The interactive USDA Plant Hardiness Map allows users to view the plants most likely to thrive in any U.S. region. Image by United States Department of Agriculture.
Chihuahuan desert plants like autumn sage, hummingbird mints, and desert willow trees thrive in the gardens that David Salman, president of Santa Fe Greenhouses, oversees.
This wouldn't be unusual...in the Chihuahuan desert. But Salman's display gardens are hundreds of miles north of the desert in Santa Fe. Thirty years ago, these plants wouldn't have survived that city's high elevation and chilly winters.
And that's not the only change in New Mexico. Santa Fe has seen better fruit and vegetable gardens over the last 10 growing seasons, and fruits like cantaloupes, which barely stood a chance before, now grow.
"Thirty years ago, trying to grow a cantaloupe was a complete waste of time," said Salman, who is president of Santa Fe Greenhouses. "We're not finding that to be the case now."
Two thousand miles east, in the New York Botanical Garden, horticulturists have also been experimenting. Their Ladies' Border plot, a south-facing, well-protected garden filled with decorative plants, was replanted ten years ago with plants that weren't normally associated with New York, like crape myrtles and camellias. Those plants are thriving now.
Travel south to Virginia and you'll find that vintners have been growing merlot grapes, a warmer weather grape that's now the second most popular variety in the state, rather than the chardonnay grapes, more tolerant to colder weather, which they were accustomed to 15 years ago. And the wine-growing regions have expanded all over the state as the season has gotten longer, said Tony Wolf, professor of viticulture at Virginia Tech.
"The warmer winters have opened doors for vintners to try varieties that were once risky," Wolf said.
Nationwide, gardeners have been experimenting with their newly assigned plant hardiness zones, a guide provided by the USDA that maps how well a plant will survive winter temperatures in a certain region.
The map divides regions into different growing zones based on the average lowest temperature. Since the 1960s, growers have used it as a guide for which plants will survive best in a given climate. This is the first update to the map since 1990.
According to the USDA, the new map is more robust. Data from over thirty years of temperature records and satellite mapping gives greater detail to the map. Users can now determine climate differences between zip codes, identifying heat and cool islands within a few miles. Heat islands are areas that absorb and radiate more heat from the sun, such as a dense urban area.
"The 1990 map was state-of-the-art at the time," said Kim Kaplan, chief of special projects for the USDA.
The results show warmer zones creeping farther north compared to the previous map. In states like Nebraska, Ohio, and Texas, the shifts can be seen almost throughout the entire state. Hawaii added a new zone 12 to their map, a warm-weather designation that didn't exist previously. Other areas like Pierre, South Dakota have gotten colder.
Kaplan cautions that the changes don't necessarily indicate a changing climate. Comparing the 1990 map to the 2012 map is apples to oranges, she said. The new map reflects more accurate data that has been ground-validated to eliminate biases, such as misleading weather stations' locations. And as for the change in zone locations, sometimes as little as 0.1 degrees Fahrenheit is enough to push a region into a new zone, she says, as was the case for Cheyenne, Wyoming.
"Some places have gotten warmer and some places have gotten more accurate," she said.
Sean Barry at the Arbor Day Foundation isn't surprised to see the changes. Horticulturists started asking the foundation for new data years ago because they felt the 1990 map was no longer relevant, he said.
"In most cases it's been a shift of half a zone, which isn't really a large magnitude," he pointed out. "But a couple of places at the margins [between zones] have been really interesting."
For example, Cleveland, Ohio has shifted its zone to a warmer winter rating of 6. Noelle Akin at Cleveland's Petitti Garden Center said they're telling their customers to stick to zone-5 rated plants as they always have, but expects they'll see more Ohioans trying their hand at plants from southern states.
"Gardening is a lot of experimenting," she said. "If you're a true gardener, you'll try new things. Just be aware this isn't a drastic change."
And it's not all good news for gardeners. Fungi and other diseases are more prominent in some areas, affecting cold weather trees like Colorado blue spruces in Ohio, Akin said. A bacterial infection that chokes vining grapes has been spreading north and west in Virginia, says Wolf. And the warmer winters have caused insect populations, such as beetles which destroy crops and trees, to swell.
The growing season in Virginia has expanded an extra three to four weeks, which means more time spent fighting the pests and diseases that pop up with the buds each spring, making the longer season more of a problem than a profit, Wolf said.
The sugar collection season for maple trees has shortened by 10 percent over the last forty years, said Timothy Perkins of the University of Vermont. If these warming trends continue, cold-loving trees like the paper birch could start migrating north, said Lois Berg Stack, professor of sustainable agriculture at the University of Maine.
But Stack warns gardeners against what she calls "zone envy."
"It's very tempting to look at the map and see a warming trend and try to grow different plants...but it's not a guarantee."
Photo credit: Salvia Maraschino, or Maraschino Cherry Sage, is a southwestern native plant that has become more widely growable because of warming winters. Photo by High Country Gardens.
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