With the arrival of cold, frosts appear . It is the great fear of the farmer or producer since it endangers both the economic and labor effort deposited in the production of the crop. At Gardenprue we will analyze the types of frost and offer techniques to reduce damage.
FROSTS: TYPES AND MEANS TO REDUCE RISKS
In white frost, frost or ice forms on the plant material (as we can see in the main photo). This does not necessarily mean freezing of the plant, since some leaves may have a very cold liquid (-4 to -12º C).
Black frost is a general cooling of the atmosphere without a dew point and consequently ice crystals form on the surface of crops. Blackish spots appear on the affected crops, which are the equivalent of cold burns.
TYPES OF FROST
We can classify frosts according to their origin. These can be:
- Radiation frosts
- Advection frost
- Evaporation frost
We are going to comment a little about each one of them:
Radiation frosts appear on clear, cloudless nights with low atmospheric water vapor concentration. In addition, these frosts happen when there is absence of wind (a cold night but without wind, producing a decrease in the thermal sensation).
In this type of frost, the altitude at which the crop is located has an important influence, so that the air temperature increases as we increase the height, with values that oscillate between 5 and 8ºC of difference with respect to 10-15 meters high (coldest on the ground surface).
Advection frosts can occur at any time of the day, not necessarily at night, which is when the atmospheric temperature is lowest. This is caused by cold drafts or arctic currents and in most cases, frost protection methods are ineffective.
The problem originates when on the surface of the plant, generally the leaves, drops of water accumulate, of atmospheric origin, such as rain or irrigation water. As these water droplets evaporate, the latent heat of evaporation is absorbed from the environment and depends on the temperature of the environment, the amount of water on the surface of the crop and the relative humidity.
METHODS TO COMBAT FROST
Previously, we made an entry evaluating the problems of low temperatures in citrus fruits , in addition to the solutions adopted to the problem. In this case we are going to discuss the protection methods to reduce the problem.
Passive protection methods
With passive protection methods we refer to those indirect measures that are chosen to reduce damage once the problem occurs. These ways of protection are the following:
This protection refers to the fact that if an area is prone to frost, avoid planting crops that are vulnerable to low temperatures or do not plant directly and choose another location.
A studied fact is that if the area is depressed in terms of altitude, is at the bottom of a valley or is surrounded by mountains, it will be more susceptible to being affected by frost.
Choice of planting time and species to cultivate
This passive protection method is related to the previous point. If you are in an area prone to frost, choose crops that are hardy or tolerable. In addition, you can also play with the planting calendar or the selection of early varieties that bear fruit before the arrival of the cold and low temperatures.
Hedge with trees
Forest-type trees such as pines or cypresses or ornamental trees such as palm trees can be used to cover the crop, preventing the action of cold winds on the crop.
Tillage of the soil
Soil plowing should be avoided in order to reduce voids that store air at low temperatures and transmit less heat.
It is convenient to eliminate the vegetal mantle that forms around the crops before the arrival of the frosts, since it favors the absorption of radiation by the soil. The difference in temperature that can exist between having a vegetal cover or bare soil can reach 2º C.
Wrap the trees
Favorable results have been demonstrated when covering the crop with insulators such as polyurethane, fiberglass or insulators with water. These insulators should not absorb water and should be arranged from the ground surface to cover them completely, whenever possible.
ACTIVE PROTECTION METHODS
Risk is a method of trying to change the environmental conditions, which are what cause frost (temperature, humidity, wind, etc.).
When water changes from a liquid to a solid state, a latent heat is released that acts as protection for the crops. At this point there are two ways to act. One of them is to have automated systems based on thermometers that activate irrigation when temperatures drop to a point. Having these devices is not normal, if so, you will not have the following way to act. It is about predicting the drop in temperature, either through weather information pages or directly in person on the ground, and from here, carry out abundant irrigation on crops susceptible to frost.
Abundant irrigation on the ground produces the effects:
- The temperature around the soil remains favorable.
- Increases the thermal conductivity of the soil ( k).
- Evaporation from the ground increases.
- Increase in the heat capacity of the soil.
Shower over the leaves
You may consider that applying water on the leaves of the crops prior to a frost is a real madness (they will freeze!). It is precisely what we are looking for. The water applied when the ambient temperature is or below zero forms a film on the leaves or branches that remains above 0ºC (water and ice at the same time), so the water vapor content increases with respect to whether irrigation would not have been carried out. The amount of water is stipulated by the following formula, known as the energy balance in the buds (Hamer, 1986):
Take this formula as a curiosity if your economy does not depend on the crop (otherwise you should stop to take a look). The only thing you have to make sure is that the irrigation has a uniformity higher than 80% and is abundant (let’s say that 7.2 times more percentage of water must be frozen than evaporated).
We are obliged to comment that irrigation on the plant surface of the crop produces an environment favorable to diseases. You have to evaluate the risks and choose, depending on the temperatures, if you prefer other methods or if the crop may be in serious danger from the frost and opt for all the options that we give.
Ground or room warming
These two methods of active protection involve a high cost, since it is opted for the incorporation of stoves or heating means, which heat the air by conduction or by convection.
These stoves can have different fuels: diesel, propane, combustion of flammable materials (plastics, tires), oils or natural gas. Of course, there is nothing ecological about the use of these methods.
Burners or stoves have to be strategically located throughout the crop, preferably between lines of trees or horticultural crops, with an approximate number of between 100 and 125 heaters per hectare or a consumption of 2.5 L of fuel (oil) per stove and time. Given the price of fuel and the duration of the frost, it can cost € 3,500 for one day and one hectare of crops. Will it compensate? (If you are close to the harvest, a very strong frost is expected and your profits exceed € 3,500, the answer is YES.)
Use of fans
As if it were a wind tower, horizontal fans are available from 10 to 12 meters high and 65-75 kW of power, 1 for every 4-5 hectares of crops. They generally use electricity for their operation.
The vertical fans (SIS) are fans that project an air mass warm up and typically consume fuel (gasoline) for operation.
I assume that very few people have private helicopters for frost protection, but why not comment on how they work. By moving hot air (which comes out of the turbine at a high temperature) at an altitude of 15-20 meters, you avoid the concentration of air at low temperature on the crop. It is usually used for high-yield crops and large hectares.
This method is the only one that is NOT effective. Before we have discussed the heaters to produce heat and one of them was the burning of objects, although it is to produce heat energy, not to create a dense cloud of smoke.
The hot air from the smoke rises to great heights where it loses all its heat, and does not prevent the heat from escaping by radiation. In addition, the next day if there is still an accumulation of smoke in the environment, it prevents the entry of sun rays (heat) and damages the crop.
In summary, passive methods are effective in the long term, as well as irrigation (on the surface of the crops or normal irrigation) is economical and its results are acceptable, taking care of the amount and uniformity of irrigation. The use of heaters or fans is more effective but the expense it represents and its economic viability must be evaluated.