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The USDA Plant Hardiness Zones are a standard that most gardeners consider when selecting plants suitable for their local climate.  These maps show us the mean annual low temperature for a given location, which helps distinguish the relative cold hardiness of plants, insects and other biota.

 Given the recent publication of the Fourth National Climate Assessment and the ominous conclusion that climate change is happening at an even faster rate than previously predicated, I have to wonder how it will impact plant hardiness zones.  How have these zones changed in the past? How will they change in the future?  What do gardeners need to consider when selecting plants now and in the future?  Over the next two editions of this column I will be exploring the answers to these questions to help all of us as gardeners draw some meaningful conclusions.

The original USDA Plant Hardiness Zone maps were published in 1960 using historical climate data from 450 weather stations.  However, similar maps depicting cold harness zones were release prior to the 1960 USDA maps, such as Alfred Rehder’s eight zone map in his 1927 “Manual of Cultivated Trees and Shrubs”.  Rehder and other’s early maps lacked a uniform temperature interval across all the zones, instead specifying zone intervals by what may make more sense geographically rather than by even delineations of the average low temperature.  This inconsistency spurred the USDA Agricultural Reach Service (ARS) to develop its own cold hardiness map in 1960 with ten zones, numbered 1 through 10 and defined by a uniform temperature interval.  Each Zone, encompasses a 10⁰F range of mean annual minimum temperature, which has been the standard applied to cold hardiness zones to this day.

In 1990, the USDA-ARS updated its Plant Hardiness Zone map using data from approximately 8,000 weather stations over a time period from 1974 to 1986.  In this release, USDA-ARS introduced the concept of 5⁰F half zones, indicated by a letter designation of “a” or “b” immediately after the zone number.  Zones 2-10 carried this half-zone designation.  A new zone (Zone 11) for the US was delineated in this addition of the map which is essentially a frost-free zone with mean annual minimum temperatures of only 40⁰F. 

Although Zone 11 does not have freezing temperatures, it still has horticultural significance since some plants do experience cold injury at temperatures only as low as about 50⁰F.   The 1990 update did shift the zones a bit with the newer dataset resulting in about a half zone (5⁰F) decrease in most areas, meaning they became slightly colder based on the approximately 13 year data set used to produce the map.

A more recent update was completed by USDA in 2012.   This update to the map features a similarly robust dataset as the 1990 update, using nearly 8,000 weather stations spanning 1976-2005.  With this update, the cold hardiness zones did shift again trending the opposite direction as the shift with the 1990 map.  The 2012 map showed that most areas were about one half zone warmer than 1990.  For the central and eastern US, this shift was primarily attributed to a warmer average low temperature, based on the dataset.   For the western US, the shift was attributed to more sophisticated data analysis techniques, greater spatial detail (meaning more stations, or data points, per unit area) and an overall, more comprehensive data set. 

The take home message to me is that we really haven’t experience a significant shift in cold hardiness zones (with respect to historical datasets of average low temperatures) in recent iterations of the USDA Plant Hardiness Zone Map since 1960.  Although a half zone shift by itself is significant (5⁰F), we have both lost and gained that amount in past map updates.  However, recent national climate assessments in 2014 and 2018 do predict a significant change in climate that will impact the map as average annual low temperatures gradually rise.  In addition, other aspects of the Illinois climate will change, along with temperature that will have an impact on gardening.

Join me next week as I discuss how gardeners should use and interpret USDA Plant Hardiness Zone Maps and how climate change in Illinois is predicted in recent national climate assessments.  I will have some interview comments from Jim Angel, State Climatologist with the Illinois State Water Survey and lead author of the Midwest Chapter of the Fourth National Climate Assessment, as he provides some insight into what the assessment means to gardeners in Illinois.