Protecting crops from freezing temperatures once involved building big smoky fires. Over time, other techniques and products have been developed to help tree and row crops survive a cold snap. No method is perfect, but under the right conditions, some can help.
PHOTO COURTESY OF SOTOYOME RESOURCE CONSERVATION DISTRICT.
Understanding frost and freeze conditions
When air temperatures drop to the freezing point of water (32 degrees Fahrenheit), a freeze (also known as a killing frost) usually occurs. Whether or not plants are actually damaged or killed depends on the duration of cold temperatures and varies from crop to crop.
Depending on atmospheric conditions at the time they occur, frosts may be classified as either advective or radiative. Advective frosts and freezes are the result of strong winds moving cold air into an area from another region. These cold fronts may drop temperatures significantly below critical levels. Radiative frosts, on the other hand, are locally produced. During the day, sun warms the soil surface. At night, heat is radiated into the cool atmosphere. Radiative frosts occur only during calm, clear nights when there is no wind to mix the ascending warm air with the descending cold air and there are no clouds to reflect heat back to the earth. Temperature change is slowed by moisture. That is why areas of low humidity, such as deserts, generally experience greater extremes between day and nighttime temperatures. Because radiative frosts at the beginning and end of growing seasons are often only a few degrees below critical levels, they usually make crop protection efforts worthwhile.
Wind speeds of more than a few miles per hour can also make frost prediction and protection difficult. This is especially true in an advective freeze. In radiational cooling conditions, a light breeze can increase temperatures at ground level by mixing warmer air with cooler air. When windy conditions exist, temperatures tend to be uniform over a greater distance, up to several miles.
Relative humidity is also a factor. As air cools, relative humidity increases. At the dew point (the temperature at which relative humidity reaches 100 percent), water vapor in the air condenses into fog or dew, which slows temperature drop. As the dew point lowers, the risk of frost becomes greater. When the dew point is below freezing, damage to plants can occur rapidly, sometimes without the typical white frost crystals. This condition is known as black frost.
How cold affects plants
Cold temperatures cause water within plants' cells to freeze. Cells then rupture. The temperature at which damage occurs varies with the crop and growth stage. The plant's water content and overall health are critical factors. Dehydration can also be a factor. Young, actively growing, flowering plants are the most susceptible to cold. The greatest damage from low temperatures usually occurs from bud break in the spring through flowering and fruit set. Blossoms are the plant part most commonly damaged by low temperatures.
Orchard-Rite Wind Machine.
PHOTO COURTESY OF ORCHARD-RITE LTD., INC.
Since weather conditions can vary significantly within a relatively small area, it is helpful to know if and how much conditions at your location tend to deviate from those at the nearest National Atmospheric and Oceanic Administration (NOAA) weather station. If you record your weather conditions daily and compare them with those of NOAA, you will have a general idea about the difference between the two and will thus know when you may need to prepare for near-freezing or freezing nighttime temperatures.
Plug-in probes and electronic devices enhance growers' ability to monitor weather conditions. Probes can, for instance, be inserted under row covers to measure temperatures there, and the results can be read remotely. Frost alarms and alerts are useful when growing areas are remote from your living and working areas. Some devices even flash alerts color-coded to the temperature.
Instruments for observing and recording weather conditions vary from simple dry or wet-bulb thermometers to complete weather stations with automatic recorders. See a selection from Thomas Jefferson's time at www.almanac.com/blog/unique-perspective-weather, and some of today's more complex units can be found at www.specmeters.com/weather-monitoring.
Cold Air Drain from Shur Farms.
PHOTO COURTESY OF SHUR FARMS.
Approaches to protecting row and tree crops from frost damage generally involve covering, applying water or heat or increasing air circulation. In the northern Sacramento Valley, where primary crops are almonds, walnuts, prunes and olives, under-tree sprinklers are most common and wind machines are rarely used.
Richard Buchner, University of California Cooperative Extension of Tahoma County, says that in this area late-blooming walnuts are the most difficult to grow. "Walnuts almost get hurt if they look at an ice cube," he says.
As anyone who has ever tried to save a crop from frost knows, most techniques are labor-intensive.
For row crops, especially when water for overhead irrigation is not available, row covers may be the answer. In advective freezes, sprinkling atop row covers may be the safest way to protect crops. Irrigation can be started after the temperature under row covers drops to near the crop's critical temperature. Because of the labor required to apply and remove row covers, this method is best suited to small acreage. Covers must be removed early the next day so drying and pollination can occur.
Weight and fiber arrangement is a factor in determining how much protection a given row cover can provide. Weights of .6 ounce per square yard have been found to give 2 to 3 degrees of protection during a radiational frost. Foam covers or a double layer of row covers may provide over 10 degrees of protection. Row covers should be placed over crops during midafternoon in order to trap heat. Row covers are widely available. One source of permeable frost fabric and thermal blankets is FrostProof (www.FrostProof.com).
In the right conditions, sprinkler irrigation works well, especially on small fruit and tree crops. Overhead sprinklers can be useful for protection from radiant freeze when temperatures are not below 25 degrees and wind speeds are light. Sprinklers having smaller droplets provide more uniform coverage. Sprinkler irrigation works because water gives off heat when it changes from a liquid to a solid, that is, when it freezes. However, the rate at which water freezes varies depending on environmental factors such as air temperature, humidity and wind speed. Wind speeds in excess of 10 mph may put crops at risk for damage from evaporative cooling. For greatest protection, be sure the system is fully functional before frost is predicted. Begin irrigating when the temperature at plant level is 4 degrees above the critical level for a given crop. Open flower buds of blueberries, for example, are damaged when the temperature falls to 27 degrees; thus irrigation should begin at 31 degrees. However, sprinkler irrigation should be delayed until more than 10 percent of blossoms (be they closed or open) are in danger of frost damage, because sprinklers can apply large amounts of water to the crop.
When warm air is present over cold air (an inversion) and there is no wind, wind machines can be used to pull warm air from above to replace the cold air at the soil surface. Since wind machines can generally raise surface temperatures by only a few degrees, they are best used when only mild frosts are predicted. Fans should be started while the temperature as measured at about 5 feet is above the crop's critical damage temperature and before the temperature at 5 feet falls below the temperature at 33 feet above the ground.
Wind machines, such as those manufactured by Orchard-Rite (www.orchard-rite.com) can cover about 5 to 14 acres.
Upside-down wind machines
Selective inverted sinks (SIS), commercially available as Cold Air Drain, are alternatives to using sprinklers for frost protection. They are especially useful in areas where water supply is limited. Basically a propeller encased in a wind tunnel mounted over a gearbox, one SIS may protect from 2 to 16 acres, depending on the size of the unit. Selective inverted sinks work when there is a layer of warmer air above the cold air closest to the ground. On cold, clear, windless nights, heat stored in the ground during the day radiates upward. Heavier cold air near the ground flows downhill, settling in low areas from which it cannot drain, causing frost damage. Inverted sinks push the cold air upward to around 300 feet. As the cold air is pushed upward, it mixes with the warmer air above. As cold air rises and disperses, the lower spots are drained of cold air.
"Shur Farms developed Cold Air Drains some 20 years ago," says vice president and owner Susie Hammersmith. "My father was growing walnuts and he needed some way to keep them from freezing. The Cold Air Drain works in his situation and for many other tree and vine crops throughout North and South America growing at least 18 to 20 inches above the ground. In flat terrain without a natural drain for cold air, we can construct barriers."
Whatever methods you use to protect your crops, be ready. Frost protection won't work if it is not used in a timely fashion.
Kathleen Hatt is a freelance writer and editor and has been a frequent contributor to Growing since its inception. She lives in Henniker, N.H.