By John Moons

Night frost is the occurrence of freezing conditions that happen at night. This article is about frosts that happen in spring and fall that can cause damage to plants or parts of plants, which can lead to a decline in yields. Night frosts happen in many parts of the world. Here in Ontario, fruit growers can be victims, but so can growers of corn, soybeans and other vegetables and those who grow or deal with annual flowers and vegetables.

We have all heard stories of orange orchards in Florida or California that were hit by night frosts or coffee trees in Brazil that suffered a similar fate.

When we consider the fact of global warming, you would expect damaging night frosts to occur less frequently. I find this only true for frosts that happen in the fall. Here in southern Ontario, we have had severe damage to tobacco crops as early as late August. I have seen a nice stand of my Zinnias killed several times by frosts in the first few days of September. Over the last 10 to 15 years, the very first frosts don’t happen until the middle of September and damaging frost is even later. I can enjoy my Zinnias into October.

When it comes to damaging frosts in the spring, I do not see any changes over the last 30 years. Every few years, we see a real killer frost in the second-half of May. Sometimes it is only one night, but that is all it takes.
 

What damage is caused by frost?

In the fall, corn and soybean plants can be killed before the kernels or pods fully ripen. This will result in lower yields. In the spring, damage is different depending on the plant. I have seen $8,000 worth of bedding plants killed in one night because people at a big box store did not have a clue what was going to happen. We have had night frosts in southern Ontario where young soybean plants were sufficiently damaged that fields then had to be reseeded. Corn plants that were 50 cm high have been hit by frost, but only the growing point was killed. You will not have any cobs, but the plant is still good for silage. Fruit growers are probably the ones who suffer the most. In the flower of apples the style is very frost-sensitive. Pollen that lands on the stigma cannot make it through the style to reach the ovary and no pollination takes place if temperatures have dropped too low. Not all cultivars show the same damage. Some cultivars have all the flowers within a few days and a good frost in that time span will do a lot of damage. One good night frost can mean no crop and no income for that year. Golden delicious apples, however, will bloom for quite some time. The first flowers might be damaged, but in the flowers that bloom later, pollination and hence a crop, is still possible. Vegetable growers can have all their young tomato or pepper plants killed by a good frost. Tree nurseries can be hit too. Here, it is mostly young shoots or leaves that freeze and can be killed. Just when you are ready to sell nice, healthy looking plants, you may find that frost has made them unsalable. The plants do not get killed. Dormant buds will wake up and give you a new crop of shoots or leaves, but that takes a number of weeks.

A few years ago, we had a really good frost in May. The strange thing about it was that a lot of the native trees lost all of their leaves, where all of the imported species kept their leaves. Later that summer we could see the trees had grown new leaves and we suspected there were more leaves than the year before. Early blooming Magnolias can have their flowers damaged by a frost. Bulbs and perennials can also be hit. A nice stand of Narcissus can be ruined by a good frost. The flower stems develop a weak spot and the whole flower will hang down and eventually dry out. Iris flowers can freeze in the bud. Fallopia japonica (this is the old Polygonum cuspidatum) freezes down to the ground here every other year, but they always come back from dormant buds in the ground.
 

What is the mechanism behind night frost?

I will not go into too much scientific detail. Heat from the sun comes to us by way of shortwave radiation. The wavelength is from 0.3 to 2.2 mu (visible light is in the range of 0.36 mu to 0.76 mu). Of course, this happens only in daytime when it is light outside. How much energy is absorbed by the ground and things on or near the ground, depends on a few factors. One important factor is albedo: this is the ratio of the amount of radiation reflected by a body and the amount of radiation that has hit it. A few values here are: fresh fluffy snow is 75-95 per cent, dense cloud cover is 60-90 per cent, sandy soil is 15-40 per cent, wood is 5-20 per cent, dark cultivated soil is 7-10 per cent. If the albedo is a high number, then not a lot of energy will reach the earth and heat it. Incoming energy can be absorbed as heat or radiated out as a longer wave radiation. Of course, another way to heat up an area is when warmer air is brought in by the wind. An area can be cooled down by two different factors. One is when cold air is brought in by the wind. This does not usually result in night frost. The earth and the atmosphere radiate heat out as long wave radiation of 6.8 mu to about 100 mu. This process takes place day and night. How much is radiated away depends on a number of factors. Some of these are: the type of soil, the amount of vegetation, loose or compacted soil, dry or wet soil and clouds or obstacles overhead.


Sometimes there are eddies just above the ground and this mixing of air can have a warming effect. The long wave radiation is by far the most important factor in the cooling process at night. The coldest spots are right at ground level.

On a grass field, we will find an inversion layer at 50 cm to 100 cm above the ground that will have warmer temperatures than at ground level. In a stand of trees the inversion layer is above the tree canopy. Above this inversion layer the air will get cooler as we go higher. Later on, we will see this inversion layer can play an important role in battling night frost damage.

The greatest drop in temperature will occur on nights without clouds or wind. The heat is radiated out into space. If there are clouds, the radiation will be absorbed by the clouds and partially sent back to earth, giving a warming effect. You can see this effect when you drive around on a winter night and check the outside temperature. In an area with no cloud cover, the temperature will be lower than an area with cloud cover.

When you drive into a valley, you will find yourself in a frost hole. Because cold air is denser and heavier than warm air, the cold air will sink through the warm air and find the lowest point in the landscape.

On long slopes you can experience the movement of this cold air as wind, especially if the air can flow out of the valley along the valley floor. There can be a big difference in the temperatures between the edge of the slope and the bottom of the slope.

The amount of radiation from the ground affecting air temperatures is also dependent on the surface of the ground. In fall, I always want to record the first time we have a night frost. When my weather station (which is mounted at 150 cm above the ground), tells me the temperature is +4.5 C, I can go out and find frost. A good place to look is on a vehicle sitting in the middle of a field. The metal of the vehicle cools down quickly because there is not enough heat coming from the ground. If I lay a piece of sheet metal on a sheet of styrofoam on the grass, that will be the first place to see frost. That means there can be a difference in temperature of 4.5 C over a vertical distance of 150 cm — and that is quite a bit. The insulating styrofoam will prevent radiation and convective heat from the ground to warm up the layer of air above the foam and frost will occur.

When there is a light frost, it is always interesting to see where there is frost and where there is not. Under a big tree with leaves there is less frost because the radiation gets intercepted and bounced back to earth. The same is true for locations close to buildings. If you have a field with deep ditches or even valleys, these areas will show you the first frost. If you have a sloping field with wind breaks all around, you will find the first frost in the lower part of the field. If you have fruit trees on that field, the ones on the lower end will have frost damage and the ones on the top could be okay. If you have wind breaks on three sides but not on the lowest side, then the cold air will flow out to the lower neighbouring field and your neighbour will get the damage.
 

Artificial protection against the frost

Artificial protection against frost begins when you choose a location for your sensitive crops. It is useful to remember the differences between advective frost; whereby cold air comes from somewhere else by way of a light wind, and radiation frost, where the cooling takes place because the heat is radiated out into space. Both types can happen at the same time. When the land is flat there is not much you can do. If your land is sloped, the cold air can move around. The cold air will collect at the bottom of the slope. By placing windbreaks higher up the hill, you can steer away the flow of cold air to the sides of the field. Make sure you do not send your cold air to your neighbour’s crop. You also have to consider the type of trees for your wind break. Windbreaks work best when they are in full leaf, so you need trees that leaf-out early or keep their leaves (dead or alive ones) all year long. It might be necessary not to plant on the bottom of the slope, especially if there is no way for the cold air to flow away at the bottom of a valley.

The top of the slope is the most likely location to stay frost free. In order to get an idea where the most frost will occur, you will have to make a lot of observations with thermometers. When laying out an orchard on a slope, pay attention to the direction of the rows so cold air does not get trapped between them. Keep the grass short for better air flow. When protecting plants, 0.5 C can make all the difference. Also, the length of time the plant (or part of the plant) is frozen is critical. Does the frost start at 10 p.m. and last until 8 a.m. the next morning? Or do you have a bit of frost by the time the sun gets up? The coldest part of the night is just past the point the sun gets up. I do not believe for a moment that frost only occurs when there is a full moon. For frost to occur you need a clear night. On a clear night, you can see the moon, but when there is a new moon you can still have a clear sky and you will have frost. Over my life, I have looked at the stars enough that I can testify to this.

Actual things you can do to prevent frost damage can be fourfold. You can cover up plants, supply heat, circulate air and bring in water through an irrigation system.
 

Covering plants can be very effective

Backyard gardeners put sheets over their tomato plants and it works. In an empty greenhouse with a double layer of plastic, the difference in temperature at 150 cm above can be as much as 4 or 5 C between the inside and outside of the greenhouse. For a single layer greenhouse, it can be up to 3 C. Put plants up on a bench away from the coldest spot on the ground. Insulated heat blankets work well, and a single sheet is already better than nothing. It is a bit of work, but worthwhile. I have seen a field of tomato and cucumber plants saved by putting black plastic containers over the plants. The pots were put over the plants by 3 p.m. The time is important. The sun had enough time to heat up the space and the soil under the pots, so all the plants survived a hard frost that night. Make sure the soil is damp. A damp soil holds more heat than a dry, dusty soil. Leave a hole near the ground. It is better to have a bigger pot than a smaller one. It will be worthwhile to move sensitive plants into a barn or shade hall so they are covered up.

 

Supplying heat

A garden centre can benefit by adding propane heaters like the ones you see on restaurant patios. They are portable and you can adjust the amount of heat. Using a fan to circulate the air will be even more effective. Use a sheet of plastic to surround a block of plants so that cold air from outside does not cool them down unnecessarily. It might be worth the cost of having someone on-guard all night for safety and security.

In vineyards and orchards, growers have used all kinds of devices to create heat. Burning stacks of firewood at regular distances can work. Years ago, it was easy to get coal to burn for heat. It is a cheap system to set up, but needs constant attention. You can burn oil in barrels too. Creating a fire with smoke is much better, because the smoke will limit the radiation and might radiate heat back to the plants. Be careful where the smoke goes. You do not want it to go over a public road and reduce visibility. Using only smoke to control frost damage is not easy because the slightest breeze can blow that protection away. In the past, you could get special oil burners that would work for many hours without a lot of supervision. In Europe, at one point, you could buy paraffin blocks with wicks in them. They could easily last one night and with enough of them you could get good protection. On a slope you will need the most heat at the bottom, so that is where you start heating first. It is always very useful to use a good number of thermometers to see where the lowest temperatures are.
 

Circulating air

On a night with frost, the lowest temperatures are near the ground. You will find warmer air higher up. By mixing that warmer air with the cooler air below, you can create temperatures above freezing around the sensitive parts of the plants.

In California, they use helicopters to fly over orchards all night long to prevent frost damage. It’s not cheap, but it works. For decades, growers have installed big fans on towers that provide similar protection. There are all kinds of configurations possible and the systems do work. When you drive around in the grape growing area of the Niagara region you will see numerous fans. A few smaller fans are better than one super fan because you spread the risk better.
 

Using an irrigation system

To understand this system you have to know that energy is released when water freezes. This energy will keep the sensitive parts of the plant at the freezing point, but not below it.

In high school physics and chemistry, we took a container half-filled with ice and half-filled with water and set it in the freezer. Then every five minutes we read the temperature. As long as there was water in the container the temperature would not go below 0 C. As soon as all the water was gone the temperature would drop quickly. This principle is the reason why it makes sense to use water to prevent frost damage. You might see a lot of ice, but you have to make sure the ice stays wet on the outside. You will have to come back every one or two minutes to spray the ice. If you stop halfway through the night you are in trouble because ice conducts the cold very quickly and your plants will be damaged.

This system can protect your orchard up to -5 C. My dad had an orchard in Holland and we always had a crop. I spent many nights wandering through the orchard and despite a huge build up of ice, we always had apples (see photos by my brother M. Moons). They did not get damaged too much. The wood of plum trees is a lot more brittle and they would be damaged a lot more. We would brace the trees before we were expecting night frost. We turned the irrigation on as soon as the lowest branches reached the zero degree mark. It was absolutely necessary to keep watering until you could see liquid water appearing between the ice on the stem and the stem itself. Sometimes we did not reach that point until two hours after sunrise. We had nights where the irrigation ran for 10 or 11 hours. You do not need a lot of water. Four or five millimetres per hour will be enough, but the sprinkler should make one rotation every 60 to 90 seconds.

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Sources:
• Barkman, J.J. and Stoutjesdijk, Ph., 1987. Mikroklimaat, vegetatie en fauna. The Netherlands: Pudoc, Wageningen.
• Geiger, Rudolf, 1965. The climate near the ground. London, England: Harvard University Press Cambridge, Mass.
• Oke, T.R., 1983. Boundary Layer Climates. New York: Methuen, London.

 

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