What are Plant Hardiness Zones And Are They Really Creeping Northward? (Credit: USDA, The New York Times)

2010 USDA Plant Hardiness Zones/ Credit: USDA

2040 NOAA Plant Hardiness Predictions

It was during the Great Depression that two researchers began creating maps to educate farmers on what crops could grow in their respective regions of the U.S. One researcher, Henry Skinner of the U.S. National Arboretum, began tailoring his map by separating the country into zones that were separated into increments of ten degrees Fahrenheit. Researchers Donald Wyman of the Arnold Arboretum began creating another map during the same time period that would split the U.S. into eight zones that could have temperature differences of either five, ten, or fifteen degrees.

It wasn’t until the 1960’s, however, that most agriculturalists adopted the U.S. National Arboretum map as the standard for gardening, despite the release of Wyman’s map in 1938. Wyman’s map was hard to follow since the zones were broad. This can cause trouble for gardeners in regions that may have diverse microclimates, something even the United State Department of Agriculture (USDA) acknowledges on their website. These microclimates can come in the form of urban heat islands, such as large cities like New York City and Los Angeles, where the overall average temperature may undergo less drastic diurnal temperature variations in comparison to surrounding areas. Areas like these will absorb more heat due to lower albedos and increased absorptivity of surfaces such as blacktop and concrete. This can cause for a warmer plant hardiness zone than areas just miles outside the typical city center, where factors such as more grassy surfaces and differences in elevation may cause a decrease in the overall average temperature, causing for a cooler plant hardiness zone.

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It is once all of the aforementioned factors such as the characteristics of microclimates are taken into account that one can think about how those exact climates may be affected by an increase in temperature, as caused by climate change. A recent analysis by the National Oceanic and Atmospheric Administration, as reported in the New York Times, brought to light how increasing temperatures due to climate change already are and may continue to have an impact on the movement of plant hardiness zones northward.

The zones are currently based on the coldest temperature of the year at various locations across the U.S., averaged over a 30 year period. The USDA’s map was revised in 2012 after over twenty years without revisions. Although the perimeters of all plant hardiness zones did creep noticeably northward, it was noted in the Times article that it was impossible to distinguish the influence of climate change on this northward progression from other systematic differences given that the USDA changed their map-making process s between the 1990’s and 20212. In other words, the USDA map is broad and fails to take into account climate change, making it impossible to make future predictions based off of just plant hardiness.

This is why the researchers at NOAA created the maps using the thirty-year technique practiced by the USDA. These maps take into account the effects of greenhouse gas emissions on the climate that the USDA’s revised maps didn’t. The maps clearly show how by 2040 certain zones designated by NOAA will have creeped much farther northward, with areas like the New York City metro area being considered as Zone 8, the same hardiness as areas as far south as Montgomery, Alabama and central Georgia. 

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©2019 Weather Forecaster Jacob Dolinger

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