What plants will survive in your garden? This map plots a warmer US state
Jason Reeves has been using lantana, a flowering shrub, as a perennial in garden beds for the past few years. This wasn’t the case a decade ago, when the plant would die back in Tennessee winters.
Now, the USDA has announced what Mr. Reeves and countless gardeners and gardeners have known for some time: that Americans are adapting to warmer weather, right in their own backyards.
“Nothing has really changed,” says Mr. Reeves, a landscape consultant and horticulturist at Western Tennessee University’s Agricultural Research and Education Center in Jackson, Tenn. “We’re just seeing it on paper.”
The USDA updated its plant hardiness zone map last week for the first time in more than a decade, showing that about half of the United States has moved to a slightly warmer zone. The hardiness map is the gold standard resource for understanding which perennials thrive where. The map divides the country into color-coded regions, each indicating the year’s average low temperature for that region.
While Christopher Daly, director of the PRISM Climate Group at Oregon State University and lead author of the map, said climate change may be a factor, he pointed to other changes in how data is collected to explain the shift.
“It’s not a forecast,” he said. “This has happened in the past as best we can describe it.”
The map is based on a 30-year average of the lowest annual winter temperatures for specific places. It is divided into 13 zones, each reflecting a 10 degree temperature range, and each zone is divided into two half zones, designated A and B.
The coldest zone, minus 60 degrees Fahrenheit, applies to remote areas of Alaska. The warmest temperature, up to 70 degrees, covers the coastal areas of Puerto Rico.
The greatest changes came in and around Arkansas, Kentucky, Missouri and Tennessee, where temperatures rose by as much as 5 degrees, Dr. Daly said.
He said some of the changes in the area reflect how data is collected, which has included the use of more weather stations and increasingly sophisticated mapping methods, all the way down to zip code.
It can be difficult to use an extreme number, such as the historical average coldest winter temperature in the region, to model climate change over time, said Art DiGaetano, director of Cornell University’s Northeast Regional Climate Center.
He added that the rise in temperatures reflected in the updated map “is very consistent, over the long term, with what we expect to see from climate change.” “Not all cold temperatures will get warmer, but on average, things will get warmer.”
This is not the first version of the map to show farming areas shifting northward as winters become milder. When the Ministry of Agriculture released the 2012 version of the map, most areas of the country had shifted by half an area from the 1990 version.
Among other uses, the plant hardiness map has applications in commercial agriculture and is used by the department’s risk management agency to set some crop insurance standards.
But gardeners are their most frequent users, and for good reason: They need to know the zone they’re in because winter temperatures will play a major role in determining which perennials will survive until spring, which ones should be taken indoors and which ones should be taken out. It cannot be grown in the first place.
Signs of adaptation are easy to find. In the New York metropolitan area, for example, some native plants, such as sugar maples, are becoming less widespread as temperatures rise. Meanwhile, some plants from the South, including the camellia, Alabama’s state flower, are beginning to bloom at the New York Botanical Garden.
The shift in the map, while slight compared to the change that occurred a decade ago, reflects longer and larger changes, said Jason De Lanier, an extension specialist at the University of Massachusetts Amherst.
“If you talk about this over a hundred years, based on what we’re seeing now, we’re seeing big differences in these hardiness zones,” he said.
He suggested looking at hardiness zones as “a kind of useful guideline.”
“We’re dealing with living organisms, so there’s nothing hard and fast,” he said. “This is an attempt to get as close as possible to a real kind of useful advice.”
That’s exactly what Mr. Reeves, who lives in Clarksburg, Tennessee, thinks.
“Nothing changed overnight,” he added. He added that if gardeners want to expand their boundaries into new areas a little, they should do so in the spring and early summer, giving the plants a chance to take root before winter.
“Just keep farming,” he said.