A new hardiness zone map has been released for growers for the first time in a decade, showing the impact of climate change.
The plant hardiness zone map is a resource issued by the U.S. Department of Agriculture to help all growers, from farmers to landscapers to backyard flower enthusiasts, know what will thrive in their region. It uses many factors, but the most important figure is the lowest likely winter temperature in a region. This tells growers what plants can survive the season, and is calculated by averaging the lowest winter temps for the past 30 years.
According to the newly released map, the first since 2012, the lowest likely winter temperature across the entire lower 48 is 2.5 degrees Fahrenheit warmer than it was then, a dramatic change.
Boston University plant ecologist Richard Primack, who was not involved in the map project, said: “Half the U.S. has shifted to a slightly warmer climatic zone than it was 10 years ago.” He called it “a very striking finding,” and also agrees that he’s seen the difference in his own garden. He no longer has to protect his fig trees in the winter, and doesn’t see new frost damage on camellias and southern magnolia trees, all plants from warmer climes.
The shifts aren’t even across the map. The Midwest has warmed more than the Southeast, and the northern reaches of the central states have changed the least, largely due to new extreme weather patterns keeping their lowest temperatures in an extreme range.
The rising winter temperatures also mean that new invasive species, of both plants and animals, can move into regions that were previously too cold for them over winter. Places which rely on harsh winters to keep down the tick and mosquito populations will find themselves very uncomfortable without those seasonal die-backs. And while less reflected on the map, hotter, drier summers will also play a heavy part in what growers can make to thrive across the entire country.