A new hardiness zone map will help American gardeners keep pace with climate change

WASHINGTON — Southern staples like magnolias and camellias may now be able to grow without frost damage in once-very cold Boston.

The USDA’s “Plant Hardiness Zone Map” was updated Wednesday for the first time in a decade, and it shows the impact of climate change on gardens and yards across the country.

Climate shifts are not equal; for example, the Midwest has warmed more than the Southeast. But the map will provide new guidance to growers about which flowers, vegetables and shrubs are most likely to thrive in a particular area.

One of the key numbers on the map is the minimum possible winter temperature in a given area, which is important for determining which plants may survive the season. It is calculated by averaging the lowest winter temperatures over the past 30 years.

Across the lower 48 states, the minimum possible temperature for winter is generally 2.5 degrees (1.4 C) warmer than it was when the last map was published in 2012, according to Chris Daly, a researcher with the PRISM climate group at Oregon State University, which It is collaborating with the USDA Agricultural Research Service to produce the map.

“Half of the United States has moved into a climate zone that is a little bit warmer than it was 10 years ago,” said Richard Primack, a plant ecologist at Boston University, who was not involved in the map project. He described this discovery as a “very surprising result.”

Primack said he noticed changes in his garden: the fig trees now survive without extensive steps to protect them from the winter cold. He also discovered camellias at a botanical garden in Boston and southern magnolia trees that had survived the past few winters without frost damage. All of these species are generally associated with warmer, southern climates.

Winter temperatures and nighttime temperatures rise faster than daytime and summer temperatures, which is why the lowest winter temperature changes faster than the temperature in the United States overall, Primack said.

As the climate changes, it may be difficult for plants and farmers to keep up with the changes.

“There are a lot of downsides to warmer winter temperatures as well,” said Theresa Crimmins, who studies climate change and growing seasons at the University of Arizona and was not involved in drawing the map. “When winter temperatures aren’t as cold, we don’t have as severe a die-off of disease-carrying insects, like ticks and mosquitoes.”

She added that hotter, drier summers in some areas could kill off plants that once thrived there.

“You wouldn’t want to plant plants that aren’t now adapted to where you live,” she said.

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