USDA’s plant hardiness zone maps got an update last year. These are the ones the agency publishes to give gardeners an idea what the average coldest temperatures are like in their region. Surprising no one, temperatures have been on the rise just about everywhere.
The 1990 version of the map put nearly the northern half of Oklahoma in Zone 6. The rest, to the south, fell into the warmer Zone 7.
The next time USDA updated the map, in 2012, Zone 7 had spread as far north as Stillwater. Zone 6 still held on in several counties east and west of there along the Kansas border. Zone 8, warmer, had gained a tiny toehold in the extreme southeastern corner of the state.
On the new map, based on data from 1991 – 2020, most of the state is still in Zone 7, but now Zone 6 has been pushed entirely into the panhandle, and Zone 8 occupies almost all the Red River counties.
Not everyone waits around for the official USDA updates to the maps. In 2006, the American Horticultural Society put out its own map, using data from 1986-2002. Three years later, the Arbor Day Foundation did the same thing. Both organizations used the same data sources and methods as the USDA.
In all cases, the overall story is the same. The climate is getting warmer, and the plant hardiness zones are shifting northward.
In addition to painting that temperature trend, the maps’ resolution has gotten finer-grained over time. The 2012 map used data from 7,983 weather stations. For the 2023 map, 13,412 weather stations contributed data.
The plant hardiness zone maps take the average lowest temperature at a given location over a thirty-year period. That’s a meaningful measurement, but it isn’t the be-all, end-all of gardening information. It’s pretty good at telling you whether a particular plant can survive the winter in a given area, but not so much whether it will consistently be able to flower or fruit.
Aside from farmers and gardeners, other users of the plant hardiness zone maps include the USDA itself, which includes the information in its calculations for setting crop insurance standards. Many scientists use the information as well, as input for modeling the spread of crop pests and invasive species.
The latest version of the USDA plant hardiness zone maps is available free in electronic format from the USDA Agricultural Research Service website, https://planthardiness.ars.usda.gov/.
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