It’s been nearly 12 years since the USDA reviewed its Plant Hardiness Zone map, used by gardeners throughout the U.S. to identify which plants have a good chance of thriving if planted according to the map’s recommendations. The new map, reflecting an overall temperature increase of a little over 2 degrees across the country, offers more detail and is more accurate than its earlier versions.
What does this mean for gardeners in and around Laconia, New Hampshire? Virtually nothing. Our zone remains the same, at 5b, but the new map, released in November, 2023, also includes a “Tips for Growers” section, which contains information and suggestions based on the government agency’s research programs. And there is much to be said for greater accuracy! The new map is based on more accurate data gathered from 13,412 weather stations instead of the 7,983 collected for the 2012 map.
For those who are new to or unfamiliar with the Hardiness Zone map, it is based on 30-year averages of the lowest winter temperatures recorded each year at specific locations. The map divides the U.S. into 13 zones, with each zone incorporating a 10-degree range of average temperature. The 13 zones are further divided into “A” and “B” zones, each representing a 5-degree range in average temperature.
About half of the country experienced a shift in the zone assigned to their location. The areas that were mostly affected by the addition of weather stations and the updated data were the Midwest, Central Plains and Alaska. The Southwest and Northeast have seen minimal changes, with only the very southern part of New Hampshire experiencing a zone shift. For example, back in 2012, parts of southern New Hampshire experienced an average winter low temperature of -15, while the updated data reveals an average winter low of -10.
The Hardiness Zone map provides gardeners with “the average annual extreme minimum temperature” at a specific location over a certain time span; in this case, 30 years. It is not based on extreme, record-setting cold temperatures, which can vary yearly by storm patterns, but on averaging together the coldest temperatures recorded. Recognizing low winter temperatures as the biggest threat to plants and crops, the hardiness zones report these average lowest winter temperatures.
What do the changes in the Hardiness Zone map have to do with global warming? Not a lot. While warming trends throughout the world must be based on trends recorded over time periods longer than 30 years, data used to produce the Hardiness Zone map depend on extreme minimum temperatures which can vary greatly each year. Also, the inclusion of more weather stations for the updated map, while adding to accuracy, introduces data that was not available in prior years.
Here is the link USDA Plant Hardiness Zone Map to view the 2023 Plant Hardiness Zone Map.