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How well are our plants sleeping?

The USDA released their most recent official Plant Hardiness Zone Map in 1990, using average annual minimum temperatures from 1974 to 1986 in the United States and Canada, and 1971 to 1984 in Mexico. This area was in zone 5, with minimums between -10ºF and -20ºF.

In 2003 the American Horticultural Society had a new map compiled, using data collected from July 1986 to March 2002. But controversy erupted because it showed many areas a full zone warmer than the 1990 map. Was the range of years too small? Were the warmer winters they recorded just part of the normal variation to be expected? The map was never officially released.

So the Arbor Day Foundation collected data from 15 years of reports by 5,000 National Climatic Data Center cooperative stations, and published the results in 2006. This map also shows warming. At http://www.arborday.org/media/mapchanges.cfm you can even see an animation of how the zones have shifted in that time. Here in the northwestern third of Middlesex County, we're still in zone 5 -- but most of zone 5 is farther north now.

Of course Hardiness Zones don't take into account other climate factors affecting the garden. Snow cover insulates, moderating temperature swings. The plants don't get quite so cold, and the ground heaves less. Plants particularly sensitive to root damage from frost heaves include Pincushion Flower (Scabiosa), Coral Bells (Heuchera), Shasta Daisy (Leucanthemum), Blanket Flower (Gaillardia), Pigsqueak (Bergenia), Coreopsis, Seathrift (Armeria), Painted Daisy (Tanacetum) -- plus fall-planted perennials that haven't had time to get roots established, especially bulbs and garden mums.

Good drainage reduces frost heaves. But even with my sandy soil I'm glad to see the grass finally covered. In February 1979 the saturated, frozen soil just north of me had a frost quake, making a crack half an inch wide, two inches deep and a quarter-mile long.

Hardiness Zones tell you where it gets too cold. In warmer parts of the country, orchardists worry about getting enough "chilling hours" (between 38ºF and 45ºF) winter to set fruit. Carl Hills of Kimball Fruit Farm in Pepperell told me around here the problem is rollercoaster temperatures, with buds breaking dormancy and then getting blasted. Mulching your garden insulates as snow does, but if you mulched this past fall you could have created a similar problem by keeping your plants so warm you prevented the dormancy they needed when it suddenly turned cold.


© Copyright 2007 Catherine Holmes Clark

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Published in the six Nashoba Publications papers on Friday, 16 March 2007

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