Earlier this past week the United States Department of Agriculture (USDA) issued a press release announcing the new cold hardiness zones for the United States. This announcement came after years of collecting data from more weather stations than they have ever used before, and the results are quite interesting.
Getting straight to the point gardeners in our part of the world we’ll find themselves in a different zone as a result of all the research that has been conducted. Previously all of Angelina County was in zone 8 B which indicated a minimum temperature between 15- and 20 degrees F. As of this past week, we are now considered 9A which is an average minimum temperature between 20- and 25-degrees F!
Cold hardiness zones are a valuable tool for gardeners and nurserymen as it helps determine which plants will survive winters in our area. This change seems significant to many in our region because the old boundary of zone 9A was limited to the Texas coast and around Houston.
This is the first time the researchers working with the USDA have changed the map since 2012. This updated map was developed jointly by the Agricultural Research Service (a branch of the USDA) and the Oregon State University’s (OSU) PRISM Climate Group. The promise of this map is that it “is more accurate and contains greater detail than prior versions”.
You can find it online by searching “USDA plant hardiness zones 2023” or at the following link: https://planthardiness.ars.usda.gov/.
These zones were first put forth by the USDA in 1960 in response to a need to standardize growing zones with some measure of accuracy. Private nurseries had developed their own “zones” more so by geographical regions as early as 1927, but no data driven standard was established.
But the USDA’s Agricultural Research Service first used historical climate data from 450 weather stations from across the nation to set up 10 “cold hardiness zones” and presented it to the public. Zone 1 was in central Alaska and zone 10 could be found in Hawaii. Each zone measured a 10-degree F delineation in the average minimum temperatures of the region. Zone 1 has a minimum average temperature of -50 F while zone 10’s minimum average temperature is 40 F.
Our region around east Texas and extending west/southwest across the state was zone 8 with an average low somewhere between 10 to 20 F.
Thirty years later in 1990, the USDA released an updated Plant Hardiness Zone Map. This map took data from close to 8,000 weather stations and used data from the period from 1974 to 1986. This increased volume of historical data also allowed each zone to be broken into a 5-degree range, designated “a” or “b”. The map was updated again in 2012 and in keeping with that 2012 map, that version has 13 zones across the United States and its territories.
The 2023 map is based on the recent 30-year averages of the lowest annual winter temperatures at specific locations. This map continues the 10-degree Fahrenheit zones as well as the continued subdivision of 5-degree Fahrenheit half-zones. Just like the increased data usage from before, the 2023 map incorporates significantly more data from 13,412 weather stations.
While approximately 80 million American gardeners and growers rely upon the Plant Hardiness Zone Map, it is important to note that there are other climate factors that should be considered. Summer heat, annual rainfall, and frost dates also play a critical role. None of those other factors are a part of the new map.
As I study the past 33 years rainfall (1990-2022) as recorded at the Angelina County Airport, the average hovers around 50 inches. When I compare that data to the USDA Soil Survey book that was published in 1988, it states that the average rainfall from 1950 to the mid 80’s was only 41 inches. I find that a significant variance.
From the press release, “Plant hardiness zone designations represent what’s known as the ‘average annual extreme minimum temperature’ at a given location during a particular time period.” This does not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time.
Significantly, the 2023 map 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 did experience warming in the range of 0-5 degrees Fahrenheit but not enough to move to another zone.
Interestingly, the USDA’s press release states, “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, as well as the use of increasingly sophisticated mapping methods and the inclusion of data from more weather stations.” Consequently, developers involved in this most recent map cautioned against crediting temperature updates made to some zones as reliable and accurate indicators of global climate change.
Although a paper version of the 2023 map may not be available for purchase, anyone may download the new map free of charge and print copies as needed.