Thread: Why spraying H2O on plants is crucial during a freeze

As we know in agriculture areas, farmers spray water on their crops to protect them before a freeze. How would you explain this , mentioning the hydrogen bonds responsible.

~Nav

Huh. I've always watered my potted plants during hard frost, I thought to keep roots from desiccating. I'm curious to learn more about this.

Yes, this is one of the questions that showed up in my AP Biology textbook. Not many people seem to know the answer. Hence I guess it remains to be answered yet. [/b]

I always assumed that it was the latent heat of fusion. The leaf is radiating heat away which will result in freezing of the cells at the surface of the leaf. The liquid water on the surface of the leaf would have to freeze first before that could happen and there isn't enough heat being lost. Of course, if it gets cold enough, you will just have your plants covered with ice.

Does a small layer of ice not act as an insulator? Protecting the sides inside the leaf.

A question really, really not sure if it's true ^_^.

Spraying water on orchard trees is a common practice in eastern Washington in the spring to protect against frosts and deep freezes.

They also put fans in places like vineyards to keep air moving and prevent freezing. This is done usually in the spring because the emerging buds are vulnerable to frost. The fans circulate low, heavy, cold air with higher, lighter, warmer air and is effective to about 25 degrees or so.

But the trick with sprinklers is to ensure the water is constantly applied until the temperature returns to back above freezing. This is because theres a tiny bit of heat that is created as water freezes... so as long as the water is continuously applied and continuously freezing, the plant is protected. This is usually effective in regions where temperatures only dip below freezing in the early morning hours and back above again by mid-morning.

Harold's explanation makes sense.

Another explanation: Think of plants as wicks, that need to keep wet. Say wicks are cold hardy to varying degrees, but no wick can stand drying. So what happens when cold shuts down the wick's circulation? Water in the fringes must evaporate or sublimate, without replacement. You would want to replenish the wick where it it drying out.

I guess.

Frost is formed by water vapour present in air freezing onto surfaces. This requires more heat from the surface than simply freezing liquid water. The plants temperature remains higher through one phase change (watering), than reverse sublimation (frost).

They are all good opinion. However Harold's opinion makes more sense. I would assume that the mere heat or energy escaping through the plant cell's epidermis would keep the internal part of the water drop moist and wet while the outer part would freeze along with the frost. Therefore when the frost does hit, it only effects the outer coating of the water drops, which are already frozen, while still keeping the hollow moist internal or lower part of the water drop close to the plant's cuticle or epidermis. This can be seen in oceans. When the water on top freezes, life under the ocean still thrives perfectly. Events that occur on the top of the frozen tundra do not have any effect on the life under the sheet of ice.

~Nav

Originally Posted by NADH dehydrogenase

They are all good opinion. However Harold's opinion makes more sense. I would assume that the mere heat or energy escaping through the plant cell's epidermis would keep the internal part of the water drop moist and wet while the outer part would freeze along with the frost. Therefore when the frost does hit, it only effects the outer coating of the water drops, which are already frozen, while still keeping the hollow moist internal or lower part of the water drop close to the plant's cuticle or epidermis.

~Nav

I also agree with that. The explanation for that gave Geo. By freezing the water on the surface, the water molecules get into a solid crystal structure. They must pass their kinetic and rotational energy. This is thermal energy, actually. That energy is transferred to the plant and keeps it warm and prevents it from freezing (at least at not to low temperatures).

Originally Posted by NADH dehydrogenase

They are all good opinion. This can be seen in oceans. When the water on top freezes, life under the ocean still thrives perfectly. Events that occur on the top of the frozen tundra do not have any effect on the life under the sheet of ice.

~Nav

That is a different effect. It deals with the water abnormality.

Originally Posted by neird

Originally Posted by NADH dehydrogenase

They are all good opinion. This can be seen in oceans. When the water on top freezes, life under the ocean still thrives perfectly. Events that occur on the top of the frozen tundra do not have any effect on the life under the sheet of ice.

~Nav

That is a different effect. It deals with the water abnormality.

Yes, I would have thought that had to do with the fact that ice floats on water (and then acting as insulation), rather than being related to the plant spraying thing.

Ice acts as an insulator. That is the only reason I have heard in some 25 years of working with plants for spraying down plants in an extreme cold snap where overnight lows are exptected to drop in the 20 F range. The objective being to keep the plant at 32 F.

Hydrogen bonds give water the property of being less dense as a solid then a liquid and so there is more empty space in ice ( ice gets bigger more volume when it freezes ) the spaces giving ice it's greater volume are filled with air and air is an excellent insulator when trapped. Air being trapped we avoid heat loss by convection. Conduction now being the only way to loose heat .. condiction being a slower way to loose heat.

I would think this strategy would work best in places where temperatures bounce into the danger zone of the temperature scale that is the temps return to a safe range when the sun rises as opposed to a long deep freeze.

Watering plants or not watering plants in regards to hydration has often been talked about but I don't think any special watering is necessary but simply apply normal water as needed and let the plant deal with the cold stress on it's own. Plants are "smart" that way as they have a complex stress physiology to help offset stress and limit strain that is permanent damage to the plant.

MB ...