By Kathy Torres
Bob Dylan wrote a song in 1963 with lyrics, “The times, they are a changin.” It was an anthem for social change directly related to the civil rights movement. The song was appropriate and true then and it resonates loudly in this century as well. While its message had a deep and serious nature, let’s switch gears and apply it to another subject, also appropriate and true…. THE WEATHER. Not to minimize the intent of the song, but nothing changes much more than the weather. And, ironically, the topic of climate change is a real social/political topic of our time. Research is indicating a slight warming of winter temperatures in the U.S. and has been documented by the U.S. Department of Agriculture in its official plant hardiness map. While the idea of fewer cold snaps that damage plants in winter is a welcome thought, we don’t know precisely how this change will impact us. It is possible that eventually we may have more in common with our coastal neighbors in terms of weather that allows for more tropical plants, BUT, it’s a bit too early to make that assumption. One thing is for sure, however… the weather, “it is a changin.”
The U.S. Department of Agriculture has recently released its 2023 Agricultural Hardiness Zone Map, a tool that provides valuable assistance to gardeners, farmers, researchers and others involved in growing plants by establishing “growing zones” or “gardening zones.” Developed by USDA’s Agricultural Research Service and Oregon State University’s PRISM Climate Group, the map provides a standard/guide for helping gardeners and farmers select plants that will thrive in their region. Data from 13,412 weather stations was collected and evaluated to support the map divisions, compared to 7,983 stations in the last revision (2012). Lowest winter temperatures are averaged and used to characterize each zone. Cold weather is not the only factor in plant survival, however it is significant, and is the basis for USDA’s map. It has proven to be a reliable planting guide throughout the United States.
The map is based on 30 years of weather data from 1991 to 2020 and consists of 13 individual zones that include the contiguous United States, Alaska, Hawaii and Puerto Rico. Zone 1, the northernmost zone, is Alaska, where temperatures can reach as low as -60 degrees Fahrenheit. Zone 13, the southernmost zone, can be found in Hawaii and Puerto Rico, where the average minimum temperature is above 60 degrees Fahrenheit. Zones represent low temperatures in 10-degree increments, plus, each zone is further broken down into additional 5-degree half zones designated “a” and “b”.
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. The shift to the next warmer half zone means those areas warmed by 0-5 degrees Fahrenheit. USDA cautions that temperature increases in plant hardiness zones are not necessarily reflective of global climate change, for several reasons… (1) Because mapping methods are more sophisticated, (2) Extreme minimum temperatures vary from year to year, and (3) Data from additional weather stations was used. According to Chris Daly, Director of the PRISM Climate Group at Oregon State University that jointly developed the map with USDA, “The reasons the 30-year data led to half-zone shifts is too complex to attribute to one cause, such as climate change. However, over the long run, we will expect to see a slow shifting northward of zones as climate change takes hold.”
The zones have shifted to slightly warmer average temperatures in parts of South Carolina, but this change shouldn’t really make a big difference for us because the average minimum temperature range indicated is still below freezing. Most of Lexington and Richland counties were previously considered Zone “8a” (average minimum temperature in winter 10-15 degrees Fahrenheit). The new version of USDA’s map indicates this area is now mostly Zone “8b” (average minimum temperature in winter 15-20 degrees Fahrenheit). There are a few areas in Lexington and Richland counties that remain Zone “8a” that are close to Aiken, Saluda, Newberry, Fairfield and Kershaw counties. Areas surrounding Lake Murray are primarily considered Zone “8b” until you get close to Saluda and Newberry counties. It’s interesting that last winter we experienced Zone “8a” temperatures that had a dramatic effect on some of the plants that usually handle winter very well here. It was unusual, however, so it is To Be Determined as to whether or not those low temperatures will affect the next 30-year average. If anything, the cold temperatures we experienced support unusual temperature fluxuation and underminds the global warming theory. Hopefully, reliable, scientific research will lead us into the future with sound evidence either way.
When purchasing plants, it’s a good idea to investigate their winter hardiness. Most of the time, you will find this information on a plant tag, but not always. Most garden centers are more conscientious about selling plants appropriate for the region, however the box stores will miss this detail occasionally. Just be sure to do your research if a plant tag is not available that includes this information. If a search on the internet doesn’t get you to the hardiness zone information, try one of these websites: Monrovia.com or NC State University Plant Toolbox.
As we prepare for the holidays, it’s unlikely our new agricultural hardiness zone assignment will affect our chances of a snow flurry (Oh, the anticipation of a White Christmas… a South Carolina dream!) If we do get that snowfall, or, if a January, February, March freeze comes upon us, just remember to cover or heavily mulch the vulnerable plants like blueberries, tropical banana trees, elephant ears, etc. And if you’re trying to overwinter your hibiscus or bougainvillea, be sure to bring them inside to the garage or sunroom. The future may take us to warmer temperatures; we just don’t know for certain, so, let’s do all we can to care for our planet in the meantime. And don’t panic over this small change in our agricultural hardiness zone – “frost” and “freeze” will continue to be a part of our winter weather language.
Here’s a link to South Carolina in the revised USDA MAP.
Best wishes to you and yours this Christmas!