A frost occurs when the temperature at ground level falls below 0°C. In a frost event, the water in between plant cells freezes. This process draws water out of surrounding cells, causing dehydration of plant tissue. The formation of sharp ice crystals, and the anomalous expansion of the water volume between the cells damages the tissue and causes the cells to burst. If such an event is followed by a sunny morning, affected leaves will suffer from oxidation and sun burn which will amplify the damage.
Spring frost (or radiation frost) is particularly harmful to fruit trees. It typically happens during the springtime when a combination of cloudless nights with little or no wind, low humidity and low temperature generate sub-zero conditions for an extended period of time. As the sun sets, heat is irradiated from the ground and reflected into the atmosphere. On regular days, this heat will protect the tree from exposure to low temperatures that are common during spring. But if there are no clouds to slow down this flow of energy, and no wind to mix air layers, a cold air layer will form close to the ground while hot air will rise up to the higher parts of the atmosphere leaving temperatures near the surface to drop below freezing point.
Damage from spring frosts may range from damaged buds or flowers, ruined fruit, poor crop quality to even full tree loss, and can be devastating to farmers and the local economy. The good news is that there are ways to mitigate and control it.
Frost protection methods keep trees at temperatures above the damage threshold.
Common methods used to protect fields and plantations from radiation frost include smokers, wind machines and propane burners. Naturally, there are pros and cons to each method. The proper choice depends on many factors such as the farm’s topography, cost of the equipment, operational costs and labor requirements.
Using sprinkler systems is among the most popular methods of frost protection due to their efficacy and cost-effectiveness.
During a radiation frost, crops are continuously sprayed with water above the canopy, that turns to ice. The freezing process releases latent heat which creates a micro climate and warms the air near the trees. For every 1 gram of water, 80 calories of energy is released. In addition, a thin layer of ice is created on the leaves that continually forms and melts so that water inside the plant cells will not freeze and the air surrounding the plant will not drop significantly below freezing point.
The frost protection system should be activated when temperatures start to fall below 2.5°C. The shutdown time is only in the morning after the frost event, when all the ice on leaves and trees is melted.
Encasing sensitive plants in a layer of ice may seem like the last thing you’d want to do. But it is highly effective in preventing or mitigating damage from spring frosts, and has several other benefits:
Frost protection using sprinklers requires the system to apply a consistent layer of water on the crop during the entire frost event until temperatures are back to safe levels.
Unlike irrigation systems that can irrigate different plots in shifts, frost protection systems need to cover the entire orchard while maintaining a high flow rate. This results in high system costs, especially from pumping, head control and mainlines. This is where targeted water application rather than full coverage can make a huge difference.
To achieve cost-effective protection, it is advised to design the system in a way that sprinklers are only applying water on the trees and not between the crop rows. For example, if the canopy area covers only 50 percent of the total surface, as in a vineyard, and the sprinklers apply water only on the canopy and not on bare ground, for an application rate of 5mm/hour required for good protection, the system main line will only need to deliver 2.5mm/hour. This will reduce the system cost by 30-40% compared to a full coverage sprinkler system.