ANNAPOLIS, Md. – In the middle of the night about five years ago, the towering red oak tree in Jenny Willoughby’s yard came crashing down, narrowly missing her house.
“It shook the ground it was so big,” said Willoughby, who lives high up on a mountain in western Maryland.
Red oak trees are well-suited to the icy chill that often grips such high elevations, said Willoughby, an avid gardener who, as the sustainability manager for the city of Frederick, often grapples with climate matters. So, she was surprised when she saw warmth-loving white oak tree saplings springing up in the vacated spot instead.
“Something is shifting on the mountain in a quick way,” she said.
Hers is a familiar tale across the Chesapeake Bay watershed and elsewhere. Greenhouse gases are warming the planet at unprecedented rates, and colder seasons are heating up faster than warmer seasons. The result: In many locations, plants and trees that prefer cooler weather are getting squeezed out by flora better adapted to warmer temperatures.
The latest Plant Hardiness Zone Map, the nationwide guide showing which plants are likely to thrive in a specific location, illustrates those changes. The color-coded map divides the country into 10-degree temperature zones based on 30-year averages of the lowest annual winter temperature.
It has 13 zones in all, ranging from Zone 1 with average winter extremes ranging from -50 to -60 degrees Fahrenheit (parts of Alaska on the Arctic Circle) to Zone 13 with average winter cold extremes of 60–70 degrees (parts of Hawaii and Puerto Rico).
It further divides those extremes into 5-degree “half zones” designated “a” and “b.” For example, if a location’s typical lowest winter temperature is between 0 and 5 degrees, such as Gettysburg, PA, it is in the “7a” zone. If the lowest temperature falls between 5 and 10 degrees, as in Richmond, it is in the “7b” zone.
The U.S. Department of Agriculture published the new map last November. It shows that about half of the country shifted to one half-zone warmer since the agency’s last map in 2012. A switch to a warmer half zone indicates that a location’s lowest average temperatures increased between 0 and 5 degrees.
Some notable locations sliding into new zones in the Bay watershed included downtown Richmond, from 7a to 7b; DC’s National Mall from 7b to 8a; Harpers Ferry, WV, from 6b to 7a; Gettysburg, from 6b to 7a; and Binghamton, NY, from 5b to 6a.
Just because a location didn’t get absorbed into a new zone doesn’t mean it’s necessarily in the clear. Some locations have warmed but not tipped into the next threshold.
The scientists responsible for the mapping caution against blaming climate change entirely for the shifts. The 2023 map drew data from 68% more weather stations than its 2012 counterpart, so some of the differences may be attributable to having more localized information, said Chris Daly of the Oregon State University research team that has worked with the USDA on the past two versions of the map.
Furthermore, the average extreme low is based on only 30 years’ worth of records, he noted. So, it wouldn’t have taken many cold snaps during the 1991–2020 period on which the new map is based to bring down the average figure.
That said, climate change is likely to have had some influence on the numbers, according to Daly. “We can’t say with certainty that the changes we see from one map to the next is solely due to climate change,” he said, “but we can feel confident that it’s a likely contributor.”
Nationwide, the typical coldest annual temperature warmed by about 2.5 degrees, Daly said. In the Chesapeake Bay region, those coldest spells ranged between 2 to 5 degrees higher compared with the 2012 map, ranking it among the fastest warming areas in the country.
Some of the most drastic shifts in the Bay region have been in Pennsylvania. Zone 5a, where temperatures could plunge as low as -20, appeared in a handful of the state’s northernmost counties in the 2012 map. As of the 2023 version, that zone had been extirpated from the state.
Between the two Pennsylvania maps, zone 5b (where temperatures range between -15 to -10 lows) contracted considerably — from covering nearly the entire northern half of the state to barely holding on in the state’s north-central and northeastern quadrants.
“I was a little bit taken aback by how quickly it happened,” said Michael Shepard, a master gardener and environmental professor at the Commonwealth University of Pennsylvania-Bloomsburg. “Those zones have probably been stable for centuries before that.”
The retraction of extreme cold puts more of the state’s 300 million ash trees at risk of deadly infestations by emerald ash borers, experts say. The invasive insect’s larvae don’t die off in the winter unless temperatures plunge to about -10 degrees.
The USDA created its first plant hardiness map in 1960. Since then, many locations have shifted a full zone warmer or more. Between 1990 and 2023, for example, Norfolk, VA, moved up from zone 7b (a low of 5–10 degrees) to 8b (a low of 15–20 degrees).
“It’s just another sign of climate change, and I don’t think it’s positive news,” said Les Parks, director of horticulture for the Norfolk Botanical Gardens.
Southeast Virginia has gotten so warm in recent years that gardeners have been able to grow sabal palmetto trees without taking measures to shield them during the coldest winter nights, Parks said. Also known as cabbage palms, sabal palmettos are the official state tree of Florida and South Carolina.
Meanwhile, Parks worries about cold-adapted species getting forced out of Virginia entirely, especially white pines and Carolina hemlocks. Now more than ever, experts advise gardeners to consult a resource such as the U.S. Forest Service’s Climate Change Tree Atlas to determine which tree species are likely to thrive or struggle in their locations in the decades ahead.
The warming is likely to make the Bay region more hospitable to invasive plants as well, experts say. Kudzu, English ivy, Japanese honeysuckle and others are expected to flourish amid the warmer temperatures, wetter weather and increased carbon dioxide concentrations in the atmosphere.
“It’s tough on gardeners because we’ve got more weeds to pull,” said Sara Tangren of the National Capital Partnership for Regional Invasive Species Management.
Same things wiped out the dinosaurs, and reformed the shapes of the continents a few years back, before EVs, windmills and solar panels.