Gardenbite: How Trees Survive Cold Winters | #GoodGrowing

Welcome to the Good Growing podcast. I am Chris Enroth, horticulture educator with University of Illinois Extension coming at you from Macomb, Illinois, and we have got a garden bite for you this week. Extreme cold and your plants. How do they survive? What happens on the inside of the plants to make it so that they don't freeze to death?

But we are going to dive into that topic today. So being a fan of the winter, I would say that I have had a great time with this stretch of really cold weather that we've had. But even so, it has been so cold that much of the people in my household, plus the animals in my household, we've all sort of been cooped up because it's been so cold that it's been almost too dangerous to go outside for prolonged periods of time. So I'll admit it. It's been pretty darn cold.

So at this point in the time, we're in the middle of winter twenty twenty six, and we have had subfreezing temperatures, even pretty common to have subzero temperatures lately, for the last week and probably for the next week here after this recording, at least as the extended forecast shows. So one question I've been hearing a lot about, and maybe you too have heard this on the social medias, is what about our plants? Maybe you've been hearing about exploding trees out in the forest. Well, if you are worried about your plants or your trees blowing up, I will say more than likely, they're going to be okay. If you are a, devout follower of the USDA cold hardiness map and you only select plants that you know are going to fit that, your cold hardiness zone, then sure.

I bet your landscape plants are gonna be just fine. Because even though it has been cold, at least from our perspective, at at this moment of time for this winter, the lows that we've been receiving is really still just sort of falling along our average trends of of a typical Illinois winter that we would often experience. I also do a lot of programming with youth, and then even my own kids, they have questions about plants. And so, you know, most of the time, and as far as my kids are concerned, they've learned their lesson in terms of should they ask their dad a plant question. But a lot of those school kids haven't learned that lesson yet.

And I do hear this question a lot is, why don't the trees die in winter? Because they see all these other herbaceous plants, you know, die back to the ground. And and by the way, a lot of kids are assuming that when a tree drops its leaves or when the plant dies back to the ground, they're assuming that entire plant is dead. That's really not the case. It's not what's happening.

So whether or not you think I'm the best or the worst person to answer this question to a, you know, you know, a youth. Well, it probably depends on your perspective and how long or your tolerance for long winded explanations. So while, yes, many children have wondered about how do plants survive the winter? What's going on inside of that plant? Scientists have been thinking about this very same question for a long, long time.

So let's focus for the most part our conversation today about trees. So how did trees develop this this hardiness, this dormancy mechanism that allows them to survive frigid temperatures in our Illinois winters. Because if you look back into geologic time when, you know, Earth's continents were at different locations on the planet, a lot of the weather was pretty tropical. That was the dominant climate. That's where a lot of our plant life came to be was in these tropical climates.

So how did plants acquire the ability to survive a big cool down? Oddly enough, it was the droughts experienced by plants in our prehistoric world that favored the evolution of plants that were better adapted to winter conditions. Because one of the biggest stresses that a plant faces, especially in the winter, is drought. Because if you think about it, the majority of our plant roots are gonna occupy those that top soil layer, which sort of depends on where you live, but it's anywhere from, like, you know, right near the surface of the soil, two to four inches down, all the way down maybe 18 inches deep. And when we look at our frost layer, how deep our soils freeze in the wintertime, that sometimes, in our neck of the woods in Central Illinois or even farther north, can go up to three to four feet deep.

So, yes, when that soil is frozen, that water, that moisture in that soil is also locked up, and the plant cannot access it. So if we think about all these millions of years, over the course of that time, deciduous and evergreen plants, they have developed these creative survival strategies to overcome drought in that tropical climate, which has then now today assisted them in our more temperate climate where we receive both hot summers and freezing cold winters. Let's take an example of a strategy that a certain species of plants use. Let's look at the evergreen rhododendron leaf. This is a super thick, rugged leaf, and it's developed that waxy coating that protects it from that that freezing temperatures, and it reduces the amount of water that is lost to evaporations.

We can also look at, like, needled evergreens, and these guys have minimized their leaf surface. So they've gone from having these big broad leaves to having these little spiky leaves so that leaf surface is much smaller, which means that there there's less leaf surface area exposed to the wind during the winter, which means they're they're not losing as much water during, a sunny or a windy winter day. If we look at the inside of our plants too, the tissue that conducts water and nutrients from the soil, from the roots up to the leaves called the xylem, it's actually gotten narrower and narrower over time. And so this has developed in response to prehistoric trouts. So these narrow xylem tubes are very useful because this the more narrow they are, it's reduces the size of, say, like, an air bubble that might break that water column in the case of a drought or in the case of frozen temperatures or frozen soil moisture.

And air bubbles in vascular tissue of our trees is not good, so we wanna try to avoid those as much as possible. And we have the thin teeny tiny xylem tissue that really minimizes the size of those air bubbles that can form. Another really effective tool that our trees have deployed are these metabolites. So there's these different chemicals that they can use to actually lower the freezing temperature within their tissue. Probably one of the most popular ones of those is sugar.

So sugar acts as kind of like an antifreeze in our plant tissues. And as winter approaches, our evergreens, what they're gonna do is they're going to, increase the sugar content of their leaves to prevent water in their cells from freezing. Deciduous trees also employ the technique of sugar in their leaves. So if you think about those red fall colors, that's partly due to the sugars in the in those leaves building up to protect that leaf long enough so that it can then resorb that energy back into the the tree tissue, the actual tree, and then the leaf can then drop to the ground. So if you remember back to biology class, our plant and our animal cells, they have a couple differences.

One of the main differences out there is that animal cells, they have a cell membrane. While plant cells, they do have a membrane, but on the outside of the membrane, they also have a cell wall. Now these plant cells, they're either walls, they're also connected each other kind of in these, like, organized like a like a column, or lack of a better term. Yes. But they're organized or stacked on top of each other.

And so picture in your mind, like, these microscopic straws traveling from roots to leaves acting as a highway for cell to cell transport of everything that a plant needs to grow. And between those plant cells, those kind of rigid, boxy, that's how they're usually illustrated, and can appear, plant cells, there's this what's called extracellular space. So it's on the outside of the cells. So the cells are alive, and the extracellular space is not alive. It's dead space, so to speak.

But this extracellular space, it is also has material in it. It has, well, air, gases, and it has water, liquids. And so the so this is that that space between our living plant cells, and this is an important thing when it comes to, this cold hardiness feature, this ability to tolerate freezing temperatures for our our plants. So as winter approaches for a tree that is cold hardy to your area, the plant begins to prepare itself for these freezing temperatures. Those metabolites, such as sugar, they are moved into the cell, which then, as we mentioned, it drops that freezing point, which prevents ice crystals from forming inside the plant cell and sort of popping it like a balloon, popping that that cell membrane.

However, ice does form in that extracellular space. But one of the the nice features is is because there is room for those crystals to form, but there's also the cell wall. It sort of develops a like, an elastic feature. Also, as as it gets colder out in the fall, it becomes more elastic, which then allows it to shrink, allows it to swell, and allows to sort of cushion that plant cell from that ice crystal formation on the outside. So the plant is still technically freezing.

It's just managing where the ice forms. And if you're wondering, yes. Animal cells, they don't do this. Our cells will just pop like a balloon when they freeze, and that's what we when we get frostbite, which is actually a pretty serious medical emergency. So, you know, your cells are exploding because it's so cold, so you you need to go get medical treatment right away when that happens.

As the temperatures get colder and colder, everything does freeze at some point. No matter how much sugar you might have packed in that cell, there is always, well, a point where something will freeze. This is when we get to our cold hardiness ratings of our plants, you know, where some actually are a bit more cold tolerant than others. It kinda points back to that drought adaptations that we talked about earlier. So if we go back inside the cells of our plants, as it gets colder, ice continues to form in that extracellular space.

This is where the and and and so as it forms in the extracellular space, that that water is now ice. It's no longer liquid water. It's no longer available to the plant. And what does water like to do? Well, water likes to be at equilibrium, and so the water in the cell, in the plant cell, will then move and into that extracellular space.

It will freeze, and it sort of dries out that plant cell. As the cell loses more water, it shrinks to a point where it actually will tear apart where its neighboring connecting cells, are located and actually breaks that straw that we talked about. And if the straw is broken, the plant cannot move water from roots to shoots, and it's essentially going to die from the same drought like effects as if there was just no moisture in the soil because the moisture is not moving in the tissue of the plant anymore because, again, the straw has been broken. Now so far this winter, we have been pretty lucky. When we had our snowfall events in December of the same winter that we're in right now, we had quite a bit of snow.

Snow is very insulating. You know, snow is 32 degrees. Even if it's minus 20 outside in the ambient air temperature, if you are underneath the snow, at least you're at 32 degrees. So it's a really good insulating the value to our landscape and to our plants and all the the critters and creatures out there as well. So, yes, snow is good for the most part.

The insulating value of dry snow can quickly be negated when we get those heavy wet snows. And, you know, the idea of branches large branches breaking or kind of altering or damaging the structure of our plants, because of heavy snows, well, then snow is definitely no longer valuable. It's now become a detriment to our landscape plants and and can be dangerous to our plants. It can kill them, damage them in ways that will make them unsightly for the rest of their lives. So, yes, snow is good, but also snow is not so good all the time.

So the other thing, you know, if we are are looking at maybe some ideas of supporting our plants through the winter is that while the, say, the aboveground portions of our deciduous plants, even our deciduous or or or herbaceous plants, They go dormant. The roots never entirely cease their active duties that they're performing, which is why a really good insulating layer of mulch is so valuable and beneficial to a lot of our plants. It's also if you want to make sure this is especially important with, like, newly planted trees and shrubs and especially evergreens because they're gonna hold on to their leaves for the winter. They're gonna hedge their bets that they're gonna be alright. Making sure that they're going from fall into winter with adequate water resources.

So we're not talking about necessarily saturating the soil there where there's just, like, standing water. We just need to have enough soil moisture in there that the plant roots can access it up to the point where the soil moisture will then freeze. So, just making sure that they got those adequate water resources as long as possible in the fall. So we don't wanna flood them, but we wanna make sure that they have water when they need it. There's a few other tricks that people will use, especially if maybe they aren't following cold hardiness guidelines so closely.

So if you're in Zone 5, but you just had to buy that crepe myrtle to see what it looked like in your yard or you fell in love with southern magnolias when you went to visit Southern Illinois and you wanted to bring them up to Northern Illinois. There are some tricks that people can use. It's not a guarantee, but things that you can try. So one thing that folks will do when they have a plant maybe they wanna protect from really, really cold weather is to site them in a Southwest exposure. This is typically going to be the warmest exposure here in Illinois, but it comes with risks, especially if you are a thin barked tree.

So just just be mindful that this is where we can get things like southwest disease, frost cracking, all that that occurs on those thin barked trees those are exposed to that warm sun, coming at them from the South Southwest during the day, and then it can cool off rapidly and cause damage to that plant in the in the cold evening. The other thing we can do is we can use natural or man made structures to block desiccating winds and take advantage of, like, radiant heat that comes off of these structures or hardscapes or or other building structures to kind of create little microclimates within our our our landscape, within our yard. And if you are curious about the cases of exploding trees that have been very popular on social media, it and if you haven't heard yet, the idea is that in extreme cold, trees are just blowing up up in Wisconsin, Minnesota, Michigan, Canada. I would say extreme cold, Yes. It is extremely cold.

The idea of exploding trees, well, that is a an extreme or maybe an exaggerated description of what truly is happening when the trunks experience frost cracking. And if you wanna know more about exploding trees or the kind of more likely the lack of explosions that are occurring in your yard, our colleague, Emily Zweihardt, she wrote an article that explains what is really happening and dispelling the myth of trees blowing up from getting too cold. Well, that was a lot of interesting information about some of those strategies that trees employ to survive our freezing winter weather. Well, the Good Growing podcast is a production of University of Illinois Extension edited this week by me, Chris Enroth. Listeners, thank you so much for doing what you do best, and that is listening or if you watched us on YouTube watching.

And as always, keep on growing.