Mist cooling for freeze protection
Bud development can be delayed by evaporative cooling using water mist.
This experimental plot at Michigan State University’s Clarksville Horticulture Experiment Station was established last year to study the Solid-Set Canopy Delivery system for spraying. Jim Flore wants to test misting as a way to slow down bud development during warm periods in late winter.
Photo by Richard Lehnert
Michigan State University horticulturist Dr. Jim Flore is exploring the idea of slowing down early bud development in tree fruits to reduce the risk of frost damage.
The concept is to use mist cooling to prevent fruit crop damage from freezes. Applying water to an orchard early in the season, when buds are just starting to swell, would activate the cooling power of evaporating water and slow down bud development, which is driven by accumulating growing degree days.
It’s not a new idea, Flore explained during the Great Lakes Fruit, Vegetable, and Farm Market Expo in December, but he wants to “revisit it” this year. It seems more feasible now than it once did, because growers could in fact apply mist, and may do so in the future for other reasons.
In a new project that started last year, researchers in Michigan, Washington, and New York are looking at what they call the Solid-Set Canopy Delivery System. Permanently installed irrigation lines and emitters would be used to spray pesticides, growth regulators, fertilizer, and other materials, without the services of conventional tractor-pulled sprayers. It could also be used to apply irrigation water and, early in the season, water in the form of mist.
The project, which was funded with a $2.5-million grant from the Specialty Crop Research Initiative, is headed by three directors: Dr. Matt Grieshop at Michigan State University, Dr. Jay Brunner at Washington State University, and Dr. Art Agnello at Cornell University. Many researchers have gathered under the project’s umbrella.
Research has shown that bud development can be set back two to four weeks by using evaporative cooling when temperatures rise above 50°F, Flore said.
Dormancy in apple trees is triggered by the shortening days of fall, he said. After the first killing frost, endodormancy increases rapidly—and this acclimation can’t be broken until trees have accumulated about a thousand hours of temperatures below 45˚F.
Deacclimation can begin when temperatures warm, and occurs before bud swelling. “As buds develop, they lose hardiness fast,” Flore said. “If we can slow down development early, we can save more crop.”
Other people have had the same idea before. Researchers in Utah were able to delay bloom in cherries, peaches, and apples by up to three weeks, Flore said. “It appears to work best in dry climates,” he said. Evaporative cooling only works when relative humidity is below 90 percent, he said.
Researchers in New Zealand nearly 40 years ago delayed bloom by 8 to 18 days by applying irrigation water whenever spring temperatures rose above 46˚F, he said, but it took application of from 19 to 36 inches of irrigation water in the two test years of 1975 and 1976. “That’s too much water,” Flore said, as it would lead to muddy orchard conditions.
Misting could be a better way. In the Solid-Set Canopy System, application of mist could be triggered automatically and frequently, and mist would readily evaporate to provide the cooling effect.
One other thing makes the idea compelling.
“The trend line shows spring temperatures in Michigan rising since 1900, and more rapidly since 1980,” Flore said. “Ice cover on the Great Lakes is falling, and bloom is coming earlier. Ice cover is one of the reasons why we grow fruit next to the Great Lakes, because the cooling effect slows bud development in the spring.”
If this trend continues, the future of fruit production in the region could be threatened unless methods are found to retard bud development and bloom, he said.
Flore’s specialty is plant growth regulators, and he has studied their effect on tree development. Ethephon applied in the fall has been shown to delay bloom two to four days, but the effect has been inconsistent. Gibberellic acid has no apparent effect. Studies with Alar indicated delays of four to six days in bloom, but this compound is no longer registered for use.
Soybean oil applied to peaches has delayed blooms two to four days, and so has abscisic acid. Materials that reflect heat and light—paints and kaolin clay—applied before bud swell also provide some delay.
Misting would have the advantage of only being needed when early spring warming conditions call for it.