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The USDA’s Plant Hardiness Map Gets an Update

In today's edition of This Week in Climate, we look at the updated USDA Hardiness Map and what it means for plants.
Abigail Bassett
Nov 24, 2023 10 min read
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The U.S. Department of Agriculture (USDA) updated its Plant Hardiness map for the first time in more than a decade this week, and it shows that growing regions across the U.S. have shifted pretty significantly - something that gardeners and farmers across the country have long suspected, according to NPR.

Of course, this shift is not isolated to just the United States. Around the world, farmers and backyard gardeners alike have noticed the creeping shift, and it's changing the way that they think about feeding a growing global population.

Climate Change and Hardiness Maps in the U.S.

Unsurprisingly, the recent update to the USDA hardiness maps has garnered significant attention from backyard gardeners and farmers alike. It’s the first in over a decade and shows considerable changes; most notably, the contiguous United States is approximately 2.5 degrees Fahrenheit warmer compared to the last map issued 11 years ago, in 2012. At the same time, it's important to note that the map takes into consideration the coldest temperatures of the year, and the USDA says that “changes in zone are not reliable evidence of wether there has been global warming.”

At the same time, the release announcing the new map goes on to note that, “Compared to the 2012 and 1990 maps, zone boundaries in this 2023 edition have shifted in many areas. The new PHZM is generally about one quarter-zone warmer than reported in the 2012 PHZM throughout much of the United States, as a result of a more recent averaging period (1976-2005 vs. 1991-2020). However, some of the changes in the zones are the results of additional data sources and improved interpolation methods. These zone shifts can sometimes result in a cooler, rather than warmer, zone. The most substantial changes produced by additional data sources and improved interpolation methods are seen in upland areas of Alaska.”

This shift in the map could further underscore some of the tangible impacts of climate change on local environments and agriculture, as gardeners in the NPR story fear. The USDA's map is an important tool for gardeners and growers in selecting plants and flowers suitable for their local climates. Shifts to the climate impact what can be grown, when, and where.

According to the USDA site, there are a few updates to how the data was parsed and collected for this new map. For one, a longer period was used (30 years of data), to smooth out weather anomalies. Additionally, the USDA says that a “very sophisticated algorithm was used to interpolate low-temperature values between actual weather reporting stations,” and more than 13,000 weather stations were used versus the previous 7900-plus weather stations used in 2012.

While the USDA says that the map is mostly accessed by gardeners in the U.S., they note that it is also used to set crop insurance standards, which inform everything from crop risk management to agricultural investment all over the country, and inform research.

A Climate Shift That's Not Just Impacting the U.S.: The Tropic Squeeze

Growing zones aren’t only changing in the U.S.; they’re shifting all over the world.

For one, growing seasons are shifting in many areas around the globe. Those in the Northern Hemisphere are generally getting longer with earlier springs and shorter autumns. The Mediterranean, and Central and South America are generally seeing less rainfall and more heat. India, Sub-Saharan Africa, and South Asia face complex changes ranging from monsoon season shifts to sustained high heat and drought, making it difficult for farmers to properly manage crop yields.

A kind of “tropic squeeze” is going on, according to a report from Yale’s School of the Environment, which has essentially makes the drier regions of the Tropic of Cancer and the Tropic of Capricorn (which border the wetter, hotter equatorial region), larger by as much as 30 miles or as this paper from 2018 reports, as much as 0.2 to 0.3 degrees of latitude per decade. As that study notes, the shift is likely to be a result of climate change.

That tropic squeeze is causing food supply changes all over the world. While warmer temperatures are making it possible to grow certain crops in regions where they previously could not thrive, there are other areas where crops that once flourished are struggling.

For example, the effects of the tropic squeeze are constricting the production of wine around the world, resulting in an overall drop of between 12% and 14% from Italy and France, the world’s largest producers. Rice production in Asia is also shifting as climate change reduces the amount of rain the area receives. Coffee and cocoa are also being impacted. As we’ve discussed before here at Climatebase, all of this is impacting the cost of food all over the world, too.

Water Stress and Hardiness Maps

It goes without saying that water stress, or the difficulty in obtaining sufficient water for human and ecological needs, is a growing concern worldwide. As the climate changes and the tropic squeeze continues to bring a wider band of dry and hot weather to the world, water stress will become an even more acute problem.

The USDA’s new Hardiness Map shows that roughly half the nation has shifted into a new zone. For example, some of those zones, like the American West, are already facing high water stress.

The most significant shift in hardiness zones occurred in the Central Plains and Midwest, where everything from corn and soybeans to dairy and livestock are cultivated. That area has experienced the largest temperature shift according to the new map.

Corn, beef, and dairy are all large industries in that area. They are all highly water-intensive goods. According to the U.S. Drought Monitor, the Central Plains and Midwest had some slight improvement in their drought conditions this past summer, but those areas are still in a drought. According to the Drought Monitor, more than 30% of the country remains in drought conditions. With the rise in temperatures across the nation, water stress is likely to continue to be an issue, even though gardeners and farmers can grow more things.

The confluence of shifting hardiness, water stress, and food demands is likely to continue to change the way we think about managing our food systems in a warming world. The update to the USDA Hardiness map may not singularly indicate climate change, but it does point to a warming trend and one that could disrupt food supplies both in the U.S. and around the world.

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The Author

Abigail Bassett