‘Plant hardiness zones’ get update

Information for this article was contributed by Christina Larson of The Associated Press and Serenah McKay of the Arkansas Democrat-Gazette.





WASHINGTON — Southern staples like magnolia trees and camellias may now be able to grow without frost damage in once-frigid Boston. The U.S. Department of Agriculture’s “plant hardiness zone map,” updated for the first time in a decade, shows the effect that climate change will have on gardens and yards across the country. Climate shifts aren’t even — the Midwest warmed more than the Southeast, for example. But the map will give new guidance to growers about which flowers, vegetables and shrubs are most likely to thrive in a particular region. One key figure on the map is the lowest likely winter temperature in a given region, which is important for determining which plants may survive the season. It’s calculated by averaging the lowest winter temperatures of the past 30 years. Across the lower 48 states, the lowest likely winter temperature overall is 2.5 degrees warmer than when the last map was published in 2012, according to Chris Daly, a researcher at Oregon State University’s PRISM Climate Group, which collaborates with the USDA’s Agricultural Research Service to produce the map. 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 that “a very striking finding.” Primack said he has noticed changes in his own garden: The fig trees are now surviving without extensive steps to protect them from winter cold. He has also spotted camellias in a Boston botanical garden and southern magnolia trees surviving the past few winters without frost damage. These species are all generally associated with warmer, more southern climates. In a video posted on Facebook, and reported by National Public Radio, Hot Springs-based gardening consultant Megan London said the update was long overdue. “I have been stating all year long, ‘This needs updating!’” London said. Central Arkansas has moved from zone 7b to zone 8a in the new map. For London, that means some tropical plants might survive Arkansas winters. “We’re excited, but in the back of our minds, we’re also a little wary,” London said in the NPR interview. “In the back of our mind, we’re like, ‘Ah, that means things are warming up. So what does this mean in the long run?’” Winter temperatures and nighttime temperatures are rising faster than daytime and summer temperatures, Primack said, which is why the lowest winter temperature is changing faster than the U.S. temperature overall. As the climate shifts, it can be tricky for plants — and growers — to keep up. “There are a lot of downsides to the warmer winter temperatures, too,” said Theresa Crimmins, who studies climate change and growing seasons at the University of Arizona and was not involved in creating the map. “When we don’t have as cold winter temperatures, we don’t have as severe diebacks of insects that carry diseases, like ticks and mosquitoes.” She said hotter, drier summers in some regions may kill plants that once thrived there. “You wouldn’t want to plant plants that aren’t adapted right now for where you’re living,” she said. The reason for the zone changes is that the USDA has data from 13,625 weather stations — the most ever, said Anthony Bowden, an extension ornamental horticulture specialist for the University of Arkansas System’s Division of Agriculture. “That gives a higher accuracy compared to what people have been seeing on the ground,” he said. The biggest takeaway, Bowden said, is that this is not a big change for Arkansas. Northwest Arkansas was changed from zone 6b to 7a, and a new zone — 8b — was added along the area from about Texarkana to the Mississippi River, he said. Bowden, who works directly with horticulture retailers and wholesalers, noted that the map shows the average lowest temperatures between 1990 and 2020. “We’re still going to get some extreme cold temperatures and extreme high temperatures,” he said. “That’s just the way it goes.” Bowden advises growers to check with their local extension offices before making plant-buying decisions “because they will have the best resources that work best for the specific area you are in the state.”