New Hardiness Zone Map

Know your updated hardiness zone for any future plants. Ashley-Belle Burns/Shutterstock

Last week the United States Department of Agriculture (USDA) released a new plant-hardiness zone map for the United States. Does the new USDA map offer proof that the climate is warming? No, it does not. Do temperatures shift warmer and colder over time? Of course they do. The temperatures over large regions fluctuate and hopefully always will. Just like the previous USDA map released in 2012, the new USDA map does not represent the new normal for climate.

The USDA website notes: “Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the (new map) represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in map zones are not reliable evidence of whether there has been global warming.”

Short-term weather is not the same thing as climate. Measuring only one aspect of weather for the short time frame of only a few decades is not reliable evidence of climate change.

There have been cold hardiness zone maps since Alfred Rehder published one in 1927. In 1960, 1965 and 1990 the USDA produced maps. Each map update used more recent temperature data instead of adding to the existing data to create a longer-term map.

The 1990 USDA map looked at 14,500 locations but only used 8,000 weather stations that had valid data, which was twice as many as the previous maps. The 1990 USDA map used the average minimum temperature for the 13 years between 1974 and 1986. The 1990 map showed much of the country as being a zone or more colder than the 1965 USDA map. The 1965 map used data from a short, warmer period of years and the 1990 map used data from a short, colder range of years.

The 2012 USDA map looked at 8,000 stations, over the 30 years of 1976-2005, and it looks a lot like the 1965 USDA map. The 2023 map used data from 13,625 weather stations from the years 1991 through 2020. A sophisticated algorithm created map information taking into account topography and coastal effects between weather stations. This map averages about a quarter of a zone warmer than the 2012 map.

Average weather is made up of highs and lows. Taking a small sampling of data could taint the data to one end or the other. If we did maps one year at a time, sometimes we would have very warm maps and sometimes very cold ones. The temperature change between two years could appear to be abrupt. A longer look at the data would yield slower changes. Each of these USDA maps has dropped older data instead of using it. This can create the appearance of rapid changes when old data is dropped.

The trees and shrubs that most people use the zone maps for live far longer than a decade or two, so maps developed from short-term data can be misleading to gardeners. All of the cold hardiness zone maps use the average annual minimum temperature during the data collection period. This is not the same as the coldest minimum temperature that occurred each year during that time. Plant long-lived plants based on the coldest temperatures in your area, not the average minimum.

Courtesy of Jeff Rugg

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