As much as I dislike cold snaps like this, I try to look for the benefits of everything. We all know that extreme cold temps can have devastating effects on plants, remember Dec. 25th 2022???. But we should also look for the value of something like cold, because it must have a function, right??
Temperate plants, or plants that evolved in temperate latitudes (like here in central Ga) are adapted to environments with distinct seasons. Therefore, they have evolved to live with periods of subfreezing temperatures, dependent on factors like elevation, latitude, and position on a continent or nearness to large bodies of water.
Let us consider Georgia's position on the North American continent. At our latitude ~33 north, the prevailing winds/ fronts are moving from the west to the east. This scenario is the general reason why San Diego (32N) rarely has subfreezing temperatures while we can have temperatures as low as the single digits.
And the further north you go the effect seems more pronounced. Great Britain is at a similar latitude as northern Quebec, yet palm trees can grow in Ireland. You'd be hard-pressed to even dig a hole through the permafrost in Northern Quebec, much less ever see a palm tree.
So being close to a large body of water certainly matters, yet it matters just as much where you happen to be positioned on the continent.
But back to the reason for this essay... While I don't understand all the science, I'll attempt to explain why cold is necessary for the "normal" development of temperate trees, perennials, and bulbs.
Agricultural scientists quantify a plant's cold requirements as its"chilling hours". And many of our fruits, especially stonefruits and apples require a minimum number of hours below a certain temperature to produce reliable flowers and quality fruits. Usually, temperatures between 45F and 35F satisfy this requirement. For reasons that are unclear to me, temperatures that fall below freezing aren't factored into this equation.
Yet when winter temperatures rise above this threshold, as they often do here in Georgia, these chilling hours can be erased, and this can create problems down the road.
As I understand it, without this consistent period of cold, plants can "malfunction", meaning they can come out of dormancy too soon, or they just don't develop normally. This is particularly evident in species that evolved or were selected in climates where winter temperatures remain consistently low, as in the PNW, the Great Lakes region, the Northeast, much of Japan, and much of Northern Europe, southern Chile, and New Zealand.
For us here in the southeast, our winters are inconsistent. We often have warm, 70F days in January/ February followed by arctic blasts of cold air that can routinely take us into the low 20s and upper teens, even in March. This often happens long after plants have broken dormancy and even in active growth.
Many of our ornamental plants also require a certain number of chilling hours to induce optimal flowering in the springtime. Tulips and other "Dutch" bulbs are great examples. Our stereotypical "fancy" tulips were bred in northern Europe and other high-latitude places. They require significant winter chilling, more than what we usually have here in central Georgia.
This is one of the reasons why understanding where a plant originated, not just its native environment, but also where it was selected by humans or hybridized, makes a huge difference in whether a plant will be successful in your garden.
This is an important concept for us to think about. Even though you may purchase a "native" plant, it does matter where said plant was selected and bred. If, for example, you purchase a cucumber tree, as I did, from a nursery in Ohio. It was likely selected from a more northern ecotype of this species' range. This can result in less than optimal growth in our warmer climate. In my case, the Ohio cucumber tree consistently leaves out 2 weeks after my tree with Georgia genetics. This results in an overall lack of competitiveness and vigor when compared to clones from southern genetics. As climate change accelerates, these genetic distinctions will be even more important.
So far I've only mentioned the benefits of cold, but as we all know, cold can also kill! I'm acutely aware of this, especially after the polar plunge of 2022! I remember it well... it defoliated and downright killed some of my favorite Zone 8 species. I lost Mexican rain lilies, some of my marginally hardy spider lilies, and many of my Gardenias and banana shrubs were also damaged. Luckily, most of my losses were plants or blubs in containers... I don't think I lost any of my in-ground perennials or shrubs. Though some were totally defoliated by winter-wind burn.
And just as cold is required for facilitating flowering in many perennials, when temperatures dip into the teens and single digits, winter foliage species will probably be damaged. And this can inhibit them from blooming the following year.
One notable example is the genus Lycoris, or spider lily. Both my x albiflora (white-flowered) and L. aurea (yellow spider lily) failed to flower this past fall because their foliage was obliterated in the polar plunge of 2022. Though the bulbs survived, repeated winter extremes like this can severely weaken the plants or even kill them.
It also matters the timing and duration of freezing temperatures. What made the December 2022 freeze so devastating was that it came relatively early in the winter before many plants had prepared their cold defenses. Though I can't explain it, there are chemical defenses that plants deploy to prevent cell death from sub-freezing temperates. But this process doesn't occur instantaneously, rather, it must develop over time (a process called hardening off). Plants can be caught off guard without this crucial preparation by even slight dips into the low 30s.
If you are interested in this topic, I'd urge you to check out more comprehensive information via the web. There are so many great videos on YouTube: https://www.youtube.com/watch?v=2LTgQ1u7EcQ
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