The Purdue Plant and Pest Diagnostic Laboratory

What's Hot at the P&PDL
January 26, 2012

New USDA Plant Hardiness Zone Map

Bruce Bordelon, Professor, and Rosie Lerner, Extension Consumer Horticulturist, Department of Horticulture and Landscape Architecture, Purdue University

The USDA recently released a new version of its Plant Hardiness Zone Map (PHZM), updating a tool used by gardeners and researchers to determine appropriate plant species and cultivars for various regions of the country. This is the first update for the map since it was originally published in 1990. According to the ARS News Service press release of January 25, 2012 (http://www.ars.usda.gov/is/pr) the new map was jointly developed by the USDA’s Agricultural Research Service and Oregon State University’s PRISM Climate Group.

There are several changes in the new version of the map.

  • The new map offers a Geographic Information System (GIS)-based interactive format specifically designed to be used over the internet.

  • The map website incorporates a "find your zone by ZIP code" function.

  • The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees Fahrenheit) and 13 (60-70 degrees Fahrenheit) (This only applies to Hawaii and Puerto Rico.)

  • The new map uses data from weather stations during the 30-year period 1976-2005. In contrast, the 1990 map was based on temperature data from only a 13-year period of 1974-1986.

  • Zone boundaries in the new map are generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States.

The slight shift in zone boundaries and improved accuracy of the map is mostly a result of using temperature data from a longer and more recent time period and from more weather stations. However, some of the changes are a result of new, more sophisticated methods for mapping zones between weather stations. These include algorithms that considered for the first time such factors as changes in elevation, nearness to large bodies of water, and position on the terrain, such as valley bottoms and ridge tops. These advances greatly improved the accuracy and detail of the map.

Plant hardiness zone designations represent the average annual minimum temperatures at a given location during a particular time period. They do not reflect the coldest it has ever been or ever will be at a specific location. Low temperature during the winter is a crucial factor in the survival of plants at specific locations. However, there are often many other factors that contribute to plant survival. For instance, in fruit crops we know that bud damage is dependent on the timing of the cold event (early, mid or late winter), rate of temperature change, and conditions immediately preceding the cold event. So while the Plant Hardiness Zone Map is useful, it is not a guarantee that a particular plant will survive our weather.

Significance for gardeners and homeowners is that this really doesn't change too much for practical gardening, other than when making choices for new plantings. All of Indiana is still in zones 5 or 6, as was the case in the 1990 Map. Books and other resources generally list plants to whole hardiness zones, not half, so for the few folks that were formerly zone 5b that will now be a 6, they might have the impression that this opens up a bunch more planting possibilities.  But in reality, plants that were marginally hardy in your area before are still just marginally hardy and will require a protected location to perform well.  Also keep in mind that this is all based on annual average low temp.  There will be years when the low temp is considerably lower than the average and typically it is these marginally hardy plants that suffer the most damage.

While about 80 million American gardeners are the largest users of the USDA Plant Hardiness Zone Map, many others need this hardiness zone information. For example, the USDA Risk Management Agency uses the USDA plant hardiness zone designations to set some crop insurance standards. Scientists use the plant hardiness zones as a data layer in many research models such as modeling the spread of exotic weeds and insects.
The new map is available online at www.planthardiness.ars.usda.gov

 

Click image to enlarge

Picture 1 caption: New USDA Plant Hardiness Zone Map for the lower 48 states

Picture 2 caption: New USDA Plant Hardiness Zone Map for Indiana

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