Frost control options
There is no perfect system, but tactics can be combined.
This propane heater was designed to be towed up and down orchard rows.
Overtree sprinklers provide the most protection from frost because they can protect the trees in both advective and low-dew-point situations, according to Dr. Robert Evans, agricultural engineer with the U.S. Department of Agriculture in Sidney, Montana. A disadvantage, however, is that a large volume of water is needed.
Overtree sprinklers have been used since the 1940s for frost protection. With an application rate of 70 to 80 gallons per acre per minute, they can protect trees down to about 24 degrees. The colder the temperature, the greater the gallonage required. The amount of protection is directly proportional to the amount of water applied, Evans said. Uniform coverage of the canopy is critical.
Undertree sprinklers require less water and are often used with wind machines. Generally, unheated water applied at a temperature of 35 to 45°F at a rate of 40 to 50 gallons per minute per acre will provide an increase of 1 to 3 degrees Fahrenheit in air temperature. Evans, who was formerly at the Irrigated Agriculture Research and Extension Center in Prosser, Washington, said that in experiments there, he found that with warm water, rates as low as 15 to 30 gallons per minute per acre could give good results, though he didn't recommend using water hotter than 120°F because it increases evaporation, which takes heat from the air and can damage the pipe system.
With undertree sprinklers, very little heat is generated from freezing. It's the drop in the temperature of the water from, say, 45°F to 32°F that generates the heat, and most of that heat goes into the ground with the water, he said. The only way the heat gets into the air is by convection, which is an inefficient method of heating. With overtree sprinklers, the heat from the freezing water is being released directly to the buds.
Undertree sprinklers, like heaters and wind machines, will only work in temperature inversions, Evans stressed.
Conventional oil and propane gas heaters are inefficient, as only 10 to 15 percent of the heat stays in the canopy and the rest goes straight up out of the orchard. It's possible to raise the orchard temperature by 4 to 5°F, depending on the inversion, though the heat gain varies with the distance from the heater.
The heaters are expensive to operate, consuming 0.3 to 1.2 gallons of fuel per heater per night. With 20 to 50 heaters per acre, that's a cost of $300 to $800 per acre per night. "You're paying an awful lot to heat the sky," Evans commented.
Oil and diesel heaters pollute the air and are banned in many areas.
Evans said growers have tried a lot of different types of heaters over the years. A propane pulse jet heater that Evans developed is the only one recommended with wind machines because it pushes hot air horizontally cross the orchard, mixing it with the cold air, rather than pushing the hot air into the atmosphere. The drawback is the noise of the jet engine.
He also described a propane heater that was towed by a tractor up and down the orchard rows to distribute heat throughout the orchard. "My opinion is it's like trying to heat a room with a candle," he said. "The amount of heat you're producing is large, but in the big scheme of things, it's insignificant because you're dealing with such a huge system." He recommended placing extra heaters on the drift side of the orchard to heat the air before it moves into the orchard.
Heaters are generally more economical when used with other frost management methods, he added. However, standard return-stack heaters should not be placed within 125 to 150 feet of a wind machine. From 150 feet outwards, he recommends placing them at a rate of 40 heaters per acre for oil, or 60 heaters per acre for propane.
The strength of the inversion determines the effectiveness of wind machines. They protect the orchard by mixing warmer air from above the orchard with the cold air below. They pull air from about twice the height of the machine, and multiple machines help each other. Short wind machines, about 15 to 20 feet high, are not very effective because they don't pull down the warm air, Evans said.
He described a wind machine that had jet engines on the ends of the blades, which was "very inventive, but noisy." It was also ineffective because heat was on the blades of the fan, and hot air rises.
For the same reason, he does not recommend placing a large heater next to a wind machine. "You're warming a plume of air, and it rises above the canopy. Most of that heat is going straight up, and you're not getting the advantage of it."
Instead, it's better to place small heaters near the edges of the area covered by the wind machine, he said.
Evans said a machine that he calls a "fountain," which is positioned in low areas on the ground, blows the cold air from the orchard up into the inversion, supposedly getting rid of it. But it actually mixes that cold air with the warmer air above, which cools down the warmer air, so it comes back down again. So it acts like a water fountain, pushing air up, and when it reaches a certain height, the air comes down again. Meanwhile, you're still getting cold air coming in, he said. At best, it might mix up the air to stop it getting cold enough to damage the trees until the sun comes up, Evans said. "All you are doing is mixing the air," Evans said. "It's a waiting game, a compromise."
Fogging systems, which are very expensive, put out very small droplets to create an artificial fog, with the aim of blocking the radiation loss. The difficulty is getting the fog deep enough, Evans said. It needs to be 30 feet deep to be effective, and it tends to be moved by the wind. If such a fog drifted across a road, it would be a huge liability issue, Evans warned. "It's a great idea technically, but there are some practical problems to it."
Frost nets can be used to create an artificial cloud cover, and heaters can be placed under them to reduce heat loss. These also are expensive, but might have multiple uses.
Other frost control methods include electric heating cables on the ground or above the canopy. These are expensive, even assuming you can get enough power, and they are not very effective for the same reason that radiant heaters are not.
Evans said there's been little research to show the effectiveness of products that are sprayed on the trees to prevent freezing or increase hardiness. "I've never seen a benefit from any of these," he said. "That's not to say something might not come along."
Evans emphasized that there is no perfect way to protect an orchard or vineyard from frost. "All frost protection systems will eventually fail, and some blocks cannot be economically protected."