Frost protection strategies
There’s nothing like a disaster to focus attention on ways to prevent freezes.
Tree and vine crops can be won or lost because of one or two critical degrees of cold temperature, so, ultimately, the best method of frost protection is good site selection. But even the best sites won’t be perfectly freeze-free.
Is there a good way to recover those one or two degrees lost on a cold night?
Research agricultural engineer Dr. Robert Evans, who recently retired from the USDA’s Agricultural Research Service’s Northern Plains Laboratory in Sidney, Montana, spent his career studying ways to do that. Before his work at USDA, he spent more than 20 years doing frost protection research at Washington State University.
Weather events in the eastern United States this year put him on the speaking circuit, where he gave growers the benefit of his accumulated experience. He spoke at the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids,Michigan, to an audience that, after a year of ill-timed freeze events, was primed to learn what it could.
“Often times, you only need one or two degrees, and it’s the difference between crop and no crop,” Evans said. “No frost prevention system is going to be perfect, but you can minimize the damage.”
On the other hand, he said, if you’re willing to pay enough, “Any crop can be protected against any cold temperature event if economically warranted.” Total coverage in a heated greenhouse is safe—but expensive.
The objective of any crop frost detection system is to keep plant tissues above their critical temperatures, which is the temperature at which tissues will be killed, Evans said. This temperature varies by stage of plant development.
Evans said that growers should approach frost protection systematically, proceeding step by step, adding increasing protection as they need and can afford, starting first with passive methods that avoid frost damage.
“The best time to protect a crop is before it is planted, and site selection is the most effective passive risk avoidance strategy,” he said.
“One of the best ways to improve the effectiveness of active frost protection methods is to understand where the cold air drainage moves into and out of a block. Windbreaks, buildings, stacks of bins, road fills, fences or other barriers, tall weeds, all retard cold air drainage and can cause cold air to pond in areas behind them. The size of the potential cold air pond will likely be four to five times greater than the height of a physical obstruction.”
Vegetation in an orchard, or weeds around the edges, can slow or stop the movement of cold air out of an orchard. Windbreaks, he said, put in the wrong place, can do more harm than good for frost events.
Other passive measures include choosing late-blooming varieties and training and pruning trees so the bearing surface is up away from where cold air accumulates. Growers may not love ladders, but on frosty sites, fruit is mostly located where ladders reach.
“There is probably no way to economically overcome poor site selection,” Evans said.
Use of water
Active frost management techniques involve: 1) adding heat to the environment using water or heaters, 2) mixing the air to move heat into the orchard, or 3) conserving heat by not allowing it to escape.
Growers need not select one method only. The effects may be additive. They can expect to gain four to six degrees of protection from overhead irrigation, one to three degrees from undertree irrigation, and one to four degrees from orchard heaters, wind machines, or helicopters. When using more than one method, the order would be to choose water first, then add wind, and, finally, add heaters, because of cost.
One of the best ways to add heat is through applications of water, he said. “Water-based methods are generally the most economical. Heat from water is also more efficient because it is released at low temperatures into the environment, is less buoyant, and may selectively warm the coldest plant parts.”
Water-based frost protection systems can also create problems with disease, saturated soils, runoff, and leaching of nutrients and other agrochemicals. And overtree sprinkling is risky and, if not done properly, can cause more damage than it prevents, and even destroy an orchard.
Water has interesting properties. Heat is released as water cools or changes phase from vapor to liquid or liquid to ice. The change from water to ice releases 1,200 BTUs per gallon. The condensation of vapor as fog releases 9,000 BTUs of energy per gallon. These are much greater amounts than released by mere cooling of the water.
High dew point temperatures are also extremely important to the success of a frost protection program, he said. When dew points are high, water condenses when it is warmer, and this condensation provides a huge amount of free heat on the coldest parts of the plant. This free heat is not helpful if it is released when temperatures have already plunged to damaging levels.
“Overtree sprinkling can provide the highest level of protection of any single available system (except field covers/greenhouses with heaters), and it does it at a reasonable cost,” Evans said.
It is the only method that does not rely on inversion strength for the amount of protection, and it can provide some protection in advective frost conditions, he said.
“The applied water must supply enough heat by freezing to compensate for all the losses due to radiation, convection, and evaporation,” he said. “Consequently, overtree sprinkling requires great amounts of water, large pipelines, and big pumps.” The irrigation must continue until the ice melts.
The system requires application of at least 70 gallons of water per acre per minute, continuously. A failure of the pump or the water supply can lead to disaster. Trying to stretch the water supply by intermittent application is not a good idea, he said.
Growers can expect to raise orchard temperatures at least 4˚F with this method if done correctly.
Undertree sprinkling uses half as much water, Evans said, and produces about half the effect—raising the temperature about 2 to 4˚F, depending on the temperature inversion. Most of the increase comes from the cooling of the applied water and not from the heat released as water freezes, he said. Therefore, the warmer the water applied, the better. Applying water heated to about 110˚F—a limit imposed by the softening point of plastic water lines—would give similar results with about half the water requirement.
“Research has shown that, depending on water temperatures and flow rates, applications of preheated water with flow-through boiler systems can be economical and only use about 20 percent of the fuel required for heaters for the same heat,” he said.
Orchard heating is an ancient method—the Romans burned prunings to heat vineyards 2,000 years ago—but it has fallen into disfavor with the rising price of fuel, the need for labor to handle the 40 or so heaters needed for every acre, concerns about air pollution, and the fact they don’t really add heat very effectively, Evans said.
Their use has been totally banned in California and in parts of Washington State, because of air quality concerns.
“Heaters are very inefficient,” Evans said. “Only 10 to 15 percent of the heat stays in the orchard; the rest goes straight up.”
Heaters are best used as a supplement to wind machines. They should be positioned at the margins of the area protected by the wind machine, 120 to 150 feet from the machine.
Evans likened mobile heaters that are towed through the orchard to “heating a room with a candle. You need to return to each place in the orchard every four to six minutes to be effective.”
Wind machines work by mixing warmer air from above with cold air accumulating at tree level, Evans said, so a source of warm air is important. They work best in radiation freezes where there is a warmer inversion layer of air from which to pull.
If there is a four-degree temperature difference between a six-foot height in the orchard and the warmer air at 60 feet, Evans said, a 35-foot tall wind machine would pull that down and contribute about a two-degree temperature rise in the orchard.
Wind machines have become more popular because they make effective use of fuel.
“Helicopters are an expensive (and sometimes dangerous) variation of a wind machine,” he said. They can be very effective because they are mobile and can find warmer air at higher levels. They can cover up to 40 acres, about three times that of a wind machine.
“A heavy helicopter will pull down a lot of air,” Evans said, “but they are expensive.”
Cold air drains, which Evans calls fountains, work like fountains, lifting cold air from the coldest part of the orchard floor and propelling it upward. Evans doubts their effectiveness because the cold air is inefficiently mixed with upper warm air and cold air falls.
“This process serves to recirculate the cold air that was pushed up earlier and has resettled back to the ground, as well as the cold air drift that continues to accumulate in the vicinity,” he said. “Over time, the cold air pool deepens, and frost injury can still occur.”