Quick – what hardiness zone are you in?
If you happen to be a farmer or gardener, you’ll know the answer right off the bat, and it will probably be “four” if you live in New Hampshire or Vermont.
Then again, maybe it won’t. Have you checked the map recently?
We’re talking about the “Plant Hardiness Zone Map,” which was first created by the United States Department of Agriculture (USDA) in 1960. The idea was to allow farmers and gardeners to get a sense of which plants were best suited to their location and to know, when shopping from a catalog, which varieties were likely to survive a northern New England winter.
The 1960 zone map was created by averaging the coldest winter temperature of each year from the previous 15 years. If your coldest winter low temperatures ranged from 12°F below zero to 25°F below zero, with an average of, say, 17 below, your location would be placed in Zone 5, which is the band from 10 below to 20 below.
On the 1960 map, the valley floors and southern mountains of New Hampshire and Vermont were mostly in Zone 5. The northern valley floors were Zone 4 (minus 20 to minus 30), and the higher ridges and northern tips of Coos County and the Northeast Kingdom were Zone 3 (minus 30 to minus 40).
A key fact to remember is that the zone map was based on data from only the previous 15 years. Every decade or so afterwards, therefore, the USDA made periodic updates to the map to take advantage of the most recent 15 years’ worth of data. The 1990 map, the most recent version produced by the USDA, is the one that most gardeners and farmers now have in the back of their minds.
It turns out that the late 1940s and 1950s were warmer-than-average winters in much of New England. The 1960 USDA map, therefore, painted a milder picture of our climate than the long-term averages would suggest. The 1970s and early 1980s, meanwhile, were colder-than-average winters, meaning that the 1990 map suggests a climate more severe than the real deal. Since then, of course, the 1990s and 2000s have turned in many of the warmest winters since recording began in the nineteenth century, though the USDA has not updated the map.
Enter the National Arbor Day Foundation. Two years ago, deciding that arborists needed a more accurate map for choosing winter-hardy trees, the Foundation performed its own update. The result created a stir among the green-thumbed: most every spot in New Hampshire and Vermont had become warmer by a whole zone, meaning that wintertime lows were now 10 degrees warmer than they had been only a few decades ago. Zone 4 was the new Zone 5, and flinty folk who nurtured pride in winter toughness now had to cope with the softer conditions formerly confined to Massachusetts and points south.
Alarmists pointed to global warming while skeptics pointed out that the new map is remarkably similar to the original 1960 map. Nobody disagrees, however, that the shift was remarkably sudden, and that while drawing lines on a map may be an interesting parlor game, the temperatures they refer to are often a key factor in determining which plants and trees grow where.
For an extreme example, consider the artichoke. The ‘choke is not native to New Hampshire or Vermont, though you can grow it here if you’re willing to engage in some trickery. It’s a tasty plant, so I usually grow a few dozen each year on our farm.
The artichoke is a biennial, meaning that it doesn’t set the delicious ‘choke until its second summer of life. But around here, there is no second summer: the fragile artichoke can scarcely survive the first frost of autumn let alone the deep freeze of winter.
Hence the need for trickery. As far as an artichoke is concerned, winter is genetically defined as 200 cumulative hours of temperatures below 50° F. Once an artichoke experiences those 200 hours, it decides that it’s two years old and goes about blooming. Every spring, therefore, I move my little flat of artichokes in and out of the greenhouse, hoping to have them outside whenever the temperature is between 33 and 50°F and inside whenever it’s 32 or below.
This past summer, for the first time in eight years, the plants never set any ‘chokes. Not enough springtime cold. I will adjust this year by moving my planting date up a week, which is easy enough for me to do. But what about our native trees and plants, amphibians and insects? Their internal zone maps have been shaped by many centuries of required winter temperatures, and they won’t be able to adapt so easily.