It’s still too cold for Japanese maples and flowering dogwoods, but warmer winters have shifted the Twin Cities into a new plant-hardiness zone, the U.S. Department of Agriculture said Wednesday.
The Twin Cities used to be Zone 4A, which meant winter temperatures plunged as low as 30 degrees below zero. Now the USDA places the Twin Cities in Zone 4B, which means winter temperatures drop as low as minus 25 degrees.
The move to a slightly balmier zone comes after the USDA recalculated its map with newer weather data for the first time since 1990.
Two decades of gradually warmer winters have shifted most of Minnesota – and much of the United States – one notch higher on the USDA’s plant-hardiness charts. The zones depict the lowest winter temperatures for each region and are used to advise gardeners which plants are safe to buy.
“The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States,” said Kim Kaplan of the USDA’s Agricultural Research Service.
Environmentalists see the changing map as illustrating the impact of climate change. The USDA, however, downplayed that discussion to focus instead on advising gardeners.
University of Minnesota Extension climatologist and meteorologist Mark Seeley said he wasn’t surprised by the shifts.
“The trend certainly has been in favor of warmer winters in Minnesota,” he said. “One of the strongest climate signals that we see in the Minnesota climate data is this upward progression in winter temperatures.”
Seeley can cite many examples. In Rochester, January daily lows used to average 1.9 degrees. In recent decades, they have averaged 7.7 degrees.
“These aren’t fractional increases in average minimum temperatures,” Seeley said. “These are shifts by whole degrees, by several degrees.”
Such changes have contributed to making the USDA’s map-drawing efforts controversial. The previous USDA plant hardiness map was based on 13 years of weather data, through 1986, and the USDA didn’t release a new map for 22 years, until Wednesday.
When horticulture groups created their own unofficial maps based on newer data, the shifts were quite dramatic. Some of those maps put the Twin Cities into Zone 5, a climate suitable for trees like flowering dogwoods and Japanese maples.
Wednesday’s USDA map avoids those dramatic shifts by expanding the weather data to include 30 years. Seeley believes that makes sense. When planting trees “that you want there the rest of your life, we should be looking at a long temperature record,” he said.
Anyway, he’s not convinced Twin Cities temperatures won’t dip below minus 25 degrees.
“I would remind everyone to go back to Feb. 1, 1996, when we were minus 33 degrees here in the Twin Cities, and they were minus 60 degrees up at Tower,” Seeley said. “That’s a little disconcerting in light of the new plant hardiness zones.”
Julie Weisenhorn, state director of the Master Gardener program through the University of Minnesota Extension, sometimes hears people claim the Twin Cities is now Zone 5. She’s a skeptic.
“People are welcome to try stuff, but they have to be prepared that it might not make it if it’s from another zone,” she said. “Japanese maples aren’t good performers in this area.”
Meanwhile, she doubts the shift from Zone 4A to Zone 4B will have much impact on gardeners.
“When a consumer goes and purchases a plant at the garden center, it’s not going to say Zone 4A, it’s going to say Zone 4-7,” she said. “So to the consumers, it’s not going to matter that much.”
Tom Webb can be reached at 651-228-5428.
