CORVALLIS, Ore. - The most sophisticated "plant hardiness zone map" ever created in the United States was unveiled Wednesday (Jan. 25) in Washington, D.C., by the U.S. Department of Agriculture, providing a new tool for the estimated 80 million gardeners in the country - and those who grow and breed plants for them.

The map was created by researchers at Oregon State University using, for the first time, geographic information system (GIS)-based software that has resulted in an interactive map that is more accurate, detailed and useable than any previous model.

The map can be viewed online at

Plant hardiness zones help provide common language for gardeners, landscapers and others to communicate where plants are best adapted for growth.

Details about the new map were announced at the U.S. National Arboretum in Washington, D.C., by Catherine Woteki, U.S. Department of Agriculture Under Secretary for Research, Education and Economics.

"Low temperature during the winter is a crucial factor in determining where particular plants will survive," said Chris Daly, a professor of engineering at OSU. "This new map updates and reflects those conditions based on temperatures from 1976 to 2005."

The temperatures don't reflect the coldest it ever has been in the area, Daly noted. Rather it is based on the average lowest winter temperature for a given geographic area during that time period.

The OSU PRISM Climate Group created the map, using technology developed by Daly. The map was produced from a digital computer grid, with each cell representing about a half-mile on a side. The PRISM software estimates the mean annual extreme minimum temperature for each cell by examining data from nearby weather stations, determining how the temperature changed with elevation, and accounting for local effects. These effects can be coastal influence, temperature inversions and the type of topography - ridges, hill slopes or valley bottoms.

Data from nearly 8,000 weather stations were incorporated into the map. Drafts of the map were reviewed by climatologists, agricultural meteorologists and horticultural experts and anything anomalous was investigated.

"Initially, zones along the Canadian border in the Northern Plains appeared too warm to several members of the review team who are experts in this region," Daly said. "We found that there are very few weather reporting stations along the border in the area. So we added data from Canadian reporting stations and the zones in that region are more accurately presented."

In addition to gardeners, plant breeders and horticulturalists, the new zone hardiness map will be used by a variety of state and federal agencies, including the USDA Risk Management Agency, which sets some crop insurance standards.

Scientists also use the data in research models analyzing the spread of exotic weeds and insects.

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Chris Daly, 541-737-2531