How to Use the USDA's Planting Zone Map

The official news that 2012 was the hottest year ever recorded in the contiguous United States probably came as no surprise to many of the 80 million American gardeners who turn to the USDA Plant Hardiness Zone Map for weather information.

The National Climatic Data Center released the temperature data just as the U.S. Department of Agriculture was about to mark the first anniversary on Jan. 25 of its latest Plant Zone Hardiness Map. The 2012 map — which has 13 10-degree Fahrenheit zones subdivided into “A” and 13 “B” zones marked by 5-degree changes — shows that hardiness zones in many places in the country are generally 5 degrees warmer than they were in the previous USDA zone map, which was released in 1990.

Remember It Shows Weather, Not Climate

But for those who think the changes in the new USDA map are proof of global warming, Kim Kaplan, a public affairs specialist for the USDA’s Agriculture Research Service in Beltsville, Md., has some words of caution: Don’t confuse weather with climate.

“People look at the new map and want to talk about climate,” said Kaplan, who was on the team that created the 2012 Plant Hardiness Zone Map. “To start with, the plant hardiness zone map is only about the average of the lowest temperatures,” she pointed out. “Climate is about high and low temperatures in a location.”

“Besides,” she added, “most garden plants don’t experience climate. They experience weather. Trees are about the only plants that experience climate because they are some of the only plants that live for a long enough period of time to do that.”

Her advice is to use the map for the purpose it was intended to serve — as a guide to determine what plants you can grow in your garden based on the average of lowest winter temperatures in your area. The 2012 USDA map does that exceptionally well.

Learn How It Differs From Other Zone Maps

There are three main differences between the way the previous map was done and how Kaplan and the rest of the team created what Catherine Woteki, Under Secretary for Research, Education and Economics, called “The most sophisticated Plant Hardiness Zone Map yet for the United States.” Let me explain those differences.

Weather Data

The temperature data used for the 2012 map is from a longer and more recent time period than the data that was used to compile the 1990 map. The 2012 map is based on the 30-year period of 1976-2005 vs. the 13-year period of 1974-1986 used for the 1990 map.

Methodology

The zones for the 2012 map were derived with a sophisticated algorithm that greatly added to the accuracy and detail of the 2012 map, especially in the mountainous regions of the western United States. For the first time, the algorithms considered such factors as changes in the slope of land, wind and proximity to bodies of water as well as data from more stations than were included in the 1990 map. In some cases, the methodology resulted in cooler, rather than warmer, zones.

Scale

The 1990 map was a four-foot square poster map. Because of developments in computer technology since 1990, the 2012 map could be created as a “Google Earth” style map in a Geographic Information System (GIS), based interactive format that lets viewers click down to one-quarter mile in scale. It also includes a “find-your-zone-by-ZIP code” function for the first time. This feature allows gardeners to click the interactive button on the map menu at the top of the home page, enter a 5-digit ZIP code, mouse over the regional map that comes up, click on the area where they live and zero in on weather data within the quarter of a mile that includes their garden. A box will pop up with their zone designation, the exact coldest average temperature for their ZIP code,the coldest average range for the ZIP code and the latitude and longitude.

“We could not have made this map and displayed it on the Internet even 10 years ago,” said Kaplan. The technology and broadband access simply weren’t widely available then, she said.
Kaplan insisted during planning meetings for the map that site visitors also be able to view and download maps as static jpegs. That’s because, she said, 50 percent of the country still doesn’t have access to broadband and, therefore, would not be able to easily navigate the interactive map. “This allows people to see their area even if they don’t have broadband access,” Kaplan said.

Know Your Garden's Microclimate

Even with the interactive map, Kaplan says gardeners, scientists and others who use the map should use it as “a guideline rather than thinking of the information it provides as the rule.

“You have to think about your own particular yard,” Kaplan said. “You are the only one who knows your garden. No one else can know it any better.”

As an example of that golden rule, Kaplan pointed out that a gardener who knows his yard will know where the water pools first or the first frost sticks. Knowing about other garden microclimates such as windbreaks or the warm spot in front of the south-facing wall may help gardeners break the “rules” about their hardiness zone and grow a plant that supposedly won’t grow well in that area.

Creating Future Planting Zone Maps

Kaplan said her running tab of users of the 2012 map shows that the site has recorded 2.5 million individual visitors and 17.2 million page hits since the current map was released on Jan. 25, 2012. The response in the first three months after the map went live was huge, she said, adding that the rate of site visits has been steady.

When will the next map be released and what new functions might it bring to American gardeners? “No decisions have been made on that,” Kaplan said. “There’s never been a schedule to produce the map. There was a map in 1960, a revision in 1966, another in 1990 and the one last year. Records of maps before 1960 are fuzzy. Producing the map doesn’t fall into anyone’s regular job description.”

And as far as updates on functionality, who knows? Perhaps the next map will be so sophisticated you’ll be able to use it to tell when your tomatoes have reached their optimum ripeness for harvesting!