USDA updates Plant Hardiness Zone Map - East Idaho News
In the Garden

USDA updates Plant Hardiness Zone Map

  Published at  | Updated at

One of the tools gardeners have used for years to select plants for their yard has been updated recently. The USDA Plant Hardiness Zone Map recently was updated.

The Plant Hardiness Zone Map (PHZM) is based on average annual extreme minimum temperatures, displayed as 10-degree F zones ranging from zone 1 (coldest) to zone 13 (warmest).

Last updated in 2012, the PHZM uses the past 30 years temperatures to establish plant growing zones. This new update used weather data from 1991-2020. The map does not represent the coldest it has ever been in an area, but the lowest average temperature.

How has the map changed?

According to the USDA, about half of the country has shifted to the next warmer half zone, and the other half has stayed in the same zone. That shift to the next warmer half zone means that those areas warmed in the range of 0-5 degrees Fahrenheit. In addition, with more weather stations being used, the map that was created used more data to identify hardiness zones.

plant zones
Differences between the 2023 and 2012 versions of the PHZM. The white indicates no change, the light brown indicates a half zone shift down, the light green shows a half zone shift change up. | USDA Plant Hardiness Zone Map

The PHZM does not act as a guarantee that a plant will survive in an area. An extremely cold snap may kill a plant that has thrived for years in your yard. Gardeners should use the map to guide them when purchasing plants. In addition, gardeners should recognize that many factors go into survivability of plants.

To thrive, plants need to be planted where they will receive the proper amount of light. A plant that requires partial shade might be injured by too much sun during the winter because it might cause rapid changes in the plant’s internal temperature.

Soil moisture varies seasonally. Plants that might be hardy in your zone may be injured if soil moisture is too dry in late autumn, and they enter dormancy while suffering moisture stress.

Plants have a range of optimal temperatures. Varieties of the same species may be injured if the temperature drops too much. The duration of cold may influence survivability as well. Many plants will not tolerate prolonged periods of cold.

To see how the map has changed, or to download a copy of the map, click here. The website has garden tips for gardeners of all levels.

USDA Zone map