Which plants will survive in your garden? This map predicts a warmer United States

Jason Reeves has been using lantana, a flowering shrub, as a perennial plant in flower beds for several years. This was not the case ten years ago, when the plant would have died during Tennessee winters.

Today, the U.S. Department of Agriculture made official what Mr. Reeves and countless other gardeners and horticulturists have known for some time: Americans are adapting to global warming, right in their backyards.

“Nothing has really changed,” said Mr. Reeves, a landscape consultant and horticulturist at the West Tennessee AgResearch and Education Center at the University of Tennessee in Jackson, Tennessee. “We just see 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 had moved to a slightly warmer zone. The hardiness map is considered the go-to resource for understanding which perennials thrive and where. The map divides the country into color-coded zones, each indicating the average low temperature of the year for that zone.

While Christopher Daly, director of the PRISM Climate Group at Oregon State University and lead author of the map, said climate change could be a factor, he pointed to other changes in how data is collected to explain this change.

“This is not a forecast,” he said. “This is what happened in the past, as best we can describe it.”

The map is based on the 30-year average of the lowest annual winter temperatures for specific locations. 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, as low as -60 degrees Fahrenheit, applies to remote areas of Alaska. The hottest, reaching 70 degrees, covers coastal areas of Puerto Rico.

Dr Daly said the biggest changes occurred in and around Arkansas, Kentucky, Missouri and Tennessee, where temperatures rose by up to 5 degrees.

Some area changes reflect how data is collected, he explained, which includes the use of more weather stations and increasingly sophisticated mapping methods, down to code postal.

Art DeGaetano, director of the Northeast Regional Climate Center at Cornell University, said it can be difficult to use an extreme number, such as a historical average of a region’s coldest winter temperature, to model climate change over time.

The warming reflected in the updated map, however, is “completely consistent, over the long term, with what we expect from climate change,” he said. “Not all cold temperatures will warm up, but on average things will warm up. »

This is not the first version of the map to show planting areas moving north as winters become milder. When the Department of Agriculture released a 2012 version of the map, most parts of the country had changed by half an area from the 1990 version.

Among other uses, the plant hardiness card has applications in commercial agriculture and is used by the department’s Risk Management Agency to establish certain crop insurance standards.

But gardeners are the most frequent users, and for good reason: They need to know what zone they are in, because winter temperatures will play a major role in determining which perennials will survive until spring, which should be brought inside and those that should be. not be planted in the first place.

The signs of adaptation are easy to find. In the New York metropolitan area, for example, some native plants, like sugar maples, become less prevalent as temperatures rise. At the same time, some Southern plants, including camellias, Alabama’s state flower, began blooming at the New York Botanical Garden.

Jason D. Lanier, an extension specialist at the University of Massachusetts Amherst, said the map change, while minor compared to that of a decade ago, reflects longer, more significant changes.

“If you talk about this in hundred-year increments, based on what we’re seeing now, we’re seeing significant differences in these hardiness zones,” he said.

He suggested viewing hardiness zones as “a kind of practical guideline.”

“We are dealing with living things, so nothing is difficult or quick,” he said. “This is an attempt to get as close as possible to some real kind of useful advice.”

That’s exactly what Mr. Reeves, who lives in Clarksburg, Tennessee, thinks.

“Nothing changed overnight,” he said. If gardeners want to push their limits a bit in new areas, he adds, they should do so in spring and early summer and give the plants a chance to take root before winter.

“Just keep planting,” he said.