A year after a record-breaking Pacific Northwest heatwave saw temperatures high enough to melt asphalt roads, tree fruit growers are reassessing their mitigation strategies for future extreme weather events.
Multiple days over 110 degrees Fahrenheit in June of 2021 withered cherries and scorched apples in key tree fruit growing regions of Washington, British Columbia and Oregon. Industry experts estimated at least 20 percent loss in the Washington state cherry industry and about 10 percent loss in apples.
“It’s not new to us to have hot summers in Washington. We are in a dry area. But it’s becoming more extreme. The growers are conscious of that and the need to have better tools to manage that heat stress,” says Bernardita Sallato, M.S., an assistant professor and tree fruit extension specialist for Washington State University.
The tree fruit industry is looking for long-term adaptations to extreme weather events, such as cultivars that are better able to withstand extreme weather events, Sallato says. For example, some cherry growers are exploring moving back toward more vigorous rootstocks such as Mazzard that has more resiliency to drought and heat.
But the climate-adapted cultivars in the pipeline still leave fruit tree growers in need of mitigation strategies for the short-term—“tools that allow us to monitor conditions of our own orchard and manage to our conditions, deficits and limiting factors,” as Sallato puts it. Those limiting factors may differ greatly between orchards even within similar environmental conditions.
Here are some key considerations for growers planning ahead for how to respond to to a heat event.
Planning for extremes in irrigation strategy
The number-one element for mitigating extreme weather events is irrigation management, Sallato says. Keeping trees adequately irrigated through a heat event allows trees to lower their temperature by regulating their internal water.
“If you do not have adequate moisture content when you have these heat events, you definitely will have negative consequences,” Sallato says.
But of course, water isn’t always available when it’s needed.
“When do we allocate irrigation? The most important time for adequate fruit quality is early in the season.” Sallato says. “But we need to maintain adequate moisture throughout the season to support photosynthesis. Especially when you get these hot spells, you have to have adequate water and sufficient soil moisture content to support water balance and temperature regulation.”
Orchardists need to factor in how much water beyond regular irrigation needs they might need to ride out an extreme heat event—including water availability for overhead cooling.
Overhead cooling is effective, but has its risks
Overhead cooling of the tree canopy is an effective strategy that has been used for many years—because many high-value cultivars are vulnerable to heat. But, depending on the system and cultivar, there can be complications.
The first issue, for orchards facing water scarcity, is simply the amount of irrigation required for an overhead sprinkler cooling system. Fogging, which uses up to 20 percent less water, is an increasingly popular alternative for this reason.
The second problem with overhead cooling is the potential impact on fruit quality. “Calcium-related disorders like bitter pit, in susceptible cultivars such as Honeycrisp, Golden Delicious or Braeburn, can be exacerbated with excessive water available during the summer,” Sallato says. To mitigate calcium-related disorders, many growers apply deficit irrigation during the summer—a strategy that can be at odds with with heat stress management practices and overhead cooling.
A third problem observed in last year’s heat wave in an orchard with both overhead cooling and a silver reflective cover was a distinct type of sunburn.
“The combination of water on top of this silver cover was like a magnifying glass for the sun, leading to photo-oxidative sunburn,” Sallato said. “You could feel it in walking the orchard. The light was very bright and if felt much hotter. The workers in the farm were all using UV glasses.”
A report looking at sunburn damage in Washington state apples during the June 2021 heatwave, written by Sallato and WSU extension specialist Chris Hedges, recommended that growers delay the installation of silver mulch until temperatures become cooler (below 85 degrees F). Some growers have indicated they punch holes in the cover to promote water drainage.
When shading orchards, flexibility is a plus
Covering an orchard with shade netting can lower temperatures by about 15 percent, but it also has its downsides. Shade cloth may affect pollination, and the reduction of direct sunlight can have an impact on early cultivars that require red fruit color.
Retractable netting systems are one solution. “In tree fruit, it is all about timing. You have to manage different techniques according to development, environmental conditions and the cultivar,” Sallato says.
Orchard training systems are another tool that can help growers respond to extreme heat events. Cherries, for example, when grown on upright offshoots, create a natural protection for fruit under the leaves that shades them from intense heat and sunlight. Training systems that produce cherries horizontally can present a higher risk of sunburn by exposing cherries and wood directly to sunlight and leaving the fruit more vulnerable to extreme heat.
“The vigor and the angle of your canopy will affect more or less the resilience to excessive heat waves and fruit quality,” Sallato says.
Ultimately, tree fruit growers have a lot to consider when planning for future heat events. They have to weigh the economics of the cultivars they plant, their production and their costs to determine their mitigation strategies to weather extremes, Sallato said.
“All these factors are related: cultivar, heat, water, nutrients, vigor, et cetera," Sallato says. "If you manage one, you have to consider indirect consequences that might affect other components, fruit quality and productivity being the most important."
Irrigation Water stress Orchards Cherries Apples Fruit trees