Winter is hard.

Yes, snow is fun—for the first few weeks, and there is an essential beauty about a land frozen in ice, but let’s face it—nearly everything requires more work and effort during the winter. Most of this effort involves staying warm, but there is also snow to shovel, darkness to endure. We humans can always retreat indoors to the warmth of the stove or furnace, but plants have to just stand there and endure it all.

 

It is amazing, really, that trees, shrubs, and wildflowers can survive the cold of a New England winter at all. The ground is frozen so they cannot absorb water and the light is so dim that photosynthesis would be pretty minimal anyway. Then of course there is the ever present cold and the ice crystals that threaten to rip their very cells asunder. So how do they survive it anyway?

 

Winter arrives, 50 million years ago

Winter is a relatively new phenomenon on planet earth. Plants have been around for about 250 million years, and for all but the last 50 million years, Earth was a warm, largely tropical place. About 50 million years ago, the Arctic Ocean was cut off from the other seas by a ring of continents, creating just the right conditions for the nitrogen-fixing aquatic fern azolla to flourish.

 

Over the span of about 800 thousand years the ferns grew on the surface of the sea, taking carbon dioxide out of the atmosphere and then sequestering it in their tissues. As these died and sank to the oxygen-free sea floor to form great deposits that turned eventually into the North Slope oil that we drill for now in Alaska. This massive 80 percent reduction in atmospheric CO2 about 50 million years ago made Earth a much colder place than it had ever been before, and winter was born. The next time you slip on the ice, remember you have a little aquatic fern to thank for winter.

 

Evolving to survive

Fortunately plants had millions of years of gradual cooling to figure out how to survive this new climate. There was a powerful evolutionary advantage conferred to species that could weather increasingly cold temperatures as vast stretches of the earth’s surface plunged into winter for at least part of the year. The trick was to develop strategies to prevent ice crystals from forming inside cells, because these sharp, expanding daggers will quickly tear a plant apart if it is caught unawares.

 

When the first killing frost turns your tomatoes limp, inside their tissues it is as if a bomb went off and all the walls that previously kept cellular contents inside were destroyed. Perennial species begin planning for the first frosts long before they actually happen. Preparing for winter is a slow and energy-intensive operation that begins in the waning days of summer.

 

Late summer strategies

After the summer solstice, the days gradually get shorter through July and then begin to really shrink in August. By September, it is pretty obvious that evening is coming on sooner and fall is just around the corner. Plants can sense the lengthening and shortening of days very accurately, and they use this information to plan flowering, seed set, and dormancy. Asters and goldenrod, for example, are triggered to flower in fall by the lengthening nights, while the same is true for the early phase of winter dormancy.

 

It is important to note that daylength on any give day varies by latitude. The farther an area is from the equator, the longer the summer days and winter nights will be. On the summer solstice, days in Homer, Alaska, are about 19 hours long, while in Houston, Texas, they are only 14 hours. By October 21, the days have dropped to just 9 ¾ hrs in Homer, while they are still 11 ¼ hrs in Houston.  

 

When you try to grow an Alaskan or Texan species in Massachusetts, either plant may misread the daylength and think that fall is either closer or farther away than it really is. This is one of the primary reasons that provenance (the specific place where a particular individual or race of plants is native to) is important from both a horticultural and ecological point of view. If you grow plants too far from their native latitude, their internal clock will be permanently out of sync with the seasonal realities of their new home.

 

Acclimation and deacclimation     

The ability to withstand sub-freezing temperatures is not something that winter hardy plants just maintain year-round. It requires a lot of energy to do this and it also interferes with growth, so as spring begets summer, even the hardiest firs and spruces lose their cold tolerance for the growing season. A fir that might withstand -50ºF in January, could be killed by 28ºF if caught unawares mid-July. To withstand midwinter cold, hardy plants must first go through an acclimation process, and before emerging into growth on the other side of winter, they go through the reverse, a deacclimation process.

 

The internal clock of the wild asters and red maples outside my window have been fine tuned by natural selection for this very place, so around about August they begin to cycle down for the year, drawing food reserves into rhizomes or roots to prepare for the coming winter. Decreasing day length combined with decreasing temperatures produces a spike in plant hormones involved in dormancy—abscisic acid and ethylene. These hormones cause aboveground growth to slow and stop while spurring the movement of food reserves to roots or underground storage organs like bulbs or tubers.

 

You may have heard that fertilizing plants too late in the season may affect their winter hardiness. Fertilizer has a way of overriding the affect of the two dormancy hormones so your plants may continue growing too long into the fall and risk fatal cold injury. Likewise, species or races from too far south may miss the cues because their clocks are set to a different latitude. They too may grow too late into fall and be killed by early cold spells.

 

Acclimation continues

Once growth has stopped and food has been reallocated below ground, the next phase of acclimation may commence. Abscisic acid continues to increase as night temperatures drop near freezing and dormant tissues start to lose water—desiccate, which increases the concentration of sugars and “cryoprotectant” proteins and alcohols in cellular sap. Like antifreeze in your car, these lower the freezing point of water to 20ºF or even 10ºF, which is enough winter protection to see the plant through the first part of the winter in New England or the whole kit and caboodle farther south.

 

If we experience a period of cold in fall followed by two to four weeks of unseasonably warm temperatures, plants can actually lose some of their cold tolerance. This happened in much of the Northeast during winter 2006–2007, when we had record warmth in December and January. In late January, the temperature abruptly fell, and February was very cold with disastrous effects on many typically hardy perennials.

 

Getting through New England winters

Cellular antifreeze is only effective down to about 10ºF (the lower end of USDA winter hardiness zone 7). With the exception of some of the southern coastal areas, New England regularly experiences temperatures below this every winter, so to be hardy here, plants have to go a step farther.

 

Very cold-hardy plants (those hardy to USDA zone 3 or - 40ºF) are able to prevent ice crystals from forming in their cells even at temperatures well below 0ºF. You have probably heard of planes seeding clouds to produce rain. In one technique, these planes spray silver iodide crystals into clouds. The crystals act as nuclei or nucleation points on which ice crystals begin to grow and fall as rain once they melt at lower levels of the atmosphere.

 

Ice crystals in cells need nucleation points to begin forming, also, so very cold hardy plants prevent them forming by removing potential nuclei from the cellular and intracellular fluids. Such fluids become in effect supercooled and can withstand temperatures as low as -40ºF, without damage. Below this temperature, ice crystals will form without nucleation points, and this explains why USDA zone 3 is the lower winter hardiness limit for many very cold-hardy species.

 

A few high alpine and tundra species like Robbin’s cinquefoil can weather temperatures around -60ºF (USDA zone 1) with a combination of supercooling and cellular dehydration that removes much of the water from cells that the ice needs to grow. Their cellular membranes, fairly flexible and resilien, enable such species to resist the thrust of ice crystals (think of them like self-repairing tires).

 

Earth helps also

The heat from Earth’s molten core radiates up through the bedrock and actually keeps the temperature of the soil substantially higher than the air temperature, especially if it is covered by an insulating mulch or blanket of snow. As a consequence, roots and other below-ground structures are not nearly as cold-hardy as evergreen leaves, stems, and buds.

 

I killed a few white oak seedlings—normally hardy to at least 25ºF when in the ground—because I left the roots exposed in pots when the temperature dropped down to about 10º.  In nature, fallen leaves, stems, and other organic detritus provide a blanket of insulation to retain some of the Earth’s heat. In the garden, mulches serve a similar role.

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While the acclimation process and maximum winter lows are important pieces of the winter hardiness puzzle, deacclimation in spring is just as critical to understand. This process will be the subject of second part of  this article.

 

Look for part two of Bill Cullina's look at plant hardiness mid-December.