History of the USDA Plant Hardiness Zone Map

farmers_great_depressionAs gardeners prepare for the upcoming gardening season, many will be viewing incoming seed catalogues along side their morning cup of coffee.  Throughout the catalogue plant hardiness zones will be mentioned but do you really know the history behind these maps?  This tale is full of intrigue, competition, and human nature.

The story begins among one of the darkest moments in United States history and that is the Great Depression.  Two individuals working for two different agencies were honing in on an idea and that was a temperature map that could guide farmers in their plantings.  Before this idea was conceived, plantings were guided by family traditions and The Farmers Almanac.

So during the 1930’s The U.S. National Arboretum was working on a map that would divide the United States into zones that were separated by 10 degrees Fahrenheit.  At the same time, Arnold Arboretum was developing its own map.  This map was divided into eight zones that could have a temperature difference of 5, 10, or 15 degrees.

In 1938, Donald Wyman of the Arnold Arboretum released the AA Plant Hardiness Zone Map but because the zone temperatures were so broad, it was hard for gardeners to use.  In doing so, the U.S. National Arboretum map was published in 1960 by Henry Skinner, who at the time was the director of this arboretum.  Immediately this map there was a conflict between the two maps.  Gardeners began to migrate to the USDA Plant Hardiness Zone Map for information but there still linger those who used the AA map.

In 1965, the USDA Plant Hardiness Zone Map was updated to include data from Florida.

Then, in 1978, the AA map was updated but by the 1990’s it fell out of vogue and the USDA Plant Hardiness Zone Map was the primary source for planting information.

In 1990, the USDA Plant Hardiness Zone Map was updated.  This update was due to the realization that many plants that grew wild in certain regions were no longer there but instead had migrated to other areas.  In other words the climate had changed.  H. Marc Cathey was in charge of this update.  The new map included Mexico and Canada.  It kept the 10 degree gradients but broke down these gradients into 5 degree increments that were labeled “a and b.”

In 2002, the map was going to be updated by the American Horticultural Society. Through temperature records kept at weather stations across the United States, it was discovered that hardiness zones were moving northward.  But political pressure prevented the new map from being published.  It was believed that this new map would give credence to the idea of global warming.

In 2012, a new map was published.  This map added two new climate zones but refused to say that this change was due to global warming.

Mindy McIntosh-Shetter has been an Agricultural Science educator, and is a horticulture and/or environmental blogger who earned a degree from Purdue University in Agriculture Education with a minor in biology, and natural resources. Presently she is finishing up her Masters in Environmental Education and Urban Planning for the University of Louisville while working on her own agriculture/environmental blog.

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