Plants do best when they can acclimatize slowly to cold. Source: libreshot.com

If you live in a cold climate, you’ve probably noticed that the first time temperatures drop to 40°F (5 ° C) in the fall, you find it really cold. It takes a winter coat, a wool hat and gloves before you feel at ease outdoors. However, if in February (in the Northern Hemisphere), the temperature is “only” 40 °F (5 ° C), you find quite balmy and may even yourself unbuttoning your coat! Between October and February, you’ve acclimatized to winter conditions. But your acclimation will disappear as spring settles in. Thus, if there’s sudden drop to 40 °F (5 ° C) again in May, you’ll haul out the winter coat, hat and gloves again … and probably complain bitterly about how cold it is!

Plants Adapt Too!

Well, hardy plants have a similar process of acclimatization and deacclimatization. The short days and the gradual drop in temperatures of fall push cold-tolerant plants into deeper and deeper dormancy. They’re at their hardiest in the middle of winter. Then, in the spring, the plant acclimates again to warmer temperatures and gradually loses its resistance to cold. Thus, a balsam fir (Abies balsamea), one of the hardiest of all trees, one that won’t flinch at -40 ° F (-40 ° C) in January, will be seriously damaged at only -10 ° C in July. It has, by then, lost its winter hardiness.

How hardy plants adapt to the cold varies from one species to another. Some develop a kind of antifreeze that prevents their sap and cells from freezing, others reduce the amount of water in their cells (water expands as it freezes and can tear and kill cells as it does so), some lose their leaves in winter and others retreat completely underground. Most combine different methods.

The Ideal Situation

In a perfect world (assuming a perfect world includes cold winters!), cold would set in gradually starting in September and temperatures would drop little by little from week to week. An Indian summer may be wonderful for humans, but it can seriously harm hardy plants, which can lose their resistance to cold if it lasts too long.

Where I live (Quebec, Canada), the fall of 2018 has been picture-perfect for plants so far: it literally has been getting colder week by week with no annoying Indian summer to set plants back. If this trend continues, the hardy plants in my region will spend the winter of 2018–2019 in excellent condition!

What Human-Imposed Winter Protection Does and Doesn’t Do

Did you know that “winter protection” (i.e. wrapping plants up for the winter), which I see many homeowners installing these days, actually doesn’t protect against the cold? Even if you wrap your shrubs, trees and evergreens in burlap or geotextile, or build a covered cage around them, night temperatures inside these protections are essentially the same as those outside.

Moreover, it’s important that these protections be light in color. Dark wrappings absorb heat during the day and can actually reduce the plant’s resistance to cold. When a really cold night follows a few days of unusual warmth, the plant can be severely damaged or even die.

So burlap, cages and geotextiles don’t keep the plants any warmer, but what they do accomplish is to minimize the drying effect of winter wind. They are therefore especially useful for plants that are freshly planted and not yet settled in or those that are planted in a very windy site.

Personally, I don’t use this kind of winter protection in my garden: it’s too much work for the slight advantage that it gives … when there is an advantage, that is; often, there is none. (I find many people wrap up plants that don’t need any protection!) In addition, winter protection tends to help plants poorly adapted to local conditions to cling to life. I prefer to use plants that like my conditions. If a plant can’t tolerate winter at my place, I’d rather it die quickly! I’ll soon find a hardier replacement. The right plant in the right place: always the best thing to do!

How to Really Keep Plants Warmer

There are, however, two “winter protections” that are very effective at protecting plants against the cold: snow and mulch.

To understand why, you need to know that the roots of most plants are much less hardy than their branches and leaves. They usually don’t need to be, as soil is normally warmer than the air above due to “bottom heat,” geothermal heat moving up from the depths of the earth all winter. But sudden drops in air temperature can cause deep freezing of unprotected soil, a disaster for plants.

Snow helps by keeping cold air from penetrating deeply into the ground. It’s an excellent insulator, being mostly composed of stagnant air. For example, when the air is at a frigid 5 ° F (-15 ° C), for example, the soil below is rarely much colder than 30 ° F (-1 ° C) under a layer of 6 inches (15 cm) of snow.

The more snow there is in winter, especially when it arrives early and stays late, the better condition your plants will be in come spring.

Mulch—a layer of organic matter covering the ground—acts in the same way as snow: it creates an insulating layer between sometimes extremely cold air and the soil below. Its advantage over the snow is that it is reliable: it doesn’t suddenly melt away in mid-January, leaving your plants exposed, as snow can do. Moreover, snow on top of a winter mulch makes an even more effective insulation: it’s a winning combination!

I mulch just about all my plantings with chopped leaves, ensuring the best winter protection that money can buy… Oops! I suppose I didn’t spend any money on my shredded leaf mulch, did I?

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Whether you to wrap your shrubs for the winter or not is up to you, but remember the best winter protection is actually free: snow and leaf mulch!