A new gardening map indicates much of our region has gotten warmer.


Wednesday, the U.S. Department of Agriculture unveiled an updated version of its plant hardiness zone map that puts most of Ohio in Zone 6A. The zone designation means that on average, the temperature during the winter dips as low as minus 5 to minus 10 Fahrenheit.


That represents a zone change for much of northern Ohio. USDA’s previous map, created in 1990, put most of the upper half of the state in the slightly colder Zone 5B, while the southern half was mostly in Zone 6A.


The bottom line is that some plants can be expected to have a better chance of surviving winter in our region than they did in the past.


“There’s no question about it. The range is moving a bit,” said Jim Chatfield, a horticulture educator with the Ohio State University Extension Center at Wooster. He said he’s seen evidence that plants can now survive a bit farther north than they did a few decades ago.


He points as an example to a Bracken’s Brown Beauty magnolia in Wooster Township’s Secrest Arboretum that used to stay fairly small because it would die back to its roots from time to time during cold snaps. Now it’s grown to 15 or 20 feet tall, he said.


“We’re not ready to say, ‘Everybody plant crape myrtles,’ ” he said, referring to a shrub that thrives in warmer areas. “But it may not be long.”


USDA spokeswoman Kim Kaplan cautioned in a news conference that the new map cannot be considered evidence of climate change. It’s based on only 30 years’ worth of weather data, which isn’t enough time to prove a trend, she said. It also considered only minimum temperatures and not maximums, she said, and it didn’t take into consideration other factors beyond temperature that would indicate climate change.


However, David W. Wolfe, professor of plant and soil ecology in Cornell University’s Department of Horticulture told the Associated Press the USDA is being too cautious and disagrees with Kaplan about whether this reflects warming.


“At a time when the ‘normal’ climate has become a moving target, this revision of the hardiness zone 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.


Kaplan noted that only a half-degree difference in average minimum temperature could be enough to push an area into a new zone, warmer or colder. “Is that significant to the plants in your yard?” she asked rhetorically. “Is that a significant weather change in terms of warming?”


Kaplan couldn’t say how much of the country had changed zones, but comparing the old map to the new shows many areas have moved to warmer designations.


Now most of Ohio is in Zone 6A, although several large areas fall mainly in Zone 5B. Those include a swath of Northeast Ohio that covers much of Lake and Ashtabula counties and a bit of Cuyahoga County; an area in the vicinity of Mansfield incorporating much of Knox County and parts of Morrow, Richland and Ashland counties; and a section of Preble and Darke counties in the western part of the state.


Areas on the edge of Lake Erie and the Ohio River are largely in warmer 6B.


Developers of the new map were able to use more sophisticated technology, so the map is far more accurate than previous versions, Kaplan said. It’s also far more detailed, assigning zones to areas as small as neighborhoods.


The developers started with weather data collected from 8,000 weather stations across the United States from 1976 to 2005, and then factored in such aspects as slope, elevation, prevailing winds and nearness to large bodies of water.


That gave them a more accurate picture of the areas between the weather stations, such as warm spots around cities called urban heat islands. “It can show smaller areas of zone delineation than ever before,” said Catherine Woteki, the USDA’s chief scientist.


The USDA put an interactive version of the map online. Users can zoom in to a ZIP code or even a neighborhood, but Kaplan noted the map can’t show tiny microclimates that can also affect how well a plant survives temperature extremes. For example, a plant might fare better than others in the same yard just because it’s close to the house, protected from the wind and warmed by heat reflected off white siding.


Both Woteki and Chatfield stressed that the zone map is only a guide for selecting plants. “Nothing is better than the gardener’s knowledge of his own garden,” Woteki said.


Chatfield doesn’t expect nurseries to suddenly start selling new types of plants just because the map changed.


He was teaching a course Wednesday at the Central Environmental Nursery Trade Show in Columbus, and he asked the 50 or so garden center representatives in the class whether they expected to change their practices because of the new map. The overwhelming answer was no.


The map only confirmed what they already know, Chatfield said. “They’re already dealing with the realities.”


The map is at www.planthardiness.ars.usda.gov. Type in a ZIP code to find out what zone most of your postal area falls in, or click on the interactive map to zoom in more closely to your neighborhood.


Users with dial-up Internet access can view a picture of the map instead of the interactive version.


Mary Beth Breckenridge can be reached at 330-996-3756 or mbrecken@thebeaconjournal.com. You can also become a fan on Facebook, follow her on Twitter @MBBreckenridge and read her blog at marybeth.ohio.com.