Frost, Critical Temperatures, and Frost Protection

It is important to minimize those occasions through site selection. In certain years, even site selection will not be sufficient, and then some physical measures may be needed to reduce the damage.

From Grow Quest

In the spring, flower damage is based on the critical temperature for flowers. The critical temperature is that temperature measured on a properly exposed and calibrated thermometer at which flowers or fruits will endure 30 minutes or less without injury. In the spring, this can range from 24° to 32°F. In the fall, damage does not usually occur unless the fruit temperature drops below 28°F

View critical temperatures

The original critical temperatures were derived in Washington State. They were developed by placing flowers in controlled freezing chambers and removing them at certain temperatures to observe the damage. This procedure mimics what may occur during a radiation frost and may not be accurate when an advective freeze occurs.

Extent of Damage

The extent of the damage to flower buds can be variable even though critical temperature thresholds are reached or exceeded. The reasons are varied, including:

• Not all buds are equally sensitive
• Not all trees are equally sensitive
• Not all cultivars are equally sensitive

Factors Impacting Critical Temperature Damage

• Flower development stage - for apples king bloom and pink appear to be more sensitive than those at full bloom. For most other tree fruit species full bloom stage is considered the most sensitive to damage. In general, it is believed the decline in sensitivity is due to increased water content in the floral parts with a concurrent increase in the ability to supercool.
• Windy conditions allow for a greater mixing of air and may not reflect the actual temperature the flower experiences.
• Humidity slows temperature changes. Under low humidity, temperature change may occur more rapidly.
• Cultivars have different susceptibilities to cold temperatures. Delicious has long been known to be more sensitive to cold temperatures and may exhibit greater damage
• Tree vigor may affect the water content of the floral parts and their ability to supercool
• Weather prior to the freeze event - the exposure to low temperatures and dry conditions prior to a frost event can induce greater hardiness and resistance to frost damage
• Rate of thawing, the slower the rate of thawing in the morning, may have less potential damage

Types of Frost

Advective Frost

Advective freezes occur when a large mass of cold air moves into the region. It may or may not be accompanied by clouds. The cold air originates over the snow-covered polar and arctic regions. The air mass then migrates into the region and is usually accompanied by winds that exceed 5 mph (8 km/h). Temperatures at the surface are below freezing and become colder with elevation. Little can be done to protect the trees other than the possibility of covering a crop. Heating is not economically feasible since there is no inversion. If the winds are not too high and the temperature of the air mass is not too low, sprinkling with water can help to reduce the damage.

Radiation Frost

Radiation frosts occur when the heat that has been stored in the upper soil surfaces is radiated back into the atmosphere at night. Wind speeds are generally low, and an inversion usually develops with temperatures near the ground surface falling to, or below, freezing.

Supplying heat from fires or self-contained heaters heats the orchard air by convection. If an inversion exists, the heat will rise until it reaches similar air temperatures and will slowly build downward until the air profile is similar in temperature. As long as that air profile is above the critical temperature for a particular fruit crop and flowering stage, then the flowers and fruitlets are safe from damage.

Factors Affecting the Temperature Drop

• Cold air is heavier than warm air; therefore, it will naturally flow down to lower elevations. Orchards or orchard pockets located at lower elevations will see more damage due to the collection of this cold air. This is the reason that site selection is critical to avoid low-lying land.
• Clouds can be an orchardist's ally. Radiation from the soil is trapped by clouds and does not pass through them. Therefore, there is less chance of a radiation frost on cloudy nights as the heat cannot radiate unimpeded. On the other hand, smoke from burning fires does not trap the radiation and the heat passes directly through to allow the surrounding air to cool.
• The rapidity and depth of the temperature drop are influenced by the dew point. Dew point is the temperature at which moisture begins to condense from an air mass. The general rate of temperature decrease due to radiative losses can be fairly rapid until the air approaches the dew point temperature, when atmospheric water begins to condense on the colder plant tissues (which reach atmospheric dew point temperature first because they are colder).
• The more water vapor in the air, the higher the dew point. Knowing the dew point can help determine how quickly a grower will need to respond to frosty conditions. When the dew point is above freezing, then the temperature drop will be much slower. If the dew point is below the critical temperature, then the temperature drop is faster. Low dew points also indicate dry air and potentially more difficulty in heating the orchard.
• Windy conditions may break up the inversion and do not allow the lower levels of the atmosphere to remain stable, allowing the buildup of heat.

Monitoring for Frost

Good environmental monitoring equipment is essential. Some fruit-producing regions may provide temperature monitoring networks (see Washington State's AgWeatherNet). It is also possible to purchase individual systems that growers can operate on their own farms. In Pennsylvania and many surrounding states, the NEWA weather monitoring system has been in place since 2013.

Thermometers should be calibrated each season. To calibrate a thermometer, in a large bucket or container, place an equal amount of ice and water and insert the thermometer in the container and swirl the thermometer in the ice bath. A properly calibrated thermometer should read 32ºF in the ice bath solution.

There are recommended standard shelters and heights at which to place these thermometers (However, that is more important for National Weather Service and scientific measurements). Place thermometers in lowest sites in the orchard at tree canopy height of fruit. If thermometers are placed too low, you may start your frost protection method too soon. Handheld digital thermometers can also be used, but should also be calibrated.

Methods of Frost Prevention: Passive

Good site selection is the most economical and effective method of frost protection. No frost protection mechanism can overcome poor site selection for the orchard.

Soils

Soil conditions make a great deal of difference in frost protection. Heat is absorbed by the soil during the day and released to warm the blossoms at night and early in the morning. Maximum exposure of the soil to sunlight is necessary to provide optimum frost protection.

Soil type

Dark-colored soils absorb a greater amount of heat during the daytime and can potentially store more heat. Gravelly soils also have a greater capacity to absorb solar radiation

Soil Water Content

Compacted bare, but moist soil can store a greater amount of daytime solar radiation heat than covered dry soil. The thermal conductivity and heat content of soils are affected greatly by the soil water content. On a daily basis, heat is transferred into and out of approximately the top 0.3 m (1 ft.) of soil. When the soil is wet, heat transfer and storage in the upper soil layer are better, so more heat is stored during daylight for release during the night. Considerable differences between thermal conductivity and heat capacity are observed between dry and moist soils. However, if the soil water content is near field capacity, wetting the soil is unnecessary. Wetting the soil to a depth below 1 foot (0.3 m) is unnecessary because diurnal temperature fluctuation is insignificant below that level.

Soil texture

Heavier soils with more clay retain heat better than sandy soils. Sandy soils are also often lighter in color and hence tend to reflect more sunlight rather than absorb it in the form of heat.

Ground Cover Management

When grass or weeds are present in an orchard or vineyard, sunlight is reflected from the surface and less heat energy is stored in the soil. A tall, dense cover crop has the potential to reduce nighttime temperatures by up to 10°F (5°C) whereas, a closely mowed cover crop is usually only about 2°F (1°C) colder than bare soil. Therefore, during the frost season, it may prove beneficial to keep the row middles in the orchard closely mowed to increase potential daytime heat absorption and reduce nighttime heat loss. Similarly, vegetative mulches usually reduce the transfer of heat into the soil and hence make crops more freeze-prone.

A bare, undisturbed moist soil with no ground cover vegetation can release enough heat to raise the temperature 2 to 3 degrees in the plant canopy as compared to a sod mulched covered soil.

Active Methods of Frost Protection

Heaters once were commonly used. However, due to fuel costs and pollution concerns have largely fallen out of use. A few may still be in use in PA and the eastern U.S. Heaters are usually found in older orchards that have been in business for a long time. There are several different types. A more common practice is the burning of pruning brush situated within the orchard. Care must be taken to avoid the spread of fires due to windy conditions.

There were basically 3 types of heaters:

• Cone-type heaters provide a little more heat.
• Return stack heaters use a little less fuel.
• Self-contained heaters with fuel in a bottom reservoir (see image below).

Mobile Propane Heater

Developed in South America and operates by adding warm air from a propane-fired heater attached to a tractor. They are sold under the trade name of "Frost Dragon". In order to properly protect an area, the heater needs to return every 12 minutes to the start point of the predetermined route through the orchard. This requirement can limit the amount of area that can be protected. Uneven terrain, which requires slower tractor speeds, further limits the potential protection area.

Wind Machines

Wind machines are becoming more popular. Depending upon the terrain of the orchard, a single wind machine can protect up to 10 acres (4.0 ha). Wind machines work best when used to maintain temperatures rather than trying to heat orchards. For heating benefits their effectiveness depends upon the existence of an inversion (heaters and small fires can work without inversion to some extent). They can be combined with heaters where the heat generated by the heaters is blown across the orchard. Small fans are also available that attach to a tractor PTO shaft and can be used for areas such as small swales.

Helicopters

Helicopters work when there are multiple sites to protect and protection is only needed occasionally. They work by mixing warmer air with the lower, cooler air so an inversion is necessary. It is necessary to contact the operators well in advance, and you should expect to pay more. Be aware of site limitations such as large, tall trees and power lines that may surround an orchard. Check local regulations for restrictions that may prohibit helicopters from flying at night. Like the Frost Dragon, the helicopter must be able to return to any one section every 5 to 6 minutes to prevent air stratification from reoccurring.

Over-Tree Irrigation

Overhead irrigation works on the principle of the latent heat of fusion. As water goes from gas to liquid to ice at each phase change there is an amount of energy given off. Application of water during a frost allows the continual formation of ice and maintains the temperature of the flower parts near 32°F (0°C). To be effective, the irrigation system must be started when the air temperature has dropped to 33°F (1°C).

• The amount of water needed depends on the temperature and environmental conditions.
• Originally as developed, it was necessary to apply water continuously once the system was started. Research by Dr. Paul Heinemann at Penn State University has shown that the water application can be cycled on and off. The result was a reduction in the use of water by up to 1/3.
• Typical water application rates are 0.10 to 0.15 inch/hour (2.5 to 3.5 mm/hr.). If needed, sometimes running the system on consecutive nights can result in excessive waterlogging in the soils.
• Overhead irrigation, at best, can only protect down to 24°F. It will not work in high winds, and if winds are strong, it may be best to not turn the system on at all.
• An overhead irrigation system for frost protection requires a permanent setup of pipes, pumps, and mainlines. However, the same system may be utilized for summer irrigation and evaporative cooling near harvest to improve fruit color.
• The system is turned off in the morning when the ice is melted and temperatures are rising. Ice-breaking free from branches indicates water is forming under the ice. Be sure that the temperatures continue to rise.
• Recent modifications of this system by Rieger in Georgia include individual risers at each tree with micro-jets that can operate at low pressures.

Under-Tree Irrigation

Irrigation applied under the tree canopy has not been used in the eastern United States, but has been tried in the state of Washington. The amount of protection from under-tree sprinkler systems depends on both the amount (mass) of the water and the temperature of the applied water, as limited by the strength of the thermal inversion.

Research in Utah also suggests that the relative humidity of the air mass may also impact the success of the system. Most of the systems use small (5/64–3/32-inch), low-trajectory sprinkler heads operating at 40–50 psi. Applications range from 0.08 to 0.12 in/hr. (40–55 gpm/A) or a little more than half of the over-tree requirements. Sprinklers are usually turned on around 32°F, or earlier if dew points are low, in order to raise the humidity as much as possible and prevent freezing of the risers and sprinkler heads.

Another variation of under-tree sprinklers is to irrigate with warm water. The application of water at 21°C in Mexico has been utilized for frost protection. Systems that heat the water have been tested in Washington and other areas, and they have been shown to be a much more efficient use of heating oil. These systems utilize large stationary boilers/heat exchangers at the side of the field and heat water for application through the existing under-tree irrigation system. The heat is thereby uniformly spread over the orchard floor.

Application of "Anti-freeze-type" Materials

There are several commercial products that claim to have frost protective properties when applied to trees (e.g., Frost Ban, Antistress, Envy, Seasol, and Teric). There has been no scientific proof that the materials listed reduce the occurrence of frost damage. However, an article in HortTechnology reported a material developed by scientists at Miami University that was shown to increase resistance to both cold damage and cold mortality of plant foliage, flowers, and fruits. The formulation has been commercialized under the trade name FreezePruf. (See HortTechnology 21(1):109 - 118). Again, it has not been tested in orchards in the mid-Atlantic region.

Frost Avoidance Through Bloom Delay

In the late 1970s, researchers at the University of Kentucky, Utah State University, and Ohio State University demonstrated that the application of intermittent overhead water when air temperatures were above 45°F (7°C) would delay flower development to avoid early-season frosts. Overhead misting effectively delayed bloom up to 10 to 14 days, but resulted in a problem of excessive water and reduced fruit set.

Application of Ethephon in Stone Fruits

Research indicates that the application of the growth regulator Ethrel to peaches and other stone fruit in the fall prior to flowering can delay bloom. The manufacturing company, however, does not want to take the liability to label the product. The application of soybean oil has been studied with support from the U.S. Soybean Board. The amount of bloom delay has varied. Work in Pennsylvania suggested as much as a 4-day delay on peach flowering and a little less in apples. The major drawback was the death of some flowers. More research is needed in this area.

Additional References

Snyder, R. L., and J.P. de Melo-Abreu . Frost Protection: fundamentals, practice and economics. FAO Corporate Document

Bootsma, A., & M. Brown. Freeze Protection Methods for Crops.

Gohil, H. & M. Muehlbauer. 2020. Frost Protection in Orchards – What Should You Monitor