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2012 USDA hardiness zone map for New Hampshire, reflecting the long-term average for winter low temperatures. Note the warmer areas along hills from Keene to Sunape - indication that the lowest temperatures in our area occur in valleys, where cold air collects. Mountains have to be much taller, asin the White Mountains, for high points to be colder in winter.
Monday, February 6, 2012

Winter low temperatures aren’t as low as they used to be

David Brooks

As your snowmobile, ice skates and snowshoes slowly rust away from lack of use, you might be wondering if our winters are getting warmer.

The U.S. Department of Agriculture has an official answer: Yes, at least in terms of average extreme low temperatures. Those averages are about five degrees warmer than 22 years ago, according to its new map of plant-hardiness zones.

Specifically, most of Greater Nashua has been shifted from Zone 5A, where the average extreme low temperature during winter (in other word, the average lowest temperature during the course of the season) are in the range of minus-15 to minus-20, to Zone 5B, in which the average low is only (only!) minus-10 to minus-15.

The eastern fringes of Greater Nashua, in Pelham and toward the Seacoast, edge into Zone 6A (a balmy minus-10 to minus-5). Previously, the New Hampshire didn’t have any Zone 6 areas.

Since plant-hardiness maps are specifically aimed to help gardeners choose what to plant, you might be wondering if it’s time to order intriguing items from the seed catalog historically too wimpy for New Hampshire winters. Local gardening experts have an answer: Not unless you’re ready to be disappointed.

“You can’t rely just on a map, you have to rely on experience,” said Kathy Gagne, co-owner of The Mixed Border Nursery and Garden Center in Hollis. “Maybe we’re getting a little bit warmer, but for Nashua, it’s not changing much. … My concern is that whatever’s published about the new zones … are going to make people less wary about trying the new things when they’re really not zone-hardy.”

More important than maps, said Gagne and other gardeners queried by this brown-thumbed columnist – including Cathy Lemire at Wilson Farms in Litchfield and Catherine Preston at House of the Side of the Road in Wilton – are local conditions.

Your property’s microclimate, such as low spots where cold air collects, is vital, as if whether you’ve planted next to a big rock or a house foundation, places that absorb heat and release it slowly. Those factors are more important than a map zone, they say.

“Garden centers can let you know when you can plant things, when they feel it’s the right time to plant. Something may work one year, but may not in the next,” said Lemire.

Despite the ski-killing warmth, this winter has been hard on outdoor plants, Lemire and Preston said. The lack of snow cover means soil has gotten colder than expected, which can harm fragile roots. A night that hits 20 below when we have 2 feet of snow is less damaging than zero degrees on bare ground.

Plant hardiness zones are an established tradition, giving a rough guide to which species are likely to grow in which area.

They are based on long-term averages of the air temperatures each winter. They indicate nothing about changes in high temperatures or summertime temperatures or average temperature over the course of the year. (Lemire says people should still wait until May before doing most outdoor planting.)

The map’s only concern is the likelihood that the thermometer will drop to the point where various kind of plants will die.

The USDA last drew up a hardiness map in 1990, based on 13 years of temperature data from roughly 8,000 weather stations throughout all North America. The new map, released in January, uses data from 8,000 stations in the United States alone (with a few exceptions). Plus it covers a 30-year period, through 2005.

This gives the new map much more detailed information, said Sandy Miller Hays, spokeswoman for the USDA Agriculture Research Service. The USDA has put the map online, searchable by ZIP code. Overall, the map’s big news is widespread warming. Most of the country shifted at least a half zone warmer, like we did. A few spots got an entire zone warmer, and USDA even had to create two new zones – 12 and 13 – to handle warmer winters in Puerto Rico and Hawaii.

This isn’t a surprise. The National Arbor Day Foundation updated its own plant-hardiness map back in 2006, and it reflected a similar increase in average winter lows. Is this climate change in action? Maybe. Some folks, including meteorologist Doug Webster, who writes The Telegraph’s monthly weather column, are dubious. Webster cautions against reading too much into such a change over a relatively short period of time, with the context of long-term weather fluctuations.

Others, including a number of climate scientists at the University of New Hampshire, Dartmouth College and other New Hampshire institutions, say it’s a sign of things to come. As man-made gases prevent more and more heat from escaping into space, temperatures are going to slowly rise, weather fluctuations are going to get wilder. It’s not just planting guides that are going to be changing, they fear.

I’m afraid they’re right. I think I’ll wait before buying new ice skates.

Granite Geek appears Mondays in the Telegraph, and online at www.granitegeek.org.

David Brooks can be reached at 594-6531 or dbrooks@nashuatelegraph.com.