Think in terms of a system, not a method.
When trying to protect orchards from frost damage, growers should think in terms of frost management, Dr. Robert Evans, an agricultural engineer with the U.S. Department of Agriculture in Sidney, Montana, said at the Washington State Horticultural Association's annual convention.
"It's an entire system, rather than frost protection, and it will be site specific and vary from block to block, or orchard to orchard."
Frost management begins with site selection, said Evans. When establishing an orchard or vineyard, it's important to select a variety and rootstock that are suitable for the site, and plant trees or vines with rows running down slopes to let the cold air drain and avoid trapping it in the orchard or vineyard. Cold air flows downhill under the force of gravity.
Windbreaks might divert cold air, but can also be a dam. When planting the windbreaks, look at where the cold air comes from and where it's going to, he suggested. Cold air can build up behind an obstruction to four or five times its height, and cause damage to the trees.
Frost management includes both passive and active methods of minimizing frost risk. Passive methods include pruning strategies and timing; keeping the alleys mowed so as not to trap the cold air; and using stacks of bins to divert cold air away from an orchard.
Active methods include heaters, sprinklers, wind machines, fountains, helicopters, covers, fogs, foams, and the application of cryoprotectants to prevent the plant tissue from freezing.
Frost management also requires a frost-monitoring network of thermometers and alarms, as well as knowledge of critical temperatures and forecast air temperatures and dew point.
The temperature at which plant tissues are damaged is known as the critical temperature. Critical temperatures are site specific and are not uniform across a block or even a variety. Different tissues, such as buds, twigs, and branches, have different critical temperatures. The amount of damage depends on the tissue temperature, not the length of time it is exposed to cold, Evans said. "It's a function of the rate of cooling and how the plant can adjust."
Plant and bud hardiness is greatly affected by what the weather has been like before the cold spell.
When protecting an orchard or vineyard from frost, it's important to keep track of the dew point, which is the temperature at which the air becomes saturated and water vapor in the air starts to condense.
Both freezing and condensation of water generate heat. As water condenses in the air, it releases a tremendous amount of heat—about ten times the amount of heat generated when water freezes—and it tends to condense on the coldest parts of the plants, such as the buds. This free heat supplied through condensation makes the impacts of heaters and other frost control measures look insignificant, Evans said.
At the dew point, the heat released from condensation replaces the radiative heat losses.
A high dew point is essential to the success of any cold temperature protection, he added. The dew point sets the minimum temperature for the night. If the dew point is 5 to 10 degrees below the critical temperature, there will be damage because there is not enough benefit from the heat generated. "You probably should stay in bed because there's nothing you can do," he said. "There are not that many methods that will be effective, even overtree sprinkling.
"If the critical temperature is below the dew point, you probably don't need to do anything because you're getting the benefit of the heat, and the temperature's not going to go down any further, and you can go back to sleep," he said.
However, typically, the dew point is fairly close to the critical temperature in the spring, and frost control measures are designed to raise the tissue temperature one or two degrees.
Evans said tools are available to measure the wet bulb temperature, which is close to the same temperature as dew point but easier to measure.
Types of freezes
There are two types of frosts or freezes: radiative, which is caused by direct heat loss to the sky and is the easiest to protect against, and advective, which occurs when the wind is stronger than five miles per hour.
Radiative freezes occur in clear conditions. During the day, heat from the sun warms the soil so that the temperature of the ground is higher than the air above. At night, because the ground is warmer than the air, the heat radiates out from the soil and tree tissues into space, making the exposed flower buds 1.5 to 3.0 degrees Fahrenheit colder than the air. As the air tries to warm up the tree tissues, it becomes colder and denser and settles to the ground. Cold air flows downhill under the force of gravity. When the temperature at ground level is colder than the air above it, this is known as a thermal inversion. Evans said inversions tend to follow the general shape of the topography and do not consist of a flat pond of air.
Advection is beneficial when the winds are above freezing temperatures, but if the temperature of the air mass is colder than the critical temperature, it can be a problem because the dew point will be low as well, Evans said. Advective conditions mix the air and do not allow inversions to form. Cold damage is caused by rapid, cold air movement that steals the heat from the plant.