For those in northern latitudes, winter is a slow time in the garden. Plants are dormant, nursery stores are bare, and for most of the winter it’s still too soon to start your seed trays for the vegetable garden.
But one thing that keeps me active through the winter months is taking advantage of snow. In Denver, a semi-arid region in the rain shadow of the Rocky Mountains, we rarely if ever get deep accumulations. Six inches is a big storm, twelve is massive. It usually melts completely before the next snow. That’s to be expected in a climate with between 10 and 15 inches of precipitation in an entire year. But when the snow comes, I’m ready, heading out to scoop the treasure and pile it in the garden.
How snow protects plants from cold
It’s counter-intuitive that something that is, by definition, frozen, helps protect the garden from winter’s dramatic temperature swings. It comes down to the ways that plants protect their dormant tissues from the destructive power of ice.
Dehydration as protection
Plant cells are mostly water, and for non-adapted plants, ice crystals forming in cells pierce and rupture them. When the plant thaws, the pulverized tissues hang limp like cooked spinach, utterly destroyed.
Cold-adapted plants have a variety of defenses. First and foremost, they prepare for cold by allowing their tissues to dehydrate. They will often do this by moving cellular water to the spaces between cells, where it can freeze safely. Inside the cells, the water becomes thick and syrupy with sugar, electrolytes, peptides and enzymes that interfere with ice crystallization. Instead of freezing at 32 degrees Fahrenheit (0 degrees Celcius) as pure water does, ice in plant cells may not form until the temperature drops to 25 degrees, 20 degrees or lower. (The limit depends on the species, the variety, and how much time the plant has had to prepare for coming frost.) As temperatures continue to drop, more water will migrate outside the cell and freeze, leaving the inside even more dehydrated. Thus, freeze tolerance in many species goes hand in hand with the plant’s ability to cope with drying out.
Eventually, temperatures drop low enough that cell contents freeze, which is lethal to tissues on plants that die to the ground each winter. Those with perennial above-ground tissues may tolerate ice inside the cell. (Cell contents are by now so dehydrated that there’s only so much damage ice crystals can do.) Other kinds of plants retreat to the ground, where the thermal mass of the Earth keeps roots and storage organs within tolerable temperature ranges.
The burdens of sun and wind
Yet, even when plants are able to survive a frozen-solid state, they have additional issues to contend with. Drying winds and sun can loosen frozen water molecules and convert them directly to vapor in a process called sublimation. (Though very tough plants can survive down to 25 or 30 percent water in their tissues, they cannot survive drying out completely.) Oxidation reactions with air can degrade tissues when the frozen plants are unable to produce antioxidant compounds to protect themselves. Ultraviolet light from the sun damages cell organs and DNA. During the growing season, plants are constantly repairing damage from these assaults. But plants can’t heal while frozen, and damage accumulates. Thus it is not just the winter’s minimum temperature but also the length and frequency of hard freezes that determines whether plant tissues are injured beyond a point of no return before spring.
Snow creates a stable, protected environment
Fortunately, most of these issues are ameliorated by snow. Because a layer of snow is mostly air—around 95 percent air after a fresh, fluffy snowfall, and at least 80 percent on old, crusty snow or heavy wet snow—it is an excellent insulator. As temperatures crash below zero outside, they stabilize at around 25 degrees Fahrenheit when measured 4 inches into the snowbank.
Next, snow blocks ultraviolet light, and keeps stems and leaves from drying in a zone of 100 percent relative humidity. Yet despite the moisture, oxygen can diffuse through and most pathogens cannot grow below freezing. Thus, snow cover can keep even fairly tender plant tissues in a state of happy, mostly suspended animation for months on end.
A few bonus benefits of shoveling your sidewalk snow onto the garden
In the Western U.S. we’ve been contending with a gradual drying trend and droughts that get more severe with climate change. At the same time, though temperatures have gotten on average warmer, they also swing more fiercely now. Sudden spring cold snaps injure plants more badly when they’d let down their defenses during a warm spell.
Snow cover helps with both these problems—keeping plants properly chilled through winter, and supplying extra moisture that is happily taken up in spring.
An additional bonus for me is that it protects the environment. Runoff from streets in winter carries pollutants such as sidewalk salts (magnesium, calcium or sodium), fertilizer, oils, nitrates and sulfates. Often, stormwater systems move runoff directly into lakes, rivers and streams. When these substances contact soil, they’re quickly scooped up by soil microbes that convert them into nutrients. Those nutrients are in turn taken up by plants. But urban runoff systems don’t often exploit that step, sending water directly into streams. There, the pollutants are far more harmful, since they interfere with aquatic organisms’ ability to use dissolved oxygen. Thus, I’m helping clean the streets at the same time I protect and fertilize my garden.