. Instead, the map is freely downloadable online, allowing widespread access and the ability to print copies as needed. This approach ensures that the latest plant hardiness information is readily available to all interested parties.

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