How can you protect your produce?

Below-normal temperature on Easter weekend 2007 will long be remembered for the killing freeze that cost growers from West Virginia to North Carolina to Texas millions of dollars in ruined fruits and vegetables. Blooms on fruit trees turned brown and never matured. Green leaves, withered and dropped off fruit trees and plants. In much of the South and Southeast, 100 percent of summer fruit was wiped out for the season. Vendors tried, often without success, to locate fruit below the freeze line for local markets. Others, more fortunate, contracted to buy truckloads of apples, peaches and pears from growers further south. Transportation costs and additional labor resulted in higher prices. Fruit picked days before its natural sweetness occurred showed up as inferior produce. Some of the citrus fruit was sold for juice. Under-ripe fruit picked before the freeze lacked the natural sweetness that occurs when left to ripen on the tree, which would make it a lower-quality produce.

Could it happen again during the spring of 2008? Of course, look at some recent headlines from major newspapers:

• “Flurries Leave Florida Citrus Unharmed”

• “Southern Farmers Scramble To Protect Crops From Freeze”

• “January Freeze: Good For Peaches, Bad News For Strawberry Crop”

American writer Mark Twain had this to say about the climate: “A great deal has been said about the weather, but very little has ever been done.” Could this 19th-century master of humor and sarcasm be wrong? Perhaps some methods of protection were available during his lifetime, but more techniques are now available. Today, growers have a variety of tools and the knowledge necessary to help protect agricultural crops from frost and freeze damage.

Protection methods

Growers have the ability to protect their crops against freezing temperatures. However, methods must be put into action before and not after the fact.

A report, entitled “Principles of Freeze Protection for Fruit Crops” and prepared by the Alabama Cooperative Extension System, says:

“Frost/freeze protection may be divided into passive and active forms. Passive protection involves methods such as site selection; variety and rootstock selection; and cultural practices, such as pruning and orchard floor preparation, that do not require expenditure of outside energy sources during the frost/freeze event. Active protection systems replace or prevent radiant heat loss by using methods such as heaters, irrigation or wind machines that require outside energy to operate during the freeze. The form of active protection chosen depends upon the crop, site, economics and other variables.”

Preventing or replacing radiant heat loss is the chosen method in frost or freeze protection. Various factors depend on the selection of equipment, as well as the location.

Start with a good site selection and evaluate the acreage before planting fruits or vegetables. Can you visualize the cold flow of air? Are low spots behind cold air dams? Are fences, hedges, windbreaks or forest areas nearby? Cold air, being denser than warm air, sinks. Therefore, low-lying areas of the planting field can be several degrees colder. This means frost may occur in these areas when it is not noticeable in other locations. Plant tender species that are prone to frost damage on higher ground or on slopes. This allows cold air to flow past the plants as it travels to lower points. As cold air does not settle easily on slopes, the area is less prone to frost. For preventive measures, choose a location with good cold air drainage. (Visualize the flow of cold air and its possible buildup in low spots or behind cold air dams, such as fences, hedges or wooded areas—avoid planting where cold air can be trapped.)

Know the last freeze date in your planting zone, but remember, record low and high temperatures can be broken each year.

Heaters to ward off freezing temperatures have been a standby for generations. Growers can delay using heaters when temperatures hover near the freezing mark, and they are useful if the temperature levels off or drops more slowly than expected. According to the North Carolina Extension Program, heaters provide the option of delaying protection measures if the temperature unexpectedly levels off or drops more slowly than predicted.

Freestanding models use fuel oil, but require no power source. Other models can be connected to a pipeline network and placed throughout the growing area. This method is based on a central pumping system, which regulates the pressure and turns the heaters off or on. The advantage of connected heaters is the ability to control the rate of burning and shut all heaters down from a central pumping station simply by adjusting the pump pressure. Pipelines operate on propane, liquid petroleum and natural gas systems.

The cost of operating heaters is generally lower than installing an irrigation system, but fuel costs must be taken into account when comparing the two methods.

Irrigation can be used for frost and freeze protection. If the system is already in place, the main cost is energy used to deliver water to the sprinklers. If there isn’t a system in place, the installation cost is relatively high. The scientific reason for using over-head irrigation is as follows: Water, sprayed on fruits and vegetables, acts as an insulator when temperatures reach the freezing point of 32 degrees F. The plant part is protected by heat of fusion. Irrigation must continue until melting begins. A backup power source is essential. This method does not provide protection if wind speeds are above 5 mph. (North Carolina State University).

There are some potential problems with this method. Low water pressure with an insufficient rate is a risk to crops; inadequate irrigation rates mean that too little water is being applied to the freeze at a rate that will provide enough heat to protect the crop. Also, ice can build up, which may cause limbs to crack and possibly break. Soils saturated from recent rains may be waterlogged from irrigation during winter months. Overwatering causes nutrients to leach from the soil.

An overhead, fixed sprinkler system is effective for low-growing crops, such as strawberries. Microjet irrigation systems are good for use in orchards or tall crops. A microjet dispenses a low-volume application of water close to the soil surface through a small microsprinkler, using a spray pattern from zero to 360 degrees.

Sprinkler systems are made from PVC pipes buried in the ground or solid set aluminum pipes that are movable for annual or perennial crops during field preparation.

According to the North Carolina Cooperative Extension Service (NCCES) bulletin, Irrigation for Apple Orchards, “In general, no system will provide protection in wind speeds greater than 5 mph for tree crops, 10 mph for low-growing crops. A backup power source is essential. Once started, irrigation must continue until the ice is melting and loose. This usually occurs soon after the morning sun hits the trees. A power failure can be devastating due to the evaporative cooling effect.”

Using only 5 to 10 percent of the energy per hour required by heaters, the wind machine capitalizes on the inversion development in radiation frost. This type of protection is not effective in an advective freeze. If the area is relatively flat and round, a single wind machine can protect about 10 acres. Mounted on a 30-foot steel tower, a machine consists of a large fan about 16 feet in diameter and is powered by an industrial engine delivering 85 to 100 hp.

Growers should be aware that wind machines are only effective under radiation frost conditions. When choosing this method of frost prevention, know the circumstances when protection will be needed. This method is not effective in winds above 5 mph.

Helicopters can also be used as wind machines. Able to hover in one spot, they raise the temperature before moving to another area. When using a helicopter, repeat visits to each location are usually necessary.

Duplicating the greenhouse effect, man-made fog has been experimented with as a method of frost protection. The object is to develop a “cloud” over the crop area to decrease the radiative cooling and prevent the plant from dropping to the critical temperature. Although some success has been achieved, no practical system has been developed. According to a report from the North Carolina State University, “The difficulty lies in producing droplets large enough to block the outgoing long-wave radiation and keeping them in the atmosphere without losing them to evaporation.”

Since the 1950s, chemicals have been examined that deal with frost or freeze protection. However, no commercially available material has withstood the scrutiny of a scientific test. Growers should use care when considering promotional claims of these materials.

Critical Temperatures

Applying water to plants to prevent damage from frost and freeze has been a common practice among growers. The following information explains what may occur with freezing temperatures.

• 32 degrees Fahrenheit (freezing point of water). Ice crystals form in the tissue of plants. Damage occurs.

• Critical temperature is lower than 32 degrees Fahrenheit.

• Damage to plants depends on duration of temperature.

• Damage to citrus occurs when critical temperature for four hours is 28 degrees Fahrenheit.

• Minimal damage if below 28 degrees Fahrenheit for one to two hours.

• Minor damage if below 28 degrees Fahrenheit for extended period: partial leaf burn, partial defoliation, but no wood damage (An NCSU report states, “for example, a citrus tree can stand a temperature of 28 degrees F for four hours before sustaining damage.).

Factors that affect critical temperatures:

• Stage of growth – early-stage or mature plants?

• Fruit set – is crop still on the plant?

• Dormancy – is plant actively growing or dormant?

• Plant water content – is plant under water stress?

• Overall plant health.

From “Irrigation as a Tool for Frost and Freeze Protection of Agricultural Crops.”


Consider the plants and location when choosing the method of frost or freeze protection. If purchasing a system from a supplier, know how the equipment works before it’s needed. Ask questions. Know how to contact the company if a problem arises. Double-check before low temperatures are expected. It may be too late when an unpredicted frost or freeze occurs late at night.

Carolyn Ross Tomlin is a frequent contributor from Jackson, Tenn.