The Opinionated Gardener: Winter Musings on Cold Hardiness and Global Warming

11 mins read

A ‘Deane’ apple still hopefully grows in Temple.

It was cold in January. Twenty-five degrees below zero cold in the hills of Temple with rumors of 35 degrees below zero down in the intervale, to say nothing of the confirmed report of 50 degrees below in northwestern Aroostook County. It was cold enough there to break the statewide record of 48 degrees below zero recorded in Van Buren in 1925.

As the Opinionated Gardener continued to feed the woodstove, she wondered if the two-year-old ‘Deane’ apple tree (see Dec. ’07 Opinionated Gardener) she planted last spring – the spindly sapling that the deer nibbled on the tips of in mid-summer – would make it through its first Temple winter.

The catalogue said ‘Deane’ was native to Maine, (see clipping below) was grown in Maine and was cold hardy* down to Zone 3, where the average annual minimum temperature ranges from minus 30 to minus 40 degrees Fahrenheit. Given the fact the tree has, to date, been subjected to a mere 25 degrees below zero, why shouldn’t it survive its first winter in Temple?

“It’s not a stretch of super sub-zero nights in January that one has to worry about, it’s a period of wildly fluctuating, rapidly dropping and rising temperatures that often occur in early fall and late spring that you should worry about,” said Maine Cooperative Extension Horticulturalist Lois Berg Stack.

For those old enough to remember – the rest of us have read about it – such conditions overtook Maine during the winter of ’33 -’34. Wildly fluctuating temperatures early in the season coupled with brutally cold record low temperatures throughout the winter all but decimated Maine’s apple industry that year.

According to Maine agricultural historian Clarence Day, some 300,000 apples trees were killed outright and perhaps as many more were rendered worthless. Zone 4 cold-hardy (30 to 40 degrees Fahrenheit below zero) Baldwin and Ben Davis apples suffered most. Census figures summed it up: In 1930 there were 1,791,981 apple trees in Maine; in 1940 there were only 550,389 apple trees.

Less dramatic but more recent were the fluctuating temperatures of winter that Maine orchardist Steve Page described in his 1886 book The Orchard Almanac. “The trees I set out six years ago were damaged at that time by a warm autumn followed by a sudden 40 degree drop on Christmas Eve. It was too cold too fast; then a January thaw compounded the injury. Those trees have been dwarfed ever since. Many growers lost sizeable portions of their orchards that year.”

Acclimating to the Cold

So how does an apple tree or any other woody plant get ready for winter and why are some hardier than others? The answer to the last question is easy. The capacity for cold hardiness is hereditary. It’s in the genes. It’s about natural selection and survival of the fittest. ‘Cortland’ and ‘McIntosh’ survive down to 30 degrees below zero (Zone 4); ‘Wealthy,’ ‘Honeygold’ and ‘Deane’ survive down to 40 degrees below zero (Zone 3). ‘Flowering Crab’ survive down to 50 degrees below zero (Zone 2).

But winter hardiness is not always present; its development is a slow and gradual process that occurs annually towards the end of each growing season. It is triggered by shortening day length and increasing cold. But because it is dependent on the temperature, it waxes and wanes throughout the winter, increasing as the temperature drops and decreasing as it rises. The trick to survival is the synchronization of hardiness with the fluctuations in temperature: to be ready for a hard freeze and to bounce back quickly after a thaw.

Understanding the mechanism woody plants use to survive sub zero temperatures is fairly simple. Scientists talk about “extracellular freezing” and “cyroprotective” compounds that produce “deep supercooling.”

In essence, woody plants survive sub zero temperatures by doing what we humans in Maine do when leaving the house in winter for an extended period of time: we drain the pipes and add antifreeze to our hot-water, base-board heating systems.

So, combining the two explanations, one can say, albeit in a sentence that is too long, that in response to shortened day-length and decreasing temperatures, water begins to move out of the plant cells through increasing permeable membranes to the spaces between the cells, where it slowly freezes (extracellular freezing), leaving behind high concentrations of salts and sugars (cyroprotective compounds) that act as a natural antifreeze, lowering the freezing point (deep supercooling) of the remaining liquid.

The O.G. can only hope ‘Deane’s’ mechanism for surviving down to – 40 degrees stays in sync with the fluctuating weather in the coming weeks as it wakes from dormancy and relaxes its guard.

The Plant Hardiness Zone Map and Global Warming

But isn’t this all moot? Aren’t we in the throes of Global Warming? “Yes, but,” said horticulturalist Stack, who is currently on sabbatical in sunny California. “Even if our winters are becoming warmer, we still have one or two very cold days in our ‘warmer’ winters. When people speak of warmer winters, they are generally speaking about averaging the temperatures throughout the winter.

“Remember, our hardiness zone maps are based on the single coldest temperature each winter. And plants have to survive every day, one day at a time. Even if the winters are generally warmer, they still have to survive those exceptionally cold individual days,” Stack said. Like those two days in January, this year, adds the OG.

But aren’t the coldest days also getting warmer? Given the fact that the Plant Hardiness Zone Map that is currently in use covers the years 1974 to 1986, shouldn’t it be updated, so we can find out whether this is true or not?

Remembering that her neighbor and landscape gardener Peter Armstrong kept daily records of the temperature, the OG turned to him for more recent data. Based on his record keeping over the past 10 years, Temple is no longer in Zone 4 (-20 to -30 degrees Fahrenheit) but has moved on up to Zone 5 (-10 to -20 degrees Fahrenheit). According to Peter, the average minimum temperature from 1998-2008 was -14 degrees Fahrenheit. So the coldest days are getting warmer?

Becoming obsessed with the issue, wondering when a new plant hardiness zone map would be issued by the USDA and what time period it would cover, the OG went online and learned that indeed the USDA had contracted out an update to the American Horticultural Society, using data from 1986 to 2002 and in fact had come out with a draft update in 2003. For a look, log onto:

It appears, however, that the USDA decided not to release the map, saying it wasn’t based on a long enough time period and has set about working on an in-house update that will include data from the years 1974 through to about 2001-2003. Spokesman for the project Tony Avent says the hardiness zones may be only slightly different from the 1990 version. (The more years you average, the less dramatic the change?)

Global warming denial?

Meanwhile, the United State National Arboretum has developed its own update, based on 1987-2001 data from the US National Climatic Data Center. It differs from the Horticultural Society’s map and shows a widespread shift of the average low temperature to the north or inland in almost all parts of the county. On that map, Temple has shifted to Zone 5 where average minimum temperatures range from -10 to -20 degrees. Check it out at Peter Armstrong may just be right.

Right or wrong, the OG is not about to order a palm tree this year.

*Cold Hardiness Zones: According to the Plant Hardiness Zone Map of Maine put out by Cooperative Extension and derived from the USDA map published in 1990, Maine spans four zones: 3, 4, 5 and 6. Each represents a ten-degree difference in average annual minimum temperatures. They range from – 40 degrees (Zone 3) to -10 degrees (Zone 6). Each zone is further divided into sections a and b with a five degree difference between them. While Temple is located in Zone 4b (-20 to -25 degrees F) other parts of Franklin County, are located in Zones 4 a (-25 to -30), 3 b (-30 to -35) and 5 a (-15 to -20). The map is intended to be a guide and does not take into account microclimates with in the zone and other factors affecting the winter hardiness of a plant.

Print Friendly, PDF & Email