Explanation I need for crops+frost+sunlight?

Question

I read long ago in a book by Laura Ingalls Wilder, set in the 1800's, about how a farmer and his family had to rush out to their potato field early one morning because an unexpected frost had come, and they had to individually douse each plant with water before the sun came up and hit the plants with light.
I remember reading that as soon as daylight hit the frosted plant, it would be ruined.

I never understood this...could someone explain please??

Thx

Answer 1

Best Answer - Chosen by Voters

Frost occurs when the temperature drops below 31.5 degrees at the plant level, and the moisture of in the air is high enough to reach the dew point. Frost can occur when the temperature at eye level is above freezing because the cold air settles and warmer air rises. This is why wind machines can sometimes be effective in frost control. They keep the air stirred up and prevent the colder from settling around the plants. Applying water through overhead sprinklers can warm up the plants because the water is above 32 degrees. This can prevent frost from forming on the plants. Before any measure can be effective they must be applied before the frost occurs on the plant. If the plant is exposed to temperatures below 31.5 degrees for as little as two hours ice crystals form within the cells, and the damage is done. The plants will look fine until the temperature reach 32 degrees or more and then the cells will lose turgor, the cell walls will break down, and lose their water. Then the plant will turn black and die. This is why the water must be applied during the night before the ice crystals are formed in the plant cells and continually applied until the sun comes up and warms the air above the freezing point. So it is the heat from the sun that will cause the damage to the plant if ice crystals are allowed to form with the plant cells, not the light from the sun.

Answer 2

When a plant has a coating of frost and the sun comes up, the sun causes a burn from the frost to the plant. The burn will often show as white dots but can do quite a bit of damage.

So, by simply adding water you turn the frost crystals into an icy coating which protects the plant from the burn of the sun.

Source(s):

Years and years of experience

Answer 3

These are some really great answers to an very interesting question.

I just want to add that the temperature of water turning into ice (and vice versa) occurs at 32 degrees F. By pouring water onto ice, the family was effectively raising the surface temperature of the ice. They probably began to carry out this process as soon as they realized that a frost had occurred. They could rest when the sun rose.

If the point of the story was that they had to beat the sun in the early morning, that is because the coldest part of the night is right before sun up.

Most plants from temperate climates (like potatoes) should be able to withstand an overnight internal temp. near 32 degrees.

Like John H. said, growers now use overhead irrigation systems for the same purpose.

Answer 4

Adding water to plants has its best value when the crop is almost cold enough to be destroyed. One often does not know whether it is quite cold enough to kill, but we warm the plant and the air around it hoping to get both warm enough to let them survive.

Mostly the water will be as high as 10C if coming from a well, a long way above freezing. One might have had +8C at sunset, so not much point adding water then. One waits until the air temperature (at one meter from the ground) drops below 4C just to avoid using too much water, waterlogging the soil, disturbing mold. If it never drops below 4C one does not do the watering.