Green Mountain Outlook

Middlebury — There has been a clear trend of temperature-change in Vermont since the 1970s, according to the U.S. Department of Agriculture.

The USDA’s new 2012 Plant Hardiness Zone Map, unveiled this week, reflects those changes in Vermont and beyond. It also provides a snapshot of the area climate over a longer period of time.

According to the new hardiness zone map, the Champlain Valley area around Middlebury, as well as the area around Rutland, appear as Zone 5a. An earlier edition of the map showed the Middlebury area within zone 5b, a colder zone.

The new map is the first revised USDA zone map in 22 years.

Dr. Peter Bretting of the USDA in Washington, D.C., said the new map incorporates detailed data sets from as recently as 2005, he said it is a better guide for helping Vermont farmers and gardeners plant commercial and ornamental species of plants.

New Market Press: Is the USDA data for the northeastern United States showing a warming trend?

Bretting: We see a consistent shift. Especially in the eastern U.S. of about a half zone warmer than in the prior map.

New Market Press: What is behind this change?

Bretting: It’s based on more data from more weather recording stations. And they were processed by a special mathematical formula that takes into account features of the landscape, such as mountains, large bodies of water, urban areas, which cause sometimes heat islands. And it was recorded over a longer period, 30 rather than 15 years.

New Market Press: Why is the new zone map such big deal for Vermonters and others?

Bretting: Well, there are more than 80 million gardeners in the United States and many of them will use this as a reference as will landscape architects, plant breeders. The USDA Risk Management Agency will use these data for certain crop-insurance parameters. One application is being able to forecast the spread of weeds or insects that might endanger agriculture.

New Market Press: Has the accuracy of this kind of climate data changed in 22 years when the last edition of the map was published?

Bretting: The precision on the map now is about a half-mile square, which is because of the new computerized technology; it is more precise than prior maps, which were based on sometimes “artistic” renditions of where zones began or ended.