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Plant hardiness zones and climate change

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The year 2012 began with a revision of the USDA Plant Hardiness Zone Map, replacing one that came out in 1990. It’s a pretty cool map. As before, its zones are based on average lowest annual winter temperature, but thanks to advances in the ways temperatures are recorded (by factoring in the effects of features such as hills and lakes, for instance) it’s far more precise. It’s also fun to read. In 1990, you could not enter your Zip code on a Web site and immediately find out your zone number. You could not place your cursor over the state you live in, zoom in and out, and trace the lines of that zone with remarkable detail.

Unless you are a beginning gardener, or are moving to a different climate, a zone map is never as accurate as what you already know. And even the makers of this one point out its limitations, because the survival and behavior of plants depends also on factors such as moisture, wind, light, duration and timing of cold temperatures. Everybody’s yard has microclimates. And food growers know that death is not the worst thing that can happen to a plant. A grape that needs more warm days to ripen than your Northern orchard provides, or an apple that needs more cold ones than descend on your Southern plantation, does not earn its keep.

Nor does the map track another vital statistic — summer temperatures, which seem to be getting ever hotter.

Nevertheless, the thing about this map that got everyone’s attention was that it seemed to confirm global warming. In every zone, since 1990, winter temperatures have risen. Interestingly, a map published in 2006 by the Arbor Day Foundation indicated the same thing. By then, the department had shelved a separate effort to update the map, prompting cries that Bush-era politics had put the elephant-in-the-room of climate change back into its cage of denial. Even the current official map comes with text ascribing the “warming” largely to improved mapping of mountainous regions.

A study by Nir Krakauer at the City College of New York’s Grove School of Engineering, on the other hand, has been mapping the changes since the period covered by the government’s 2012 map (1975 to 2005) and says the rate of warming has quickened.

Admittedly, a map such as the winter hardiness one can, at best, tell only a small part of the story. It does not track climate destabilization, jokingly referred to as global weirding. Others are gathering data on the quantity and severity of extreme weather events, from typhoons to droughts. And certainly views of disappearing glaciers and the snows of Kilimanjaro, or your car floating out of its garage, are the most compelling. The new map will certainly help wholesalers decide when and where to ship strawberry plants or guide a home gardener in the purchase of a hydrangea. But if its implications are correct, they’ll soon have a lot more urgent things to worry about.

Damrosch's new book, "The Four Season Farm Gardener's Cookbook," will be published in March.

Tip of the week Certain vegetables should be started from seed as early as January, including cabbages, broccoli and leeks. Think about buying — or making — a light stand for seed starting. — Adrian Higgins

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