Magazine for Northwest Arkansas

Have USDA Plant Hardiness Zone Maps Fallen Victim to Climate Change?

With all of the controversy surrounding what used to be called global warming, and still is in some particularly invested circles, have you ever wondered what effect this phenomenon - whether real, imagined or contrived - may have had on the Plant Hardiness Zone Maps produced by the U.S. Department of Agriculture? You know the ones. When you go to the garden center to pick out plants there is usually a tag on them that shows the plants'  respective hardiness zones. The zones reflect the average yearly lowest temperature that one might expect in a given geographical area and a particular plant's suitability for it. We thought we would take a look and see if the political football that is climate change (that is what it is called these days) has had any effect on the zone maps for Northwest Arkansas.

Evolution of The USDA Zone Map

The USDA produced its first Plant Zone Hardiness map in 1960, with a revision released in 1965 (Fig. 1). Unfortunately, we are not exactly certain what years' data were used to produce either the 1960, nor the 1965 revised maps. The map was divided into zones that were in ten-degree increments (Fahrenheit), the higher the zone number, the higher the average lowest yearly temperature. On this map, all of Northwest Arkansas was in Zone 7, with an average yearly minimum temperature of between 0 and 10 degrees F.

Fig. 1.  1965 revision of the 1960 USDA Plant Hardiness Zone Map

The 1990 map (Fig. 2)) was more accurate than the 1960/1965 versions for a couple of reasons: There were a great many more reporting stations providing more detailed data and the zones were split into "a" and "b" sub-zones. These sub-zones reflected average minimum yearly temperatures in five-degree increments, providing for even more local accuracy. Unlike the 1960 map, we do know the time-period used for the making of the 1990 map, which was 1974-1986. Northwest Arkansas was split into two subgroups, with the northern part of Benton county being in Zone 6a (-5 to 0F),  and the southern part of Benton County, as well as all of Washington County, being in Zone 6b (0-5F). Interestingly, Northwest Arkansas had moved an entire zone colder than on the 1960/1965 maps.

Fig. 2  1990 USDA Plant Hardiness Zone Map.

Next, enter the 2012 USDA Plant Hardiness Zone Map (Fig. 3). This version used data from a much longer period, covering the years 1976 to 2005. That raises a couple of questions: For one thing, why the much longer sampling period? Also - and a little more curious - is why the 2012 map sampling period stopped seven years before the map was produced? Clearly, by 2012, the global warming controversy had been raging for years. That was not the case with the earlier maps. Since it has been demonstrated (but seldom reported) that there has been no global temperature increase in almost two decades, was the older data included and newer data (2006-2012) excluded to support the global warming position? As the USDA is a department of the federal executive, which presently subscribes to global warming/climate change as undeniable fact, these two things certainly merit a closer look.


Fig. 3  2012 USDA Plant Hardiness Zone Map

To the USDA's credit, the department passed on the opportunity to validate the need for the 2012 map, by blaming it on global warming. They explain the 2012 map quite well on their website, with no mention of global warming or climate change, whatsoever. You can link to the USDA page HERE and see for yourself. There have been, however, plenty of voices chiming in on the subject from the global warming camp, most notably American Horticultural Society (AHS) and National Arbor Day Foundation, both of whom produced their own zone maps. AHS produced a zone map in 2003, using data from July, 1986 to March, 2002. This was a period of warmer winters than the 1974-1986 period used by the USDA for their 1990 map. In some cases, the AHS raised areas a full zone from the 1990 map. That was a whopping 10 degrees F. In 2006, National Arbor Day Foundation followed suit with its own zone map. The result were nearly identical to the 2003 AHS map. Gardeners, horticulturists and the USDA, itself, dismissed both maps as coarse and inaccurate.

By the way, Northwest Arkansas is still in Zone 6a and 6b on the 2012 map. In fact, with a closer look at the map for Arkansas, specifically, which is shown in enhanced detail, below (Fig. 4), you can see that part of Benton and the northern part of Washington County, including Fayetteville, is in Zone 6b, which is represents an average yearly minimum of between -5 and 0 degrees F.

Fig. 4  Arkansas detail of 2012 USDA Zone Map

What About the Last Eight Years?

This is not exactly scientific, but Boston & Ozarks had a look at the yearly minimums at Drake Field in Fayetteville for each year between 2006 and 2013. The average was 3.25 degrees Fahrenheit. That is where it has been since the first map in 1960.

In reality, the zone maps are such a broad generalization of things, that their real usefulness is somewhat limited. The seasoned NWA gardener knows from experience that one should expect temperatures here to drop into the -5 to 0 degree F. range with some regularity. If you select plants that are good to Zone 6b you will be okay - most of the time. As those of us that have been here for a while know, there are occasional rather extreme drops in temperature, like on February 12, 2011. The low at Drake Field that morning was 17 degrees BELOW ZERO! Botanical Garden of the Ozarks, also in Fayetteville, reported -22F, that morning. Friends, that is cold.

The saving grace in that situation was that there was two feet of snow on the ground from the previous day. Snow cover actually acts as an insulator. While we were freezing, most of the plants were covered in a lifesaving blanket of snow. It should be noted, however, that in our part of the country, one cannot count on snow cover like folks can in other areas of the country. When it gets terribly cold here, it will likely be without the insulating benefit of snow.

Another thing to keep in mind is that in Northwest Arkansas the temperature just doesn't remain at its extreme low for long. The skies are necessarily clear for temperatures to sink that low. That means that the sun will be out (when it comes up) and temperatures will begin to rise, quickly. That is good news for plants. Really, the biggest temperature problems we experience here are related to very late-season freezes. When we get a hard freeze after trees and shrubs have begun to bud, we get a bunch of damaged or dead trees and shrubs! We saw that play out in April, 2007. Unfortunately, like the rare low far below our zone's norm, the late-season freeze is impossible to avoid. They have and will continue to occur.

Summing up, it will drop to single digits in Northwest Arkansas every year. Also, it will drop below zero by at least a few degrees every few years. Zone 6b is most appropriate for our area.

Copyright 2014, James Kevin Connell

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