A few months ago, I wrote about the USDA plant hardiness zone map. This map was last updated in 1990. The new map is expected to reveal that warmer planting zones have moved northward. This could have an effect on the 82 million households that garden in the United States (National Gardening Association). It could also influence what plants and crops will grow where.
"Familiar to anyone who has paged through a nursery catalogue, the USDA hardiness map divides North America into 11 latitudinal zones, each representing a 10 degree Fahrenheit range of 'average annual minimum temperature' " the coldest lows that can be expected in that area. Zones 2 through 10 are each subdivided into two sections " 'a' and 'b' " that represent 5 degree Fahrenheit ranges." (J. Weeks for dailyclimate.org)
The USDA zones are updated every 10 years based on temperature, precipitation, heating and cooling degree day data gathered by U.S. Commerce Department National Climate Data Center, which collects information from thousands of U.S. weather stations. In 2003, when the center examined the differences between average minimum winter temperatures in the U.S. from 1961 to 1990 to those from 1971 to 2000, several areas in the continental U.S. showed warmer winter lows. In parts of central and southern California, the Rocky Mountains and the northern Great Plains, temperatures rose as much as 2.5 degrees. Some areas had not warmed at all. It appears that zones are shifting north.
The new map will be based on factors that influence temperatures such as altitude and the presence of water bodies, and will incorporate new geographic information system data. These variables were not part of the data set in previous maps. The scientists at Oregon State updating the map will be able to better define the edges of the zones and identify areas with heat island effects, which can alter zone information in specific locations.
Updated maps will provide gardeners with more accurate information about appropriate plant choices for their general area. These are guidelines only. Know about the plants you select for your yard. What are their temperature, light and water requirements? How hardy are they? There are many microclimates in Nevada; pay attention to the hot and cold spots in your yard and note the weather to garden successfully.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension. She may be reached at (775) 887-2252 or skellyj@unce.unr.edu
