WASHINGTON, D.C. — The USDA, in February, introduced its first-ever total revision of the Plant Hardiness Zone Map, a reference used for determining what plants and trees are winter-hardy enough to survive in a particular location.

In this revision, many areas will be shown as a half-zone warmer than the initial map. The zone changes theoretically mean the average lowest annual temperature at a given location is probably about 5 degrees F. warmer than it would have been 60 years ago, according to the formula used for calculating the zones.

Some parts of the country, such as the Ozarks and western Montana, have experienced much more climate change than average over the same time period. Throughout the flatter parts of the Midwest, though, a change of about 3-4 degrees was experienced. For example, Paducah, Ky., was in Zone 6b or “upper Zone 6,” whereas now that same city is technically in Zone 7, meaning in an average winter the temperature does not get below zero.

The U.S. zone map’s new changes follow Canada’s, whose similar zoning system was updated four years ago.

Garden companies and seed houses, it turns out, are spending millions of dollars studying climate change. Fedco, Ferry-Morse and Baker Creek Seed are just three of the seed companies whose literature has mentioned the need or effort to adapt existing crop varieties to possible changing weather norms.

Response to a warming climate raises questions about perennial plants, including how cold of a winter period the plants need. Plant breeders are also becoming involved, working on new varieties of cool-season vegetables that can withstand higher heat, as well as perennial landscape plants and fruit trees that can as well.

The zone map, however, fails to include the possible decreasing number of “chill hours” available each fall and spring. Chill hours are defined as the number of hours each year temperatures remain between 32-45 degrees F. These hours are necessary for many fruit trees and shrubs, including peach, apple, cherry and blueberry, to bloom and set fruit. Warming climates mean fewer chill hours, and an absence of the necessary biochemical stimulation for buds to swell and open.

Existing trees and bushes have limited climatic flexibility. Fortunately, it is possible to plant trees that are already sufficiently adapted to warm climates. Plant scientists have identified “low-chill” cultivars for many common orchard crops. These strains can respond adequately to warmer winters and shorter springs.

A question still unsettled is whether slow changes in climate have any structural effect on long-established trees. A recent National Public Radio report quoting U.S. Forest Service Forester Paul Hennon stated some foresters have extrapolated from the warming-related death of cedars in Alaska to assume global warming will cause massive death of American forest trees in the future. There is no concrete evidence to support this. It is known, however, that subtropical weeds and fungal diseases are being found at more northerly locations, as are some crop pests.

A warming climate means a wide variety of plants could be grown in the future that could not have been grown before at that location. Figs are now raised at a few Illinois locations – a feat that would not have been successful a century ago.

A Cornell University study recently showed that soybeans are more profitable crop in upstate New York than in the past. Peaches and nectarines may thrive in Wisconsin and northern Michigan the future.

And 115-day corn is now being cultivated in parts of Ohio where old agricultural manuals claim only 95-day varieties should be planted.

The zone map system is often misunderstood. Farmers and gardeners frequently interpret numbers to be an indication of how cold it can get in a particular place.

But Dr. Gordon Carriker, agricultural business specialist with the University of Missouri extension, explained that is not the case. “It’s based on records, but it’s averages over (the) years,” he said, “not (record cold).”

The zone map is based on the mean annual low temperature in a 30-year period.

If the coldest a particular town gets in an average winter is -12 degrees F., for instance, that town falls in Zone 5 or, more accurately, Zone 5b. Many growers misunderstand the phrase “based on records,” and think their zone is determined by the record coldest temperature seen there during a given period.

Therefore, zones must not be construed as a guarantee a certain cold temperature will not be exceeded. For example, parts of Kentucky and Missouri listed as Zone 7 saw temperatures below -10 degrees F. during the blizzard of February 2011; they experienced Zone 5 temperatures.

Most trees and plants will not be killed by short jaunts into a zone of extra cold, but buds may die and other plant stresses may result.

Although the original USDA Zone Map will soon be replaced in official USDA use by the new one, it is likely the old map will continue to be printed by many private nursery and plant-catalog publishers, partly for that reason.

The old map may serve as an approximate guide to what temperatures are possible or likely in a given place – the purpose for which it was incorrectly used all along. For more accuracy, however, 50- and 100-year coldest temperature records are usually available from regional offices of the National Weather Service.

The USDA has not yet published the map in poster or wall format, but to identify zones, call the Agricultural Research Service at 202-245-2726 or visit http://planthardiness.ars.usda. gov/PHZMWeb