plant cold hardiness

Winter is once again descending on the Vermont landscape. When I venture outside now I’m well-wrapped in warm jacket, hat and mittens. But there are no cozy scarves or down parkas for trees and other plants to don against the cold. How do they make it through until spring?

Plant cold hardiness is pretty amazing. Plants that are adapted to my New England winters (and other cold winter climates) have evolved ways of making it through the long months of cold and surviving some pretty frigid temperatures. A plant that might be injured or killed by below-freezing temperatures during active growth will undergo physiological changes that enable it to withstand winter temperatures many degrees below zero. These changes happen in part in response to signals from the environment that the season is changing. Shorter days (it's actually the longer nights that plants are responding to) and falling temperatures send a message that winter is on its way. That's why it's important not to cover plants like roses with winter protection too early in the fall. They need to be exposed to the environmental signals to develop as much of their natural hardiness as they can.

However, sometimes Mother Nature throws meteorological curve balls that can affect how well plants weather the cold. Plants develop the full extent of their hardiness by degrees, going through a period of acclimation before settling into full dormancy. When the onset of cold weather is early and sudden, plants that haven't had a chance to prepare their full defenses may be injured. When a midwinter thaw is warmer and longer than expected, when midwinter temperatures plunge unusually deep into the negative numbers, or when temperatures fluctuate rapidly from cold to warm and back again, especially early and late in the winter, plants can suffer.

If there is a period of cold weather followed by a return to warmer than usual temperatures in late fall, some plants get "confused" and begin to shed their winter hardiness in expectation of spring. Sometimes they even begin growing actively, only to be injured when the cold returns. This is why you may see early spring bloomers like forsythia unfurling a few blossoms in November and December warm spells. Usually enough flower buds remain dormant to preserve the springtime show.

The soil movement that occurs during periods of alternate thawing and freezing during winter warm spells can result in plants being heaved out of the ground, leaving their roots exposed to injury from cold and drying air. When we spread protective mulch over perennials in the garden, we are not trying to keep them warm -- we are actually trying to keep them cold, locked safely in consistently frozen soil. That is why gardeners are advised to wait until the ground is frozen to cover plants with mulch and why a consistent blanket of snow provides such good winter protection.

Of course, plants can be injured by extreme cold temperatures as well. To use forsythia as an example again, the flower buds are not as hardy as the leaf buds. Often winter temperatures here in Vermont are cold enough to kill forsythia flower buds, resulting in a burst of foliage in spring, but few blossoms. In fact, sometimes the only flowers seen are at the base of the plant -- where snow cover provided insulation to those lower buds. 

Like other woody plants, fruit trees adapted to cold winter climates spend the winter in a period of dormancy. In fact, they are programmed to require exposure to a certain amount of cold before their flower and leaf buds are ready to open, nature's way of making sure they don't open before spring. This is called their '"chilling requirement," the number of hours of below 45 degree F temperatures the buds must be exposed to in order to come out of dormancy. But once this requirement has been met, the buds, especially on stone fruits such as cherries, peaches, and plums, can begin to lose their cold hardiness if there is an extended spell of warm weather in mid to late winter. When this is followed by more cold weather, the flower buds may be injured. The same type of injury can also occur in spring as the plant begins to deacclimate to the cold -- a late frost can nip future fruits “in the bud." As in the fall when plants are developing their winter hardiness, a gradual and consistent pattern of temperature change suits plants best.

Robert Frost, our beloved New England poet, was a keen observer of the natural world and its cycles and uncertainties. So I'll leave you with his lovely and seasonal meditation on trees, winter, and the acceptance of the vagaries of nature, something we gardeners must all confront.

Good-by and Keep Cold

by Robert Frost

This saying good-by on the edge of the dark
And the cold to an orchard so young in the bark
Reminds me of all that can happen to harm
An orchard away at the end of the farm
All winter, cut off by a hill from the house.
I don't want it girdled by rabbit and mouse,
I don't want it dreamily nibbled for browse
By deer, and I don't want it budded by grouse.
(If certain it wouldn't be idle to call
I'd summon grouse, rabbit, and deer to the wall
And warn them away with a stick for a gun.)
I don't want it stirred by the heat of the sun.
(We made it secure against being, I hope,
By setting it out on a northerly slope.)
No orchard's the worse for the wintriest storm;
But one thing about it, it mustn't get warm.
"How often already you've had to be told,
Keep cold, young orchard. Good-by and keep cold.
Dread fifty above more than fifty below."
I have to be gone for a season or so.
My business awhile is with different trees,
Less carefully nurtured, less fruitful than these,
And such as is done to their wood with an ax--
Maples and birches and tamaracks.
I wish I could promise to lie in the night
And think of an orchard's arboreal plight
When slowly (and nobody comes with a light)
Its heart sinks lower under the sod.
But something has to be left to God.

From: New Hampshire, 1923

Blog by: Susan Littlefield

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