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New USDA map shows Oklahoma's shifting plant zones

U.S. Department of Agriculture

Oklahoma gardeners can rest easy when planning their spring garden - an updated zone map is out.

The last time the United States Department of Agriculture (USDA) updated its Plant Hardiness Zone Map, it was 2012 and now the outlook is warmer. Some people use it to decide what plants can flourish in a location and is the resource USDA uses to set certain crop insurance standards.

The map is based on 30-year averages of the lowest annual winter temperatures. Like other areas of the country, growing zones shifted in parts of the state. David Hillock, an OSU horticulture extension specialist, has been in Oklahoma for 27 years and noticed changes with each map renewal.

“I don't know if I’d say it’s “significant” (changes),” Hillock said. “But you know, it's definitely showing a trend of things getting a little bit warmer.”

The main shifts the map shows are near the state’s northern and southern borders. For instance, most of Oklahoma’s southern edge moved from zone 7b to 8a.

Hillock said growers might be successful at raising different plants, but it's important to note a cold snap can bring temperatures below average and damage plants.

“It does mean that we can maybe be a little bit more successful with some plants that we haven’t been able to grow in the past that aren’t cold hearty enough,” Hillock said. “But we have to remember too that these are average minimum temperatures. And so, we all know that we can go well below those average temperatures.”

The map’s developers, USDA Agricultural Research Services (ARS) and Oregon State University’s PRISM Climate Group, claim the map is more accurate. To make the 2012 map, information was pulled from nearly 8,000 weather stations. This time developers used about 13,400 stations.

Although researchers warn the map is not necessarily reflective of climate change, the average recorded temperatures were warmer, causing the plant zones to change.

Oklahoma Climatologist Gary McManus said research, like the National Climate Assessment, shows increasing average temperatures and aridity are expectedto continue through the next few decades.

“When you’re talking about increasing temperatures, we’re talking about average temperatures,” McManus said. “But a lot of that is, I guess you would say, an increase in the lower temperatures. So not getting as cold as you would normally expect. And then so extreme temperatures in summers would also become more extreme."

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Anna Pope is a reporter covering agriculture and rural issues at KOSU as a corps member with Report for America.
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