Vineyard Frost Protection
Active Methods for Managing Frost
Active frost protection methods involve modification of the vineyard climate immediately before and during frost occurrences. The climate of the vineyard may be altered by 1) utilization of atmospheric heat (wind machines, helicopters); 2) addition of heat (heaters, over-vine sprinklers); and 3) a combination of both methods. In selecting a method(s) to modify cold air temperatures, the grower must consider temperatures and expected durations, occurrence and strength of inversions, soil conditions and temperatures, wind (drift) directions and changes, cloud covers, dew point temperatures, critical bud temperatures, vine condition and age, land contours, and vineyard cultural practices.
Wind machines (See Figure 27.2) are considered one of the best protections against radiant frosts. Wind machines work by directing warmer air from above the inversion layer downward around the vines, and at the same time, they displace the colder air on the ground away from the vineyard.
Effective operation of wind machines requires that a sufficiently strong temperature inversion exists and within reach of the machine. Ideally, a temperature inversion of 5 to 9 degrees F (3 to 5ºC) must exist between the warm air and the cold air situated at ground level in the vineyard.
Start-up and Shut-down Temperatures
Many growers turn on wind machines at about 32 to 34 degrees F (0 to 1°C), which is appropriate for many radiative frost situations.
Critical temperature is generally defined as the temperature at which vine tissues will be damaged by cold weather. Critical temperatures will vary depending on the variety and stage of development (e.g., bud swell, budburst, etc.).
A wind machine consists of a tower 30 to 80 feet (10 to 12.5m) in height, with a propeller mounted at the top. Drive units are usually located at the bottom of the tower for ease of servicing. An efficient wind machine must have the rotor installed high enough to obtain better mixing of the inversion layer.
Wind Machine Location in Vineyard
A single machine can take care of approximately 10 to 12 acres with the effective range depending on the site contour, the power of the wind machine, and the proximity of other wind machines. Multiple wind machines increase the range of effectiveness of individual machines.
Wind machines are fairly expensive to install, costing upwards of $30,000 per tower. Wind machines can be powered by electric motors, gasoline/liquefied gas (LP) powered engines, or diesel engines.
Frost Monitoring Systems
Distributed temperature monitoring stations across a vineyard are an essential part of frost risk management by providing warnings and data for tracking long term trends.
Helicopters are another way in which the inversion layer may be mixed. The area covered by a single helicopter depends on the helicopter size, weight, and the weather conditions. Usually helicopters are owned and operated by service companies. They are leased, with a pilot, for particular operations and are relatively expensive compared to other methods, but growers faced with devastating frost losses find them very effective.
Inverted Sink Fans
Inverted sink fans (See Figure 27.3) are most effective in vineyards with low-lying areas where cold air drains or pools. A fan is mounted horizontally inside a bell-like housing and unlike a wind machine, blows air straight up to a height of 300 feet (90m).
Heaters, or more commonly known as smudge pots (See Figure 27.4), provide frost protection by heating the air as uniformly as possible up to the inversion layer. As the heated air rises, it cools until it reaches the height where the ambient air has the same temperature of the inversion layer. Than the air spreads out and, eventually, the air descends again creating a circulation pattern much like that of a gravity furnace. When there is a strong inversion, the heated volume is smaller, and the heaters are more effective at raising the temperature whereas heaters are less efficient in weak inversion conditions because there is a bigger volume of air to heat. Most of the heat produced (convective energy) during a weak inversion goes straight up, and the only benefit from the heaters is the radiant energy generated.
Over-vine Sprinkler Systems
Over-vine sprinkler systems remains among the most reliable methods of frost-protection, since it does not rely on access to warm air above the vineyard, and provided sufficient water is applied, is able to protect against severe frosts. Over-vine sprinkler systems involve spraying the vines with a fine mist of water as the temperature falls to freezing. This water then freezes encasing the canes and buds in ice (See Figure 27.5). As the water changes to ice on the surface of the vine, it releases a small amount of heat (known as latent heat) that protects the vine from any damage. Latent heat prevents the surface temperature of the vine tissue from falling below 32 degrees F (0°C). Conversely, when ice melts, or water evaporates, the temperature around the water is cooled. Water evaporating from the surface of a vine will draw heat from that vine.
Start-up and Shut-down Temperatures
When frost is expected, the usual practice is to start the sprinklers when the temperature drops to 34 degrees F (1ºC), thus providing a margin of safety (Weaver, 1976). However, it may be necessary to start the sprinklers at a higher temperature than 34 degrees F (1ºC) when very dry atmospheric conditions prevail (low dewpoint temperature) in order to increase the relative humidity before frost occurs.
Wet-bulb Temperature: The wet-bulb temperature represents the temperature a wet surface will cool to as the water evaporates.
Dew-point Temperature: The dew point is the temperature at which moisture condenses from the air to form dew.
Start-up Temperature: Table 27.2 can be used to determine the absolute minimum irrigation turn- on temperature based on the critical temperature of the vines and dew-point temperature.
Shut-down Temperature: The turn-off temperature can be as critical as the turn-on temperature.
The application rate required for overhead sprinkling depends on the sprinkler rotation rate, wind speed, and the dew point temperature. The general recommendation for over-vine systems in central California calls for about 0.11 in/hr (2.8 mm/hr) which will protect to about 27.5 degrees F (-2.5°C) [Evans, 2000].
Frost protection with overhead sprinklers is dependent upon uniformly supplying water to the ice-encased surface on the vines at regular, rapid intervals. Several types of sprinkler designs are commonly found in the vineyard.
Sprinkler Rotation Speeds
Sprinkler rotation rates are important because the temperature of wet vine parts initially rises as the water freezes and releases latent heat, but then it falls to near the wet-bulb temperature, due to evaporation, before the vine is hit again with another pulse of water.
Estimating Water Needs
There are a number of critical issues to consider when planning a frost protection system for a vineyard. First and most important is determining the available water supply in terms of gallons per minute and how many gallons of water are needed in the case of a frost event. Flow rate is important to know because many frost protection systems target a water application rate of at least 0.1 inches per hour (in/h) to help protect the vines.
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