After 11 years, USDA comes out with new planting zone map

It’s taken more than a decade, but the US Department of Agriculture has finally released an updated Plant Hardiness Zone Map that spans the nation in such detail that gardeners can now zoom right into their local areas.

This map is used by around 80 million growers to work out what plants have the best chance in the garden, with sellers such as nurseries adopting the zone code to help people find the right fits. The interactive map, developed by PRISM Climate Group at Oregon State University and the Agricultural Research Service, is the most accurate and detailed the USDA has ever released.

The update is well overdue, with many changes across 11 years. Aside from geographic information technological advancements, the 2023 map features data taken from a 30-year period between 1991 and 2020.

"Overall, the 2023 map is about 2.5 degrees warmer than the 2012 map across the conterminous United States," said lead author Christopher Daly, director of the PRISM Climate Group. "This translated into about half of the country shifting to a warmer 5-degree half zone, and half remaining in the same half zone. The central plains and Midwest generally warmed the most, with the southwestern US warming very little."

There are 13 zones on the new US map, each with a 10 °F range. The zones have been divvied up further, with each one split in two, for a more precise 5 °F range. So plants marked ‘a’ will have a 5 °F warmer range than ones labeled ‘b’ within the same number zone.

Essentially, each zone details the lowest to highest cold a plant can withstand. Gardeners then use these zones to choose plants based on a temperature spectrum. For example, if a plant suits zones three to seven, it can survive some cold but not extreme periods of low temperatures. 

The new online map is available at the USDA's official website.