What Makes Frost a Killer?

The Early Season Killer: How Frost and Low Temperatures Affect Young Plants

John Baranick
By  John Baranick , DTN Meteorologist
The typical last-frost date in the U.S. varies from north to south, but local conditions also have an effect. (NOAA graphic)

Risks can be mitigated, but they cannot be eliminated. Ask any farmer and the farmer will likely tell you that the biggest risk during a growing season is the weather. Heat, drought, severe storms and flooding all strain a producer's psyche.

One weather factor that is especially important this time of year is frost.

On Friday, I had written about what is the frost risk potential this spring, especially in places like the Corn Belt. (See https://www.dtnpf.com/…).

But what exactly happens to plants when there's frost? And can plants survive when the temperature is below the freezing point?

LIKE A WATER BOTTLE

Many of us have seen how frosts and freezes have damaged plants, even killed them sometimes. At a cellular level, frost damages plants by literally bursting cells. When water freezes, it expands. If it expands too much, it will cause its container (the cell) to burst. If you've ever left a water bottle in a freezer, you know what I am talking about.

Cold and frost damage make plant material look like it is soaking wet, due to the inner fluids being burst out of the cells. After a few days, the effects become much more noticeable as the plant turns brown or black and wilts.

But frost can have a secondary effect if it forms on the outside of the plant. When temperatures get too low, available water freezes and forms ice crystals. Those ice crystals grow outward and can make for a pretty appearance, but that water has to come from somewhere. It may come straight from the air, but it can also draw water out of plant stoma, desiccating the leaves.

HOW DO PLANTS SURVIVE FREEZING TEMPERATURES?

Water freezes naturally at 32 degrees Fahrenheit (zero Celsius), but plants can sustain lower temperatures than that sometimes for several hours. So how do plants in temperate climates survive the freezing point? They have a few neat ways of warding off the frosts and freezes, and some plants are better than others.

The website "How Plants Work" summarizes some of the neat tricks that plants use to escape the horrible effects of frost damage here: https://www.howplantswork.com/….

Here are a few notable ways that plants can survive temperatures below freezing.

1. We deal with ice on roadways and sidewalks by spreading salt on the ice. Plants can do something a little similar. By accumulating more dissolvable solids like proteins, sugars and other minerals, they can create their own slurry that lowers the freezing point of water by sometimes several degrees.

2. Plants can create "anti-freeze" proteins and push them to the cell walls or excrete them to limit the ability of ice crystals to attach to the leaf or stem surfaces.

3. Some plants can create proteins called "dehydrins" that bind to water molecules and make it more difficult for water to align itself in a crystalline structure that is associated with freezing.

4. They can alter their lipid composition in their fluids to make it more viscous, again making it more difficult for water to form crystalline structures that results from freezing.

5. They can generate a little bit of heat. Though it is not like animals, plants also consume some of the sugars they eat to perform specific functions for growth and development. That consumption generates heat, just like it does for us. When you work hard, you get hot. Plants can do a little of that as well.

WHAT FACTORS DETERMINE FROST DAMAGE?

Even with the potential to resist frost, if it gets too cold then those defenses can be overcome. For agricultural purposes, frost is most damaging when plants are limited in their ability to produce fruit. Therefore, the further developed a plant is, the more damaging frost can be. Defoliation can usually be repaired and replaced, but if the growing point of the plant or reproductive parts are damaged or destroyed, it is game over.

Wheat, corn and soybeans all behave differently to the effects of sub-freezing temperatures. There is a reason why wheat can be sown in the fall and allowed to overwinter, but corn and soybeans cannot.

Wheat is a hardy plant, and its ability to overwinter provides it with multiple ways to avoid frost damage. However, frost can still damage the plants, and the farther along the plant is developmentally, the less effective its coping strategies become.

According to the University of Kentucky Extension, wheat in the tillering stage can regularly withstand temperatures down to 12 F (minus 11 C) before damage occurs. With the growing points starting to come up above the soil surface in the jointing stage, the wheat becomes much more susceptible to frost. Damage can occur down to 24 F (minus 4 C) and in the boot stage, only 28 F (minus 2 C). But when the plant starts heading and the reproductive parts are exposed to the air, damage occurs at just 30 F (minus 1 C). Still, that means wheat can survive sub-freezing temperatures even throughout its life cycle -- it is just a matter of how well.

For corn, the crop is pretty hardy as the thick stalks make it difficult for frost to penetrate to the growing point. The growth point of the plant does not start to emerge above the soil surface until about V5-6 so even frost-damaged leaves before then can be replaced. According to the University of Wisconsin, frost damage that occurs before the V6 stage is usually not an issue, but can result in yield losses up to 8%. However, the cost of replanting and the lower expected yield by using a shorter-season hybrid do not offset the losses by frost. After the V6 time period, however, the plant cannot tolerate temperatures below 28 F and even an extended period temperatures below 32 F can cause damage.

Soybeans are the most delicate plant when it comes to frost damage. Instead of keeping its growing point underneath the soil for a period of its growth cycle, it is just about the first thing to pop up out of the ground. According to the North Carolina State University Extension, it is in the crook stage, when the hypocotyl arch is pulling the cotyledons out of the soil, that soybeans are most susceptible to frost damage. And it makes sense, because once a plant's growing point is destroyed, the plant can only use the leaves it has already produced. But so early in its life cycle, it has yet to produce any leaves, so the entire plant dies.

WHAT WEATHER CONDITIONS PROMOTE COLD AND FROST?

Living in Minnesota, I am used to low temperatures extending through April and sometimes May as well. Waking up and seeing frost on the tips of the grass can be pretty. But for a farmer, it could turn a beautiful morning rather glum. So what produces these low temperatures that harm plants?

The easiest way for cold weather to occur is just by having them move in from farther north. In Saskatchewan, Canada, temperatures in April can be like Iowa in March. When storm systems bring colder air from the north down through the region, the resulting temperatures could be enough to produce cold morning conditions that do frost or freeze damage. Any air sourced from the Arctic stands a better chance to produce more widespread sub-freezing temperatures throughout the Plains eastward to the Midwest and sometimes down toward the Gulf of Mexico, too. These sorts of events typically bring widespread cold damage, but you rarely see the ice crystals forming on leaves. The air is typically drier coming from the north, and temperatures rarely make it down to the dew point where frost would form.

To get those ice crystals, you usually need to be under good "radiational cooling" conditions. These types of frost events usually produce more localized damage as local conditions tend to favor some areas over others. When high pressure settles overhead, skies clear out, and winds stay calm, overnight temperatures fall like a rock as that heat is radiated away toward space. Clouds act somewhat like a blanket, emitting infrared radiation (most people would call this heat) back down to the ground to keep it a bit warmer overnight.

Winds can help mix up temperatures that are a bit warmer above the surface back down to it. But in the absence of both of those, radiational cooling leads to sometimes surprisingly low morning temperatures. When the temperatures fall to the dew point temperature while it is below freezing, those pretty ice crystals form frost.

Radiational cooling can also be influenced by surface cover. If there is snow on the ground, dry soils, or bare soils, radiational cooling is typically enhanced. Snow reflects more radiation than the ground does, and also emits more infrared energy (heat) away. Water on the other hand, is a much better insulator. Wet soils typically hold on to heat better than dry soils. If there are growing plants or dead plant material on the surface, those can act to emit more radiation back to the soils to keep the whole system warm. Even soil texture can play a role whereby sandier soils cool off faster than soils with a lot of clay.

WHAT ROLE DOES ELEVATION PLAY?

Those living in valleys probably understand that elevation can also play a role. If you compare your temperature with your neighbors, you will likely see that those lower in elevation tend to be cooler than those at relatively higher elevation. Well, the heat in the valley radiates upward, which includes to the surrounding hillside. Being down in a valley also shields locations from even slight breezes, keeping the air more still than up on the hill.

And then there is a potential "draining" effect that may occur as well. Cold air is more dense than warm air; it sinks and can flow into the valleys, helping to intensify the cooling that occurs there. Deeper pits or depressions in the landscape will feel similar effects.

Cold and frost can form in a variety of ways, but the easiest and most common are being brought in from the north, or having a clear, calm night. That does the trick, but could bring young plants a host of issues.

Read more about wheat tolerance from the University of Kentucky here: https://graincrops.ca.uky.edu/…

Read more about corn tolerance from the University of Wisconsin here: http://corn.agronomy.wisc.edu/…

Read more about soybean tolerance from the North Carolina State University Extension here: https://content.ces.ncsu.edu/…

John Baranick can be reached at john.baranick@dtn.com

John Baranick