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The U.S. Department of Agriculture recently released an updated plant hardiness zone map and the new map suggests Rockbridge County is slightly warmer than suggested by the previous plant hardiness zone map released in 2012.
Where much of the western and northern reaches of Rockbridge County were considered in plant hardiness zone 6b, most all Rockbridge is now firmly in zone 7a with zone 7b designated in parts of the southern extents of Rockbridge.
The plant hardiness zone map is based on the average annual extreme minimum winter temperature, displayed as 10-degree Fahrenheit (F) zones ranging from zone 1 (coldest) to zone 13 (warmest). Each zone is divided into half zones designated as “a” and “b.” For example, 7a and 7b are 5-degree F increments representing the colder and warmer halves of zone 7, respectively.
These designations serve as convenient labels and shorthand for communicating and comparing the extreme winter temperatures within the United States. All plant hardiness zone maps should serve as general guides for growing perennial plants. Zones in this edition of the map are based on 1991-2020 weather data. This does not represent the coldest it has ever been or ever will be in an area, but it simply is the average lowest winter temperatures for a given location for this 30-year span (1991-2020).
The slight shifts in zones for Rockbridge County means that where much of the county could expect to reach between -5 and 0 degrees F during its coldest typical winter weather, the data now suggests these same regions of the county can expect cold temperature extremes for a season to be between 0 and +5 degrees F.
Does the new map tell us anything about climate change? Not really. Climate changes are usually based on trends in overall annual average temperatures recorded over 50-100 years. Because the hardiness zone map represents 30-year averages of what are essentially “typical” extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.
Compared to the 2012 and 1990 maps, zone boundaries in this 2023 edition have shifted in many areas. The new map is generally about one quarter-zone warmer than reported in the 2012 version throughout much of the United States, as a result of a more recent averaging period (1976-2005 for the old map vs. 1991-2020 for the new map). However, some of the changes in the zones are the results of additional data sources and improved interpolation methods. These zone shifts can sometimes result in a cooler, rather than a warmer, zone. The most substantial changes in the new map produced by additional data sources and improved interpolation methods are seen in upland areas of Alaska.
Much of the information in this week’s column is taken directly from the USDA Plant Hardiness Zone website (https://planthardiness. ars.usda.gov/) where you can read a much more in-depth description of the map, its proper interpretation and use, and a range of informational topics related to weather, growing plants, and how the maps are developed.
