USDA updates plant hardiness zone map for gardeners due to climate change

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” was updated Wednesday for the first time in a decade, and it shows the impact that climate change will have on gardens and yards across the country, including on Long Island.

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, including on Long Island.

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 Fahrenheit 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.

On the 2012 map, most of Long Island was in zone 7a, with winter lows of 0 to 5 degrees.. On the updated map, the region is nearly all zone 7b, with winter lows of 5 to 10 degrees. 

Nurseries on Long Island have already taken note of the warming temperatures.

“Frost dates have changed,” said Sig Feile, one of the owners of the Atlantic Nursery in Freeport. The first frost arrives later in the winter, and the last freeze comes earlier in the spring. The nursery revised its recommended schedules for planting vegetables several years ago, moving the dates up by one to two weeks.

Some plants that Feile once sold with a warning that they may not be fully hardy — the crepe myrtle, for example — he can now sell with a guarantee that it can survive a Long Island winter. “It’s now one of the more popular plants on Long Island,” Feile said.

There are also "a handful of plants that are starting to show heat stress," according to Pam Tobol, the greenhouse manager at Hicks Nurseries in Westbury. The paper birch, for example, is retreating north: "It's just getting too hot for that tree," Tobol said.

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.

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 added that hotter, drier summers in some regions may kill plants that once thrived there.

The shifting seasons can also cause problems for native pollinators, says Raju Rajan, president of ReWild Long Island, a Port Washington-based nonprofit that promotes sustainable landscaping.

“Ecosystem changes don’t happen uniformly,” so some species' migration and hibernation schedules are no longer aligned with their food sources, he said. Ground-nesting bees, for example, are emerging earlier in the spring as soils warm earlier, but “the flowers they depend on may not be blooming yet.”

It’s unclear, Rajan said, how this will play out.

With Tracy Tullis