American Horticultural Society Draft Changes to the 1990 USDA Hardiness Zone Map

In 2002 USDA contracted with the American Horticultural Society (AHS) to revise the 1990 Hardiness Zone Map and "better reflect minor regional variances in temperature that have occurred in the last decade" AHS. The AHS, under the direction of Dr. H. Marc Cathey, who lead the development of the 1990 USDA Map, issued a draft of the update in the May/June 2003 issue of "The American Gardener".

The AHS 2003 Draft differs in several ways from the 1990 version. Mexico and Canada are no longer represented. The number of zones has expanded from 11 to 15 to address ideal growing climates for sub-tropical and tropical plants. The 5 degree a/b zones were dropped in order to make the map easier to read. The AHS version is based on 16 years of data (1986 to 2002), while 1990 Map was based on 13 years of data (1974 to 1986).

The most striking aspect of the AHS Draft is that many of the hardiness zones have moved northward reflecting a general warming trend during the winters over the 16 year measurement period. 40% of the 4,600 weather stations used in developing the map showed a higher zone number compared to the 1990 USDA version. For example, St. Petersburg, Florida has moved from Zone 9 to Zone 10. In the maps for Florida below notice how Zone 9 has migrated northward into Georgia and away from the central coast of Florida. The two small spots within Zone 9 on the AHS Draft represent the effects of urban warming (Orlando and Lakeland).

USDA 1990 Hardiness Zone Map
AHS 2003 Draft Hardiness Zone Map

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. Regardless of whether the implication is true or not, the revision process has dragged on far too long. Additionall, several horticulturists and nurserymen were also concerned that the AHS Map might cause confusion for gardeners when compared to the USDA version and lead many to make incorrect decisions about plants suitable for their area.

The one positive result from the USDA's rejection of the AHS Draft is that it provided an opportunity for alternative approaches to fill the void. Click here for a discussion of some of these alternative approaches.

The following links deal with global warming and the AHS Draft:

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