This is the first update since the map was created in 1990. The USDA’s Agricultural Research Service teamed with Oregon State University’s PRISM Climate Group to develop the new map. The team created drafts of the map and then consulted with climatic experts across the country to make revisions for the final map.
What the map records
The Plant Hardiness Zone Map splits the United States into zones that represent the average annual lowest winter temperature in that location. Each zone spans 10 degrees (F) and is further divided into 5-degree “a” and “b” sections.
For instance, in USDA Zone 6, the average lowest winter temperature over the 30 years the researchers studied fell between –10˚F and 0˚F. Zone 6a saw the average minimum of –10 to –5, while Zone 6b’s average minimum was –5 to 0.
Changes since 1990
The new map shows shifts in the boundaries of the zones drawn in 1990. The majority of locations are now a half-zone warmer (5 degrees) than indicated on the 1990 map. Some locations are now in cooler zones.
The USDA and OSU attribute the changes to these facts:
- They used data measured at weather stations over a 30 year period (1976–2005), while the 1990 map used data from a 13-year period (1974–1986).
- They used new measurement methods, including algorithms that considered changes in elevation, proximity to large bodies of water and extremes of terrain (peaks and valleys), which all affect temperature.
- They drew data from more weather stations than in 1990.
New features for the gardener
The map was designed to be more interactive online. Gardeners can go to the map’s website and search for their zone simply by entering their ZIP code. There are also large, separate images of individual states. There’s an “Interactive Map” with a “zoom in” function and you can select whether to display roads, terrain or a satellite image. The site also includes static images of the map and downloadable versions for use in print publications and extension fliers, so the map will reach gardeners without Internet access.
You can explore the new Plant Hardiness Zone Map at http://planthardiness.ars.usda.gov/PHZMWeb/.