Arizona mostly holds ground, but plant hardiness zones get major national adjustments

Nov 18, 2023, 6:20 PM | Updated: Nov 20, 2023, 12:30 pm

U.S. plant hardiness zones...

Southwest U.S. plant hardiness zones. (USDA)


PHOENIX — The U.S. Department of Agriculture’s “plant hardiness zone map” was updated Wednesday for the first time in a decade, depicting the impact climate change will have on gardens and yards across the country.

Although changes are substantial in some regions of the country — Southern staples like magnolia trees and camellias may now be able to grow without frost damage in once-frigid Boston, for example – in Arizona and the Southwest, changes are limited.

Minor changes in Arizona

Most of the Phoenix area remains in 9b and 10a zones. Zone 9b is an area with average annual extreme minimum temperatures of 25 to 30 degrees.

Zone 10a is an area with average annual extreme minimum temperatures of 30 to 35 degrees.

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.”

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 die-backs of insects that carry diseases, like ticks and mosquitoes.”

She added that 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.

Uneven climate

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.

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.

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.

The Associated Press contributed to this report.

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Arizona mostly holds ground, but plant hardiness zones get major national adjustments