MOVING NORTH: Climate zones have shifted north in the U.S., according to both gardeners and the U.S. Department of Agriculture's new plant hardiness zone map. Image: Aunt Owwee, courtesy Flickr
The gardening cycle has been thrown off the rails in Debbie Ricigliano's Howard County, Md., vegetable garden in past years. Shrubs are blooming earlier, cherry buds are opening in the fall and flower bulbs are emerging when they shouldn't.
"I definitely would say our climate has gotten warmer," said Ricigliano, a horticulture consultant at the University of Maryland's Horticulture and Garden Information Center. Scientists have long established that climate change will affect, either positively or negatively, the future of agricultural crops. To date, there have been no tools to help gardeners adapt.
Yesterday, the Agriculture Department's Agricultural Research Service (ARS) released the first revisions of the Plant Hardiness Zone Map since 1990. Adorning the back of seed packets, the map is one of a few authoritative guides for gardeners to know what to plant where and when.
The designations on the map represent the average annual extreme minimum temperatures during a particular period, rather than record cold or hot temperatures. "Bands" of temperature ranges are attributed a number, 1 to 13, and subdesignations of a or b show smaller ranges in temperature. The new version includes 13 zones -- two more than the 1990 map -- for temperature bands of 50 to 60 degrees Fahrenheit and 60 to 70 degrees Fahrenheit.
Indeed, the new map shows an upward shift north. Zone 8 used to run from South Carolina to the Texas Panhandle. Now, it has reached the top of North Carolina and covers the bottom third of Arkansas, a state it barely entered in the 1990 map. Zone 6's northernmost point was once southern Connecticut. Now, it's the southern tip of Maine, more than 200 miles away. Overall, the Northeast is in a zone that is 5 degrees warmer than before.
The change is clear to David Ellis, director of communications at the American Horticultural Society and a gardener. For example, Indian hawthorn, a hedge shrub that typically grows in South Carolina and Georgia, has thrived in the society's northern Virginia gardens.
"There's some clear signs of changes in average minimum temperatures," said Ellis. "Gardeners know, [through] their own observation, that average temperatures are warming."
A reluctance to acknowledge climate change
But the ARS was reluctant to showcase its freshly minted map as a symbol of global climate change. Technical accuracy, not environmental change, is the reason for a different map, said ARS spokeswoman Kim Kaplan.
"It's not whether or not there has been global climate change, it's that the map is simply not a good instrument for trying to address it," she said.
To start, the new map is far more sophisticated, relying on data between 1976 and 2005. The 1990 version only spanned a 13-year period between 1974 and 1986. The developers of the new map created an algorithm that incorporated four factors into the local climate: slope, elevation, prevailing winds and nearness to large bodies of water. The scale is smaller, as well.
"To look at climate change, you really have to look at the whole cycle," she said, at least a 50- or 200-year cycle. Weather events like El Niño and La Niña, which can cause unusually heavy rains or droughts, also affect the zoning.
While the map may show regions that have fewer cold days, it does not show data on the number of hot days per year in a certain zone. The American Horticultural Society does provide a heat index map, but it has not been updated since its release. This heat index was not included in the Plant Hardiness map because it could not be translated to the new version's interactive platform, said Kaplan.
David Wolfe, a professor of plant ecology and chairman of the Climate Change Focus Group at Cornell University, commended ARS's efforts to craft a new map but said he would have been bolder on strengthening the climate change link.