Gardeners, Check Your Hardiness Zone: It’s Warmer

Grist, an environmental blog with the tagline “A beacon in the smog,” ran this headline recently: ”If you’re 27 or younger, you’ve never experienced a colder-than-average month.”

While technically impossible—people feel temperature in situ, not globally, which is the scope of the cited data—the basic point is bulletproof: Global warming is the prevailing trend, has been for some years, and is not about to let up. Even our science-averse president spoke of it in his victory speech and again at his post-election Q&A.

Some politicians have been impervious to the argument. Senator James Inhofe said it was “the second-largest hoax ever played on the American people.”  (The first, he said? The separation of church and state.) 

Plants don’t have the luxury of opinions or political beliefs, the only culture they claim is hortus, and they don’t do what Jim Inhofe or the Heartland Institute say. Their loyalties are ancient and unchanging. For them, climate is not an ideological issue: it is destiny. They follow the sun and sip the rain, respond to the soil’s warmth and shrink from frost. They are finely tuned organisms that thrive under certain conditions only.

So it stands to reason that if plants are making adjustments, we should take note. 

Lately, as alert gardeners have begun to suspect, horticulture in late autumn around here will soon be wearing sunglasses and a swim suit.

Since homo sapiens first stuck a seed in the ground, gardening has been an act of faith. To improve a farmer’s odds in crop selection and timing, the U.S. Department of Agriculture in 1960 established a national map of hardiness zones based on mean temperature at thousands of weather stations. Gardeners have been ancillary beneficiaries. This designation is to be trifled with at a planter’s peril. Imposing a milder zone on a plant—a smattering of lovely Zone 9 Coleus or Agapanthus in Zone 5, say—will only cheer some Georgia nursery as a chill morning in a Smithfield garden reveals withered voids.

I’ve kept track of the first bloom of a dozen plants around my Millerton house since the late 1990s, and I estimate spring now begins nearly two weeks earlier.  This doesn’t seem to accord with Zone 5.

A paper published last May adds science to my backyard observations. Written by Nir K. Krakauer of the City College of New York and funded by the National Oceanic and Atmospheric Administration*, it points out that the latest Hardiness Zone map, updated last January for the first time since 1990, is already as much as 3.6 degrees F. out of whack in places and off an average of 2.2 degrees F. overall.

The problem, Krakauer discovered, is that the USDA designates hardiness zones by determining the mean of temperatures around the country over a lengthy period, most recently from 1976 to 2005. But close examination reveals a warming trend underneath that the period’s mean temperature disguises. In fact, recent annual minimum temperature in the U.S. is increasing 2.5 times faster than the long-term mean. All indications are that this trend will continue.

Suffice to say that the math behind Krakauer’s calculations is a deft assemblage of weightings and estimates, statistics used with great precision. For us fumbleheads, he offers two simple tools. The first repositions an area within the correct Hardiness Zone, based on major weather stations. The station closest to Millbrook in his calculator is Albany. Dialed in, it shows that the area in 2012 has shifted from Zone 5a to 6b, a warm-up of 5.2 degrees F. (plus or minus 2.0 degrees). Find it here:

The second tool, once you enter your latitude and longitude (another tool provides it), estimates how much warmer your location actually is from the USDA’s published Hardiness Zone mean. Mine turns out to be 2.4 degrees Fahrenheit. The uncertainty is plus or minus 1.1 degree F. The URL:

The temperature discrepancy in my case between the area figure represented by Albany (+5.2) and my location’s figure south of Millerton (+2.4) urges caution—I’ll use the conservative figure. But the higher number will surely be achieved and probably surpassed. Be patient, Coleus.

I asked Dr. Krakauer if the USDA had been in touch with him about updating their Hardiness Zones. “No,” he said quietly, “but they might interested.” Especially since a quick check among Northeast stations shows that the majority are mapped in cooler Zones than they should be. I guess one shouldn’t be surprised that the climate is fleeter of foot than the USDA. 

Is it coincidence that the Zones were adjusted last January, a month before James Inhofe published his book The Greatest Hoax: How the Global Warming Conspiracy Threatens Your Future? If plants judged the chance that they’d flourish by their current Hardiness Zone, they’d be as confused as a good science student at a James Inhofe talk.

So, as you do your garden planning this winter, pouring over seed, plant, and nursery catalogs, adjust yourself southward a good half a Hardiness Zone—despite what you may hear from the Administration or Congress. Your garden has. 

* Nir K. Krakauer, “Estimating Climate Trends: Application to United States Plant Hardiness Zones,” Advances in Meteorology 2012