One of the crucial moments of the cherry season is the end of dormancy and the beginning of the earliest phenological stages, when plants are most susceptible to cold damage; having a control system for this type of event is essential for the success of the season.
Week 37 of the year and cherry orchards, depending on their variety and geographic location, are showing different phenological states. While in the north they are preparing to start the harvest in one more month, in the central part of the country you can see everything from green tips to flowering.
It is essential at this time of year to consider the occurrence of frost episodes and their proper control; although there are several definitions around this climatic phenomenon, INIA in its document “Frosts. Factors, trends and effects on fruit trees and vines” defines it as follows: “Frost is considered meteorological when the air temperature at 1.50 m from the ground drops to a value equal to or lower than the freezing point of water, that is, 0 °C, regardless of its duration or intensity. From an agrometeorological point of view, frost could be defined as the temperature at which plant tissues begin to suffer damage.”
The lower the temperatures and the longer the frost episode lasts, the greater the damage to plants can be. While frosts usually occur during the early morning or at sunrise, in winter these episodes can last for several hours.
If we look specifically at the agricultural area, INIA in the aforementioned document indicates that: “The existence of agronomic frost may be indicated, which is defined as when the air temperature drops to critical levels for crops without necessarily reaching 0 ºC. However, a temperature equal to or less than 0°C is considered the signal for decision-making in the management measures used to minimize possible frost damage.”
Types of frost
Frosts can be classified according to synoptic origin, time of occurrence, and visual appearance. The first classification, corresponding to synoptic origin, considers:
Advective frost: They usually occur after the passage of a frontal system, when a mass of cold air with high pressures from polar areas enters; that is, they are produced due to the movement of a mass of cold air over a specific region. Among their main characteristics are: clear skies, variable wind, frost in large areas and persistent days.
Radiative frost: They occur during clear nights and are caused by the loss of radiation from the surface during a clear night and dry atmosphere. Their characteristics include: heat loss, night cooling, clear skies, weak winds, local frosts.
Mixed Frost: They are a combination of advection and radiation frosts; rather than being a type of frost, they are a transition between the cold air mass and the loss of heat from the ground through radiation.
Evaporation frost: They are produced due to the evaporation of water from the surface of plants. They usually occur when the relative atmospheric humidity decreases and the dew formed on the plants evaporates, a process that requires heat; this is provided by the plant, generating a subsequent cooling of the same.
Frosts can also be classified according to the time of occurrence:
Spring or late frosts: They are recorded once winter is over and mainly affect annual cycle crops and fruit trees in their reproductive process.
Autumn or early frosts: Episodes prior to the arrival of winter and can be harmful to crops that are in the harvest period.
Winter frosts: They are generated when there is a significant decrease in temperature. They can affect all types of crops, however they are not harmful to deciduous fruit trees because they are in a dormant period.
According to their visual appearance, this type of climatic phenomena is classified as:
Black Frost: There is no frost and the affected plants turn black the day after the phenomenon; the damaged parts of the plants die. They occur when there is a low concentration of water vapor in the atmosphere, which in turn causes lethal temperatures for crops. This is the type of frost that can cause the most damage to agriculture.
White frost or hoarfrost: A less severe climatic phenomenon than black frost. In simple terms, it is the freezing of dew, which occurs when the surface of bodies reaches a temperature below or equal to 0° C.
Frost damage
Plants are susceptible to damage from low temperatures and/or freezing, however the severity of these depends on the type of crop and variety, even when they are subjected to the same temperature and phenological state.
In cherry trees, for example, there are phenological stages that are more vulnerable to frost damage. According to the Avium team's 13 years of experience in cherry production consulting, there are significant records of damage between the swollen bud and exposed branch stages, and more advanced stages, such as white bud or full flower.
Among these states (swollen bud-exposed bouquet) cherry plants are more vulnerable to cold damage; therefore, in the event of episodes of low temperatures, below 0.5ºC for more than an hour, it is necessary to control this type of phenomenon using the different methods that exist and that will be analyzed later.
Frost damage occurs when ice forms inside plant tissue, causing damage to cells: “The formation of ice crystals within the protoplasm of plant cells is direct frost damage (intracellular frost), while indirect damage is the formation of ice inside the plant but outside the cells (extracellular frost), and it is the formation of ice and not the cold temperatures that really damages plants,” states the article “Theoretical and practical basis of the effect of frost on cherry production” prepared by Avium.
Photo 1. Visual appearance of floral primordia in cherry buds, before (A) and after
(B) from exposure to a frost event. (Source: Valenzuela et al. 2020).
The degree of frost damage is related to the freezing and thawing process of water. If the cooling is rapid, its effect is irreversible and the damage is severe, since ice forms quickly in the spaces between cells (meats) and inside the cells, and the cells are unable to adapt.
Subsequently, the cell membrane and protoplasm components break down, causing cell death and blackening. In another case, if cooling is slow, cells can adapt to the changes; in addition, once the extracellular water freezes, an outflow of intracellular water occurs, which compensates for the osmotic deficit. In this case, tissues can survive and recover, however, cells can die due to both denaturation processes and mechanical effects.
The adaptation of plants to cold temperatures in the face of frost is called “hardening” and, as the Avium team explains, “This process is probably related to a higher solute content in plant tissue or to a decrease in the concentration of bacteria active in ice nucleation during cold periods, or it may actually be a combination of both components. The accumulation of sugars or sugar alcohols would lower the freezing temperature of tissues and increase supercooling in many deciduous or leafy trees.”
perennial at low air temperatures.”
During the post-harvest of cherry trees, while preventing abiotic stress caused by high temperatures through different strategies, indirectly it is contributing to improving the “tolerance” of the reproductive structures, which is technically known as “hardening” at low temperatures, due to the better conformation of the buds in the cultivar.
Frost control systems
As the years go by, technology has made new frost control methods available to the cherry industry. The most commonly used methods are briefly summarized below, covering the simplest and most sophisticated tools. It should be noted that there are passive and active methods for frost control, the first type being a strategy related to adapting the plantations to the climate, choosing the best varieties, a planting site and appropriate orchard management.
This time, we will delve into active methods of frost control, the aim of which is to create a microclimate around the plant and prevent cold damage.
Sprinklers: It consists of a high-altitude sprinkling system that allows large quantities of water to be applied to cover the plant; when the water cools, it releases heat into the environment and allows the plants to maintain the temperature at 0ºC and not reach the freezing point that causes damage. It is one of the most widely used methods, as well as being quite economical compared to other strategies.
Flood: It is based on the same principle as spray control, but can only be used in certain orchards. It consists of flooding the soil, thus preventing it from cooling, but it is only applicable to crops that are tolerant to root asphyxiation.
Combustion: Hot air, due to its chemical composition, always tends to be located in layers above cold air; thus, when generating combustion with lighter-type burners that use fuel, a barrier is created that prevents the mixing of air at different temperatures. Special care must be taken with direct heat to the plants, considering this important aspect when placing heaters in the garden.
Fire and smoke: Although it is still a widely used method, research has determined that its effect is practically null: “Bonfires produce whirlwinds within a radius of influence of no more than 20 meters and the area heated by radiation is not significant on a surface under cultivation. The use of bonfires has a limited effect on frost control. On the other hand, smoke produced by burning tires, wood or weeds is not effective in controlling frost,” according to the article “Frost Control in Agriculture – Part I” by INIA Tierra adentro.
Fans: They are the most modern and effective alternative for frost control; their blades mix the hotter air that is at a height of between 10 and 20 metres above the surface of the orchard with the cold air that is on the ground. This allows the temperature of the air in contact with the crop to be raised by one or two degrees and prevent damage from frost.
Helicopters: They have the same effect as fans, moving layers of warmer air to lower areas; however, the scarcity of helicopters and the high costs of use mean that this method is rarely used, except in highly profitable crops.
Covers: They are versatile and increasingly offer more technology, allowing orchards to be covered with structures that protect crops from inclement weather, such as rain, hail or frost, while providing optimal ventilation inside. However, they may need to be supplemented with another type of frost control in the event of more intense episodes.
