In November, the U.S. Department of Agriculture released its latest version of the Hardiness Zone Map.
This mapping system categorizes geographic regions according to their average annual minimum winter temperatures. The map helps by assisting gardeners, horticulturists and landscaping experts in choosing plants suited to specific climates.
The USDA Hardiness Zone Map has a history dating back several decades, and it has undergone multiple revisions to reflect the changes in climate knowledge and technology. The first version of the map was developed in 1938 by then-Secretary of Agriculture Henry Wallace. The early map divided the United States into 10 temperature zones based on average annual minimum winter temperatures.
It wasn’t until the 1960s that the USDA updated the map. This revision included more detailed temperature zones and took into account additional factors such as elevation, wind exposure and proximity to large bodies of water.
The most well-known and widely used version of the USDA Hardiness Zone Map was released in 1990. This version, developed by the National Arbor Day Foundation in cooperation with the USDA, further refined the zones and incorporated more sophisticated technology, including the use of computer-based Geographic Information System (GIS) mapping.
The USDA last released an updated version of the map in 2012. This revision was based on more recent weather data and trends. It also introduced half-zone increments to better represent small-scale climate variations.
The new version was released in November in response to ongoing climate changes and advancements in climate science. It’s essential to periodically update the map to reflect shifts in temperature patterns and to provide accurate information for gardeners and horticulturists.
Louisiana’s 2023 zoning has been adjusted, with north Louisiana now classified as zone 8b and the southernmost tip as zone 10a — each moving up by half a zone.
Each zone number represents a 10-degree Fahrenheit difference in average low temperatures in winter. The “a” and “b” denote half-zone increments, representing a 5-degree difference in average minimum winter temperatures within a main zone. Zones labeled with an “a” indicate the colder part of a numbered zone while “b” designates the warmer part of a zone. This provides more specific information for tailored plant selection in areas with temperature variations.
Each zone has a temperature range associated with it. For example, zone 8 has a range of 10 to 20 degrees. This means that plants recommended for zone 8 are generally able to withstand winter temperatures within this range.
The zones of Louisiana range from 8 to 10. In zone 9, average minimum winter temperatures range from 20 to 30 degrees, and in zone 10, the range is 30 to 40 degrees.
For reference, here are the average low temperatures for all 13 hardiness zones, which stretch from Alaska to the north and Hawaii and Puerto Rico to the south:
Zone 1: Below -50 degrees
Zone 2: -50 to -40 degrees
Zone 3: -40 to -30 degrees
Zone 4: -30 to -20 degrees
Zone 5: -20 to -10 degrees
Zone 6: -10 to 0 degrees
Zone 7: 0 to 10 degrees
Zone 8: 10 to 20 degrees
Zone 9: 20 to 30 degrees
Zone 10: 30 to 40 degrees
Zone 11: 40 to 50 degrees
Zone 12: 50 to 60 degrees
Zone 13: 60 to 70 degrees
The USDA Hardiness Zone Map is available online at planthardiness.ars.usda.gov. It is a valuable tool for gardeners and plant enthusiasts, helping them choose plants that are well-suited to the specific climate conditions of their region. For us in Louisiana, the recent changes to the map may mean it’s time to incorporate more semitropical and tropical plant species.
