The United States Department of Agriculture (USDA) has released a new version of its “Plant Hardiness Zone Map,” updating this valuable tool for gardeners and researchers for the first time since 2012.

USDA’s Plant Hardness Zone Map is the standard by which gardeners can determine which plants are most likely to thrive at a given location.

The new map is more accurate and contains greater detail than previous versions.

The map is available online:

https://planthardiness.ars.usda.gov/

In addition to the map updates, the Plant Hardiness Zone Map website was expanded in 2023 to include a “Tips for Growers” section, which provides information about research programs of interest to gardeners and others who grow and breed plants.

The 2023 map is based on 30-year averages of the lowest annual winter temperatures at specific locations. The map is divided into 10-degree Fahrenheit zones and further divided into 5-degree Fahrenheit half-zones.

Approximately 80 million gardeners and growers in the United States represent the most frequent users of the USDA Plant Hardiness Zone Map.

However, they’re not the only ones with a need for this hardiness information.

For example, the USDA Risk Management Agency refers to the map’s hardiness zone designations to set certain crop insurance standards.

Additionally, scientists incorporate the plant hardiness zones as a data layer in many research models, such as those modeling the spread of exotic weeds and insects.

Plant hardiness zone designations represent what’s known as the “average annual extreme minimum temperature” at a given location during a particular time period (30 years, in this case).

Put another way, the designations do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time.

Low temperatures during the winter are a crucial factor in the survival of plants at specific locations.

That, in turn, not only affects which trees, shrubs, evergreens and perennial flowers a gardener plants, but which ones garden centers carry in the first place. Hardiness zones are routinely listed on plant tags and labels.

As with the 2012 map, the new version has 13 zones across the United States and its territories. Each zone is broken into half zones, designated as “A” and “B.”

For example, Zone 7 is divided into 7a and 7b half-zones. When compared to the 2012 map, the 2023 version reveals that about half of the United States shifted to the next warmer half zone, and the other half of the country remained in the same half zone.

The shift to the next warmer half zone means those areas warmed somewhere in the range of 0-5 degrees Fahrenheit.

Some locations experienced warming in the range of 0-5 Fahrenheit without moving to another half zone.

The Lehigh Valley has gone from 6b to 7a. The USDA’s new Plant Hardiness Map confirms what gardeners have been noticing for years: Lehigh Valley winters aren’t as cold as they used to be.

As USDA officials point out, the map is based on average lows over a given period, not all-time lows. That means a cold spell can go below the listed averages, killing a plant that otherwise would survive an average winter.

“Growing Green” is contributed by Diane Dorn, Lehigh County Extension Office Staff, and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613.