On Wednesday, the U.S. Department of Agriculture released an updated guide for the color-coded map of planting zones often seen on the back of seed packets. Half of the cities included on the guide are now in warmer zones, which many are saying reflects global warming.
The headline used by the Associated Press was “New map for what to plant reflects global warming”.
But AP reports later in the article that USDA spokeswoman Kim Kaplan, who was on the team that created the map, didn’t want the new zones on the to be associated with global warming. AP reports Kaplan as saying even though some areas of the country are now in warmer zones, the map “is simply not a good instrument” to be used to support global climate change. Kaplan says this is because the map is based on the coldest days of the year, not average temperatures.
On the flip side, AP goes on to report David Wolfe, a professor of plant and soil ecology at Cornell University, as saying that the map clearly reflects global warming:
The revised map “gives us a clear picture of the ‘new normal’ and will be an essential tool for gardeners, farmers and natural resource managers as they begin to cope with rapid climate change,” Wolfe said in an email.
According to USDA’s press release, which does not mention the words “global warming” or “climate change,” this update to the plant hardiness zone map comes two decades after the last update in 1990. The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees F) and 13 (60-70 degrees F). Each zone is a 10-degree Fahrenheit band, further divided into A and B 5-degree Fahrenheit zones.
AP points out that entire states, such as Ohio, Nebraska and Texas, are now in warmer zones. Here are some examples AP uses to show the changes in zones:
For example, Des Moines used to be in zone 5a, meaning the lowest temperature on average was between minus 15 and minus 20 degrees. Now it’s 5b, which has a lowest temperature of 10 to 15 degrees below zero. Jerry Holub, manager of a Des Monies plant nursery, said folks there might now be able to safely grow passion flowers.
Griffin, Ga., used to be in zone 7b, where the coldest day would average between 5 and 10 degrees. But the city is now in zone 8a, averaging a coldest day of 10 to 15 degrees. So growing bay laurel becomes possible. It wasn’t recommended on the old map.
The new guide also uses better weather data and offers more interactive technology. For example, gardeners using the online version can enter their ZIP code and get the exact average coldest temperature.
Also, for the first time, calculations include more detailed factors such as prevailing winds, the presence of nearby bodies of water, the slope of the land, and the way cities are hotter than suburbs and rural areas.
The press release states that the new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period; the new map uses data measured at weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.
The Associated Press contributed to this report.