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Title: Development of a new USDA Plant Hardiness Zone Map for the United States

Author
item DALY, CHRISTOPHER - Oregon State University
item Widrlechner, Mark
item HALBLEIB, MICHAEL - Oregon State University
item SMITH, JOSEPH - Oregon State University
item GIBSON, WAYNE - Oregon State University

Submitted to: Journal of Applied Meteorology and Climatology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2010
Publication Date: 2/8/2012
Citation: Daly, C., Widrlechner, M.P., Halbleib, M.D., Smith, J.I. 2012. Development of a new USDA Plant Hardiness Zone Map for the United States. Journal of Applied Meteorology and Climatology. 51:242-264. Available: http://dx.doi.org/10.1175/2010JAMC2536.1

Interpretive Summary: In many regions of the world, extreme low temperatures in winter limit the geographic ranges of perennial plants and also limit their successful cultivation. In the U.S., the USDA Plant Hardiness Zone Map (PHZM) is the primary reference that gardeners, nurseries, horticultural producers, climatologists, and plant scientists use to define geographic patterns of extreme winter cold. This paper describes the approach we followed to produce a new version of the USDA PHZM, which was last updated in 1990. The new map is based on 1976-2005 mean annual extreme minimum temperature (the lowest temperature recorded in a year), displayed in 5°F half-zones, for the US and Puerto Rico. Low-temperature data from nearly 8000 weather stations were interpolated to a grid with the PRISM climate-mapping system, which accounts for the effects of elevation, air-mass blockage by mountains, coastal effects, temperature inversions, and cold-air pooling in valleys. A map showing year-to-year variability in low temperatures and two 15-year maps for 1976-1990 and 1991-2005 were used to evaluate geographic variation, which was substantial in much of the western U.S. The new PHZM is generally one half-zone (5°F) warmer than the 1990 map throughout much of the U.S., as a result of differences in averaging periods. The new map was able to more accurately represent zone patterns, especially in the western U.S., by using sophisticated interpolation methods, better topographic detail, and a more comprehensive set of weather stations. The new PHZM will be a valuable tool for gardeners, nurseries, and horticultural producers for matching the suitability of plants to locations and for plant scientists to understand the role that low temperatures plays in plant adaptation.

Technical Abstract: In many regions of the world, the extremes of winter cold are a major determinant of the geographic distribution of perennial plant species and of their successful cultivation. In the United States, the USDA Plant Hardiness Zone Map (PHZM) is the primary reference for defining geospatial patterns of extreme winter cold for the horticulture and nursery industries, home gardeners, agro-meteorologists, and plant scientists. This paper describes the approaches followed for updating the USDA PHZM, the last version of which was published in 1990. The new PHZM depicts 1976-2005 mean annual extreme minimum temperature, in 2.8°C (5°F) half-zones, for the conterminous U.S., Alaska, Hawaii, and Puerto Rico. Station data were interpolated to a grid with the PRISM climate-mapping system. PRISM accounts for the effects of elevation, terrain-induced air-mass blockage, coastal effects, temperature inversions, and cold-air pooling on extreme minimum temperature patterns. Climatologically-aided interpolation was applied, based on the 1971-2000 mean minimum temperature of the coldest month as the predictor grid. Evaluation of a standard-deviation map and two 15-year maps (1976-1990 and 1991-2005 averaging periods) revealed substantial vertical and horizontal gradients in trend and variability, especially in complex terrain. The new PHZM is generally one 2.8°C (5°F) half-zone warmer than the previous PHZM throughout much of the U.S., as a result of differences in averaging periods. However, a more sophisticated interpolation technique, greater physiographic detail, and more comprehensive station data were the main causes of zonal changes in complex terrain, especially in the western U.S.