The USDA 2012 Hardiness Zone Map and Some of Its Predecessors

The initial hardiness zone maps for the United States were developed during the 1920's and 30's. The first USDA Map was published in 1960 (USDA Miscellaneous Publication No. 814 "Plant Hardiness Zone Map.") and was developed by Henry Skinner while he was the Director of the U.S. National Arboretum. The map showed ten broad hardiness zones based on 10 degree F. gradients.

1960 USDA Hardiness Zone Map

The 1960 map was updated in 1965 to add temperature data that had been missing from the original. Click here for a close up of the Florida Zones.

1965 USDA Hardiness Zone Map

During this period the Arnold Arboretum also published a Hardiness Zone Map. Donald Wyman of the Arnold Arboretum (AA) published the first version in 1938 (the map to the right was published by AA circa 1969). There are subtle differences between the AA and USDA maps (including the use of different temperature ranges), which lead to some confusion for gardeners. There are two excellent articles from the Arnold Arboretum that outline the historical development of the maps and include several images:

Arnold Arboretum Hardiness Zone Map

Here is a 1978 update to the AA Map. By 1990 the AA version had fallen into disuse and the USDA version became the primary source for zone identification.

Arnold Arboretum Hardiness Zone Map

In 1990 a major overhaul of the USDA map was completed by H. Marc Cathey (USDA Miscellaneous Publication No. 1475) using temperature data from 1974 to 1986. One new zone was added to coincide with adding Mexico and Canada to the map. In addition, the prior 10 degree gradients were broken down into 5 degree a and b zones.

One of the primary reasons given for the update was that, "We have been losing from our landscapes plants that apparently survived the 1940's to the 1960's.  Many of the hardiness zone classifications of plants are no longer considered valid.  In North America, the ranges of temperature and moisture for the past decade were wider than those recorded for the 1940's through the 1960's." This is an intriguing statement, since it presages the current debate over the proposed update to the 1990 USDA map and the implication that the hardiness zones need to reflect recent global warming.

In 2002 the USDA initiated a project with the American Horticultural Society to update the 1990 map. A year later the AHS released a draft of the update which showed that many of the hardiness zones had moved northward reflecting a general warming trend. The USDA, after a brief review, decided to reject the draft and gave little justification for its decision. It's terse dismissal raised the possibility that it was a political decision - the USDA was concerned that the AHS Draft gave support to global warming proponents and decided to suppress the project to avoid controversy and not embarrass the Bush Administration. Ten years later the USDA released the 2012 Hardiness Zone Map which added two new climate zones and showed a general warming trend across the country. The USDA, however, noted that, "Changes in zones are not reliable evidence of whether there has been global warming."

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