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Frost Protection for Tree Fruit

Frost causes the loss of a certain percentage of tree fruit crops almost every year. Susceptibility to frost damage depends on a tree's stage of development, variety, and location, but certain preventative measures can be taken. The goal of all frost protection methods is to maintain the blossom temperature above the critical temperature.

Only a few frost protection methods have been consistently effective over the years. These vary considerably in cost, management time, and effectiveness. It is very important to get a good estimate of actual economic losses caused by frost before deciding whether to implement a frost protection system. A small average annual loss of crop to frost may not be worth the time and money invested in a protection system.

Selecting the Right Site
When air near the ground cools, it becomes denser. If the ground is sloped, the colder air will flow downhill into a pocket or onto level ground. Therefore, planting the orchard on a hill is the first step in frost protection, because there is nothing to trap the cold air as it drains. In some cases, cold air drainage may protect the crop from frost enough that no other method is necessary. However, if a good sloped site is not available or a site already established is experiencing frost damage, other protection measures may be required.

Heating
Burning combustible materials to protect crops from frost originated thousands of years ago. This is the most effective way to maintain the temperature above the critical level, but it is also the most expensive and environmentally damaging. Stack heaters, still available from selected distributors, burn fuel oil stored in the heater's base or injected through nozzles from a fuel line. The heaters often have a diffuser on the top to spread out the flume. Trees are heated through radiative transfer directly from the heaters or from air warmed by them.

Sprinkling
Using water to protect blossoms from cold has gained popularity over the last couple decades. Compared to heating, irrigation for frost protection lowers expenses and reduces environmental damage. Irrigation lines can also be used for soil moisture maintenance, chemical injection, and heat suppression. Although water use can protect against frost, it involves greater risks than heating. Overuse of water can saturate soils, increasing the likelihood of diseases, and build up ice, which may damage trees. Water application rates depend on the desired bud temperature (buds in earlier stages of development are hardier and therefore have lower critical temperatures), air temperature, humidity, and wind speed.

Overtree
Applying water directly to flower buds allows the heat released from freezing water to maintain a bud temperature near freezing, which is a few degrees above the critical damaging level. As long as the rate of water being applied and the rate of freezing are balanced, bud temperature will remain close to the freezing mark. Insufficient application can do more damage than no protection at all, because evaporation may cause flower bud temperature to drop below air temperature.

One problem with overtree sprinkling is not knowing how much water to apply. Application rate models have been developed by modeling the energy balance between heat lost from the buds as a result of environmental conditions, and heat gained from freezing of the water. Rates for different conditions have been determined using the sprinkler application rate model FROSTPRO (Perry, 1986) and are shown in the table below. Note that lower humidities will increase the application rate.

Pulsing
The principle behind pulsing (or cycling) of the irrigation system is that the rate by which water is delivered to the orchard can be varied by turning the water on and off in short cycles, e.g., 2 minutes on and 2 minutes off. First, determine the fixed rate of the sprinklers. If it is not known, you can determine the application rate by placing buckets in a grid pattern in the orchard and sprinkling for one hour. Measure the depth of water in the buckets, and use the average application rate in inches per hour. For example, if the fixed rate is 0.20 inch per hour, and the recommended rate is 0.10 inch per hour, the cycle would be something like 2 minutes on and 2 minutes off. The on time should not be less than the time for a sprinkler head to make a complete rotation, and it is not recommended that the off time exceed 3 minutes.

Undertree
A sprinkling technique that is gaining acceptance is the use of undertree sprinklers to protect trees from frost. How this method works is not completely understood, but it is believed that the heat released as water vapor condenses on leaves and blossoms keeps buds above the critical temperature. This approach uses less water, and there is little or no damage to the tree as a result of ice buildup; but certain blossoms, especially those at the top and exposed to the sky, may receive inadequate protection.

Wind Machines
The lowest several hundred feet of the atmosphere become stratified under calm, clear, frost conditions. An inversion condition thus exists, meaning that temperature increases as it rises to the top of the inversion layer. A wind machine mixes the warmer air from the upper portions of the inversion layer with the colder air near the ground, raising air temperatures around the trees by a few degrees.

Wind machines are motor driven and therefore consume fuel, although not nearly as much as stack heaters. They work under calm, clear conditions as long as the frost is not too "deep"; that is, temperatures are not more than three or four degrees below the critical temperature. Wind machines do not work under cold, windy conditions, because the wind usually mixes the atmosphere enough to prevent an inversion layer from developing.

When using wind machines, it is important that the machines are turned on when the air temperature in the orchard is still above critical temperatures. If air temperature is being monitored in a protected shelter within or outside of the orchard, the machines should be initiated when the air temperature is still above 32°F. It is very possible that bud temperatures may be several degrees below the air temperature due to radiative cooling, and they can experience damage even if the air is still above freezing. More information about choosing and implementing frost protection systems can be obtained from your county extension specialist or from commercial dealers that offer frost protection systems and components.

Critical Temperatures for Various Fruits
The temperature at which fruit buds are injured depends primarily on their stage of development. As flowers begin to swell and expand into blossoms, they become less resistant to freeze injury. Not all blossoms on a tree are equally tender. Resistance to freeze injury varies within trees as it does between orchards, cultivars, and crops. Buds that develop slowly tend to be more resistant. As a result, some buds are usually killed at higher temperatures, while others are resistant at much lower temperatures. Table 1-25 shows the average temperatures required to kill 10 percent and 90 percent of buds. Consideration should also be given to weather conditions preceding cold nights. Prolonged cool weather tends to increase bud hardiness during the early stages of bud development.

Management practices for apples and stone fruits are summarized in Table 1-26.