Chambersburg's famous peaches could now grow in Clearfield and other points north because Pennsylvania's coldest days aren't as cold as they once were.
That warming trend, generally extending across the United States, is detailed in the U.S. Department of Agriculture's new "plant hardiness map," a rainbow-banded depiction of growing zones often found on the back of hardware store seed packets and used by farmers and gardeners to determine which fruits, vegetables and ornamental trees and shrubbery can grow in which regions.
The map, released by the USDA on Wednesday, charts the average lowest winter temperature for a location, a crucial factor in determining if certain plants will survive or freeze and die.
"The map is useful to home gardeners and provides one of the ways people figure out what plants will grow in their front or back yards or gardens and which will survive the winter," said Nina Bassuk, professor of horticulture at Cornell University.
William Miller, a Cornell professor of horticulture, said the new map "won't change what will grow but will give gardeners more confidence that they are planting plants that will survive."
It is the first update of the plant hardiness map since 1990 and reflects shifting and warming climate patterns across the U.S.
The new map is based on weather station data gathered during the 30-year period 1976-2005 and for the first time incorporates mapping techniques that factor in elevation change, proximity to large bodies of water, valley bottoms and ridge tops. It moves most of Ohio, Nebraska and Texas to warmer zones. Each zone spans a 10-degree Fahrenheit range.
In Pennsylvania, the growing zones have been adjusted to reflect average lowest temperature data that is about 5-degrees Fahrenheit warmer. The shift moves two-thirds of the state out of Zone 5 into Zones 6A or 6B.
Sean Barry, a spokesman for the Arbor Day Foundation, which published its own plant hardiness map in 2006, said the USDA's old map had outlived its usefulness long ago.
"People were ordering trees from us based on that old map and telling us that it wasn't right," Mr. Barry said, citing the southern magnolia, white flowering dogwood, certain cherry and peach trees as examples of trees recommended for Zone 6 and above but not Zone 5.
"We have peach trees recommended for planting in Zones 5 through 8, but Zone 5 is right on the edge," he said. "The old map put parts of Pennsylvania on the colder side of Zone 5. Now it's mostly a Zone 6, giving a truer indication that peach trees would probably do fine now."
Dave Abler, a professor of agricultural economics at Penn State University, said the warming trend will not cause major changes in agricultural practices in the short term but could lead to longer-term reductions in the acreage planted for cooler crops such as potatoes and apples and increased planting of warmer weather crops such as sweet corn.
Wineries in Erie County may replace native American varieties of grapes that like cooler weather with European varieties, Mr. Abler said, and garden nursery plant stocks will likely need to change, too.
For agriculture, the overall effect of warmer temperatures will be "mixed," he said, with warmer temperatures causing more heat stress on crops and livestock, but likely increases in precipitation will reduce droughts. The longer growing season will allow for greater diversity of crops and longer pasture times for livestock but will also allow for more insect pests and non-native weed growth.
"If you take all the changes together, Pennsylvania might be better off," Mr. Abler said, "but that would not necessarily be true for states to our south."
According to the USDA, the nation's 80 million gardeners and nurseries that grow, breed and sell plants are the primary users of the plant hardiness maps. But it also provides researchers with another indicator that the climate is generally warming in Pennsylvania and across the United States.
USDA officials stopped short of linking the new map to climate change, but David Wolfe, professor of plant and soil ecology at Cornell, said he "wouldn't be so cautious."
"In isolation one map doesn't prove climate change, but taken with other evidence where it charts milder temperatures over a number of years it corroborates that evidence that there is climate change occurring," Mr. Wolfe said.
He said farm crops respond to climate signals just like backyard garden and ornamental plantings do.
"Peaches, grapes, apples, as well as weed and insect pests are all impacted by changing climate, and their survival is affected by how cold it gets," Mr. Wolfe said. "So this map and the changes it shows have implications beyond the backyard gardener."
Art DeGaetano, a climatologist and professor of earth and atmospheric sciences at Cornell and director of the National Oceanic and Atmospheric Administration's Northeast Regional Climate Center, said the map is another tool for researchers.
"It's not the Holy Grail for identifying climate change," he said. "However, the changes it identifies are additional information that together with changes in species range and snow cover and insect appearances all support the fact that the climate has been changing."
The new USDA plant hardiness zone map and the Pennsylvania map can be accessed at http://planthardiness.ars.usda.gov/PHZMWeb/
Don Hopey: firstname.lastname@example.org or 412-263-1983.