USDA updates Plant Hardiness Zone Map

The updated PHZM shows a significant shift in Iowa's average annual minimum temperatures. Image courtesy of the USDA.

The United States Department of Agriculture released an updated version of the Plant Hardiness Zone Map (PHZM) this week, a tool often used by gardeners and researchers to determine the average annual lowest temperature at a given location during a particular time period.

This update marks the first time the USDA has modified the PHZM since 1990, and the changes are significant. In some instances, entire states have been shifted into warmer zones. This would imply that some plant species can now survive farther north than before.

“People who grow plants are well aware of the fact that temperatures have gotten more mild throughout the year, particularly in the wintertime,” said Boston University biology professor Richard Primack. “There’s a lot of things you can grow now that you couldn’t grow before.”

In Iowa’s case, the northern half of the state has shifted from a 4b designation, which represents -20 to -25 degrees, to a 5a designation of -15 to -20 degrees. A similar transition occurred in southern Iowa, where the previous 5a designation has been updated to 5b, -10 to -15 degrees.

The Washington Post has an interactive map which can be used to observe the changes on a national scale.

For more information, read the full article at The Press-Citizen, and the official USDA news release.

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