Choosing the right frost protection for your situation can be a daunting task. Estimating the risk of damage – how much, how often – and balancing that with the cost and benefits of mitigation techniques is a necessary first step.

Whether the crops would benefit most from irrigation, having a heat source, creating wind or draining out the cold air will be based on a variety of factors, and every situation is different.

Sprinkers used under trees.

The irony to frost protection: doing it incorrectly – at the wrong time or under the wrong conditions – is more damaging than doing nothing at all. An understanding of the basic meteorological conditions during which frosts occur, as well as an idea of how different methods provide protection, plus accurate equipment to help assess the microclimate unique to your fields, are all crucial to successful frost protection.

Temperature plus

Once passive measures of frost protection such as site selection, variety, pruning and cultivation techniques are in place, the accurate monitoring of weather conditions is essential. Specialized temperature monitoring equipment can provide the guidance needed to know when to put your active frost protection plan in place. Knowing what to measure, and when and how to measure it, can mean the difference between protecting the crop and damaging it.

Active frost protection begins and ends with these readings. Having a properly calibrated thermometer is essential, of course, but so is reading it accurately. Various types of thermometers are available, including liquid-in-glass, thermocouple or thermistor thermometers. Each comes with its own set of pros and cons, but the thermometer’s accuracy and ease of use (which ensures you will actually use it correctly) should be determining factors in selection. Having a properly sheltered thermometer that is measuring the actual temperature conditions at the appropriate locations in the fields is the primary tool in the implementation of any active frost protection system.

The wet-bulb temperature, which can be measured using a psychrometer or calculated by knowing the dew point and the air temperature, is critical for some types of frost protection, particularly sprinkler systems. The wet-bulb temperature is the temperature to which the air temperature will fall when a sprinkler system is first started. Then, as the water freezes on the ground and plants, heat is released and the temperature rises.

The difference in wet-bulb temperature and air temperature depends on the humidity level. If the dew point, which is the point at which the water molecules are evaporating from a flat water surface at the same rate as water molecules are condensing on the surface, is equal to the air temperature, the relative humidity is 100 percent, and the wet and dry-bulb temperatures will be equal. Other than that they are never the same and cannot be used interchangeably.

Orchard following overhead irrigation for frost protection.

Hand-held pocket weather meters can help growers measure these critical factors, but old-fashioned methods, such as stirring ice water in a metal can with a thermometer and noting when condensation forms, are inexpensive, low-tech means of keeping tabs on the potential for damage.

Alarm systems exist that can register critical temperature readings and send a signal – phone call, text, strobe light, audible alarm – to alert you of impending frost situations, depending on your needs.

Frost events

The air temperature is a crucial factor in all frost situations, but the difference between the temperature at ground level and aboveground, called inversion, is of the utmost importance to many frost protection methods. In a radiation frost, the air will be coldest at ground level and warmer as the height increases up to the ceiling, which is the point where the temperature begins to drop again. Knowing the strength of the typical inversion in various field locations can assist with placement of mitigation equipment. Inversion can be measured using a thermometer tied with fishing line to a helium balloon 40 feet above the soil for a minimum of 15 minutes and comparing that with the temperature at the soil line in the same location.

In situations where there is typically a strong inversion, water application, wind machines or cold air drains can keep the air temperature around crops above critical points. During an advection frost, active methods are not effective as cooler air flows in and replaces the warmer air with little inversion. Advection frosts typically occur on cold, windy nights, whether or not there is cloud coverage. The temperature will remain at or below freezing even during the day. In comparison, a radiation frost occurs under calm, clear skies with low dew point temperatures, where the temperature changes rapidly. Mitigation involves heat transfer, which is accomplished in a variety of ways.

Because of the basic physiological properties of plants and events such as evaporation, condensation and freezing, applying water, wind or heat at the wrong time or in the wrong amounts can be catastrophic. Whether adding heat, conserving heat or mixing warm air from an inversion, doing it right requires more than turning on the equipment.

Overhead sprinkler used for frost protection.

Knowing the critical damage temperature for the crop, which is dependent upon its growth stage as well as the preceding day’s weather, is of primary importance in any type of frost situation. Frost is defined as the condition that occurs below 32 degrees Fahrenheit, but there are mitigating factors that determine what type of action, if any, should be taken.

Protecting crops with blankets or row covers is a basic way to gain protection against cold temperatures. A variety of blankets or covers are readily available on the market and offer varying degrees of protection. Coverings are not practical in many crop situations, and even when utilized, more aggressive measures may need to be deployed in conjunction with the covers. In certain situations, such as an advection frost, covers may be the best choice, as many other methods rely on the temperature inversion, which is not present during this type of frost event.

Adding heat

Win Cowgill, regional fruit agent for Rutgers Cooperative Extension and the New Jersey Agricultural Experiment Station, reports that the most frequent type of frost protection put into use during the late spring frost events that occurred in New Jersey this year was open burning.

“Basically, growers can burn to raise temperature. If there is an inversion they can mix the air with wind machines or move the air around with big fans,” Cowgill explained.

Burning is not normally permitted except under extreme conditions and must be approved by the New Jersey Department of Environmental Protection, Cowgill said. Due to the mild winter, fruit trees were in bloom four weeks ahead of normal and were at risk of severe damage. According to Cowgill, one northern New Jersey grower had success preventing damage to peach trees by utilizing a system of 400 small, open fires.

Many small fires scattered throughout a field will be more beneficial than one large fire in maintaining the air temperature at crop level above the freezing point, and it works best under calm conditions. One large fire can cause convective energy, which will break the inversion layer, and warm air will be released into the atmosphere, allowing cold air to replace it.

A variety of heaters (diesel, propane or other fuel sources) can also be employed. Heaters need to be placed strategically throughout the fields, particularly in low spots where cold air settles, as well as around the perimeters. Strong inversion situations are required for the most effective protection using supplemental heaters. Heaters, which provide radiant heat as well as burn heat, can be important in high wind situations, where many other systems will fail. Adding heat via open burning or heaters can be effective in an advection situation where there is no inversion and other methods of protection cannot be used.

Overhead sprinkler system layout.

Water systems

Crops can also be protected from frost via the application of water. Whether providing surface or overhead irrigation, the evaporation rate is critical. If water is not applied at a fast enough rate to overcome the heat loss due to evaporation, any heat gain from the applied water cooling and freezing will be lost. The colder the water prior to the frost – and irrigation must begin before the critical temperature is reached – the more depth the water must have to offer the same level of protection in a surface irrigation system or the higher the application rate in an overhead sprinkler system.

There is a great risk of damage to crops due to improper application rates in irrigation systems, potentially even more damage than if no frost protection were provided. The cold, applied water should not be recaptured and reapplied when using irrigation to protect from frost. Starting and stopping the system when the wet-bulb temperature is greater than the critical damage temperature is imperative.

Caution must be taken with surface irrigation, which can provide a 5 to 7-degree temperature increase, so that root asphyxiation or disease does not occur. Erosion issues are also a concern with irrigation systems, and irrigating for frost protection is not practical where water use is restricted or the water supply is inadequate to maintain the high application rates needed.

Trickle irrigation systems, particularly if used with heated water, can also offer some degree of frost protection. Micro sprinkler systems, which utilize less water at a lower pressure, are another alternative and may be recommended where fruit trees need frost protection and the risk of overhead sprinklers causing ice formation that can lead to branch break is great.

Foggers use pressurized water to create fog droplets to absorb upward radiation from the ground, which are then re-emitted at a higher temperature. A fog machine can be used for protection during low wind conditions and works best in high-humidity situations.

Wind machines

Most machines move the air horizontally, mixing the warm air above with the surface air. In any wind-based system, proper placement is critical. Their use depends upon there being an inversion, and they can maintain a 3 or 4-degree temperature increase at crop level. Frost fans move the wind at ground level, while tower wind machines draw warm air from above and mix it with the colder, lower layer of air. As the inversion weakens, wind machines are less effective and can cause major crop damage if used during an advective frost situation.

A Phil Brown frost fan used for frost protection.

Cold air drain units force cold air upward, expelling it away from the crops. In hilly terrain, a cold air drain is placed at the lowest points, where the coldest air accumulates. On flat terrain, the drains are located at the perimeters of the area to be protected. By capturing and expelling the cold air upward at the edges of the field, the cold air is prevented from penetrating the crop area, maintaining a slightly warmer temperature than otherwise. This vertical movement of air means that the cold surface air is forced upward and mixed with the higher, warmer air, where it then becomes warmer itself and continues to rise.

For greater protection, wind machines can be utilized in conjunction with added heat, with the wind being the primary method and heaters added if more protection is needed. Heaters should be located a distance from the machines to have the desired effect of increasing the heat available to the plants. Stopping and starting wind machines must occur at the appropriate temperatures or, just like with sprinklers, more damage than good can occur.

Helicopters can be used to force warm air down to the ground level during an inversion. The temperature increases when air is compressed down by the helicopter. Helicopters can be used in conjunction with other methods to increase effectiveness during severe frosts. They need to fly at the height of the warmest air and make regular passes to keep the warm air near the surface or more damage will occur than if no active frost protection methods were used.

“Note that a wide variety of products are being marketed for active frost protection, but that not all of them have been evaluated with independent, peer-reviewed research to verify their actual benefits under field conditions,” Mark Battany, viticulture farm advisor, University of California Cooperative Extension San Luis Obispo County, cautioned.

Initial costs of the various systems, operating costs, time and labor needed to effectively run and maintain the system, and the risks of crop loss are all factors that must be considered before making any active frost protection decision. Once the best system is chosen and installed to maximize benefits to the crop, it must be used correctly. In certain conditions, it may still be best not to use active protection at all. Wind speed, air temperature, evaporation rate, dew point and more all impact which system should be used and when it needs to be implemented. Prior to investing in any system, growers need a thorough assessment and understanding of their microclimate, normal weather patterns and typical frost risks in order to decide which, if any, active frost protection measures should be implemented.

The author is a freelance contributor based in New Jersey.