Commit to cool

Derk VanKonynenburg, center, shares cooling details.

Evaporative cooling can help improve fruit quality, reduce tree stress, and promote return bloom, a California orchardist reports. But the decision to use evaporative cooling should not be taken lightly.

"The amount of time it takes to design and install, and properly manage a cooling system is enormous," explains Derk VanKonynenburg of Britton VanKonynenburg Farms, in Modesto, California.

It’s a big commitment, and once you start cooling, you cannot quit–not even for a day if the weather is hot–otherwise you risk having more sunburn than a grower who doesn’t cool at all, he told growers at the International Fruit Tree Association’s annual conference in California.

VanKonynenburg said his company has been using evaporative cooling for about 14 years. When they started, all they knew about it was from research that had been done in the eastern United States or Washington State, where growing conditions were substantially different. At the outset, their goal was to enhance fruit color, but they soon learned that for growers in California, the real benefit was sunburn prevention. There were unexpected benefits, too, such as less tree stress, larger fruit, and possibly less likelihood of alternate bearing.

Dual system

Britton VanKonynenburg Farms uses a dual sprinkler system so that cooling and irrigation can be controlled automatically. The orchard can be irrigated at night and cooled during the day. Many microsprinkler systems have enough capacity to be converted to an evaporative cooling system, but the problem is being able to go from irrigation to cooling and back to irrigation, VanKonynenburg said. Cooling alone doesn’t apply enough water for the trees to be adequately irrigated.

His company started out using a spinner-type system that required 64 gallons per acre on a 104 degree F day, but changed to a rotor-style system, that gives good results with only 43 to 49 gallons of water per acre, although on extremely hot days, lower rates might not keep the fruit cool enough to avoid damage, he said.

Cooling can result in a quarter of an inch of water a day reaching the ground during very hot weather. If there’s a long hot spell, there’s a risk of overirrigating, VanKonynenburg warned. It’s important to monitor the amount of water applied for both irrigation and cooling. After using various methods, including a coffee can, he now uses a system called PureSense that allows real-time monitoring of soil moisture.

Water must be available every day during hot weather, VanKonynenburg stressed, so a back-up source or reservoir is needed. In addition, a diesel pump or generator is needed as a back-up for electrical pumps and controllers in case of power outages.

Every sprinkler must work. VanKonynenburg said sprinklers can become clogged, and at their orchard, coyotes chew on the hoses. An employee checks each sprinkler in the orchard every 10 to 14 days to make sure they’re working properly.

The pH of the water needs to be maintained at around 6.5, otherwise even low amounts of bicarbonates in the water will result in residues on the fruit that are difficult to remove at the packing house. VanKonynenburg said the water he uses comes from an irrigation canal, and the pH can vary between 6.5 and 9.0. He uses an automated acid injection system to keep the pH at the right level.

Cooling period

To minimize water use, VanKonynenburg uses a combination of cooling and a particle-film product called Raynox. The cooling period begins 45 days before harvest for each variety. Usually, with Fuji, there’s about a month of temperatures above 90 degrees°F before that time, during which the sun-guard is applied to protect the fruit. With Pink Lady, there are two months during which the fruit need to be protected before cooling begins, so up to three applications of particle film can be needed.

To determine what time of day to switch on the cooling, VanKonynenburg uses an infrared sensor that he bought at an auto parts store to read the fruit surface temperature. Cooling begins when the fruit surface is about 106°F, which is usually when the ambient temperature is between 86 and 88 degrees F. A fruit surface temperature above 112 degrees F will damage the fruit skin. Cooling is switched to the next block when the fruit skin temperature drops to about 96 degrees F.

A common problem is starting the cooling too late, VanKonynenburg warned. In California, midmorning temperatures can be higher than people think. "You must use an in-field temperature sensor to control starts, cycle changes, and stops," he said.

During the day, the cooling goes through three cycles per hour (7 minutes on and 14 minutes off) until the ambient temperature drops to 91 degrees F. Then, the cycles switch to 10 minutes on and 10 minutes off.

Research that VanKonynenburg had read before using evaporative cooling had focused on longer cycles of, say, 15 minutes on and 30 minutes off, or 10 minutes on and 20 minutes off. In his experience, longer cycles allow the fruit to warm up too much to keep it in the desirable temperature range. "In central California, we just can’t do that," he said. "That gets you in trouble."

Cooling should be stopped in the evening when the fruit surface temperature begins to drop rapidly due to decreased solar radiation, he said, and that’s at about 6 p.m. The correct shutdown time should be checked at least once a week. This can be done by standing in the field with an infrared sensor.

One question the grower needs to address is whether the cooling can be shut off during harvest in small areas or whether employees will be willing to get wet.

Coloring

Research indicates that coloring of fruit occurs during the coolest evening and morning hours, VanKonynenburg said. So, theoretically, evaporative cooling in the evening and morning should increase the length of time that the fruit are in the temperature range that enhances color. VanKonynenburg runs one or two seven-minute cycles on Fuji apples at 8 p.m. when the temperature is over 96 degrees F. On Pink Lady, which are harvested in late September, he applies two cycles in the morning, just after dawn, and one at 8 p.m. Applications before dawn would warm the fruit because the water temperature is about 70 degrees F.

"We have no proof that it does any good, but everything we read says it does," he said.

VanKonynenburg said there are many things about evaporative cooling that can only be learned in the field. The system controller must be fully automated so that starts, stops, cycling, and shutdowns are automatic. However, a person must be there to make sure the automation works. "In ten years, I think we’ve had the system not come on three times, but if it doesn’t come on and you don’t go check, it’s a disaster," he said. "You have to make a decision that whatever it takes, you’re going to run it every day."

Occasionally, a ten-inch tee will blow up at VanKonynenburg’s orchard. It takes about four hours to repair it. One time when it happened, the temperature was over 100 degrees F and the system couldn’t be switched on until 2 p.m., whereas it would normally be on at 10:30 a.m.

"We fired the system up at 2 p.m. and went right to ten minutes on and ten off, and the next morning we saw sunburn," he said. "It wasn’t much, but if we’d missed that whole day, it would have been a disaster."

There have been times when temperatures have been over 105 degrees F for over ten days. VanKonynenburg said surrounding orchards that didn’t cool ended up with burned fruit, but they did not. "It can pay off if it works," he said. "