Apr 11, 2023

Orchard covers, netting solutions have varied impact

Editor’s note: This article is an expanded version of the article that appeared in print.

Orchard protective covers and netting used in fruit production can reduce hail damage and provide sunburn protection, along with other crop enhancement considerations.

“Just 45 minutes of sunburn pressure can damage the fruit surface,” said Lee Kalcsits, assistant professor, tree fruit physiology at Washington State University.

Colored nets don’t have a big impact on the environment within an orchard, but it does reduce wind speed by 40%, even if it’s not closed. It also diffuses lighting and has an effect on fruit quality.

Kalcsits was part a panel on orchard technologies at the International Fruit Tree Association’s annual meeting Feb. 12-15 in Grand Rapids, Michigan.

“Under netting, the effects on tree physiology include less stress, better canopy growth and larger fruit,” he said. “There are no losses in carbon assimilation if the light intensity remains above the saturation level for photosynthesis.”

Netting also provides cooler soil temperatures for longer into the season at depths up to 16 inches. Less stress on tree roots, the ability to control water delivery (in irrigated areas) and water conservation are common when using standard nets with 20% shading.

Design options

Fruit tree netting is used to protect the fruit from birds, bats and other pests. It is a mesh netting that is draped over fruit trees or individual plants. There are two main types of fruit tree netting materials: polyethylene and polypropylene.

Continuous over-the-top structures are not all done the same, Kalcsits said. They can be stitched together down the row or put on cable systems as groups of panels sliding down the row. Retractable systems can be retracted quickly from the ground and are more labor friendly.

“Exclusion netting offers the most protection and can be used on sites where wind damage is a risk,” he said. “It’s most effective in a north-south tree row orientation; (providing) protection to the west side of the tree that is exposed to damaging solar radiation in the afternoon. It’s not full hail protection because it’s only protecting half the tree.”

Kalcsits said drape netting is the easiest to deploy and costs the least. It is the most difficult to work under because it doesn’t have a superstructure. It’s also the least durable.

Retracting the nets 10 days before harvest allows apples develop a redder color without exhibiting sunburn, he said.

Considerations for growers should include available labor for deployment and retraction, along with post-bloom and postharvest needs.

Designing and retrofitting in new orchards is much more expensive and labor intensive, so operators need to plan ahead.

“Engineering is essential,” he said. “Costs are a factor. It’s only going to have a role for high-volume varieties in key blocks where you have higher margins.”

Microclimate impact

Dugald Close, associate head global at the Tasmanian Institute of Agriculture, focused his presentation on self-ventilating rain cover effects on orchard microclimate, along with sweet cherry water use and fruit quality.

His research shows that rain coverings lead to bigger, sweeter fruit and more tree vigor. Less air flow led to higher humidity, and covered fruit matured more quickly. Coverings produced softer fruit and was related to the length of the row (distance to the edge).

“Microclimate stability under covers led to reduced wind speeds, warmer (but moderate) temperatures and more humidity,” Close said.

Water use under covers was 70% less than in netted trees, and was fairly consistent at different locations that were studied.

Fruit quality under covers showed that mature fruit was softer on a warm site, while fruit was firmer and sweeter at a cooler site.

Close continues to evaluate why firmness is lost when temperatures and humidity are more extreme. The role of calcium uptake also is an important factor.

Production impacts

Richard Vollebregt, president/CEO of Cravo Equipment Ltd. of Brantford, Ontario, Canada, said

seasonal orchard covers protect against weather extremes, but also have some negative effects.

“In 2012, we had growers ask us, can you put cherries under a retractable roof? We said you can put anything under a roof. What’s the impact on yield, quality and timing?”

In 2013, Cravo worked with Greg Lang, Michigan State University (MSU) horticulture professor, to install a retractable roof at MSU’s Clarksville Research Station “simply to learn because we didn’t know anything about growing cherries. If you’ve never been in a cherry orchard before, you don’t really know about climate optimization to know what the weather extremes can do to your crop.”

“Some of the quick, key learnings of what I’ve been able to gather in dealing with growers from across the world who have used these types of structures, not only for cherries, but for raspberries, blueberries and other crops are that these principles apply equally to all types of crops,” Vollebregt said.

The project was designed to document the effects on bringing a crop to maturity faster.

“We were able to show a one- to two-week change in flowering,” he said. “There’s a client in New Zealand who did three weeks, and another one in Hungary who did four weeks. That carries its own challenges, because growers have the risk of frost when they’re going four weeks earlier.”

Vollebregt said it’s important to consider that ambient temperatures may differ than the temperature of the tree.

“The temperature of a trunk in the winter was 48˚ F, but it was 68˚ F in the sun,” he said. “… When someone says, ‘what’s the temperature,’ we talk about air temperature. When we’re talking about chill units or chill unit hours, if you have a really sunny climate, your tree is going to be getting really warm in the daytime and you’re going to try to cool it off at night, which will reduce your overall chill efficiency.”

Every type of covering has positive and negative effects, which makes having the ability to regulate the effects by retracting it an important factor.

“The problem is every covering has positive effects and negative effects,” he said. “… A covering is going to change your light quantity, light quality, wind, leaf and fruit temperature, soil temperature, humidity, transpiration and water stress. It basically changes everything.”

Trellis design vital

Mark DeKleine founded TrellX in 2015. The company develops engineering guidelines for horticulture trellis planning.

“We boil everything down to math and equations that define the requirements for a trellis,” DeKleine said. “As we start adding structures, as they start getting taller, more loads that have to be accounted for, that’s what we do. That’s what we specialize in.”

The company’s trellis systems are in North America, South America, and in limited use in Europe, and are designed with the environment of the growing area in mind.

“We have special cases. In gorges, for example, out in Washington there’s a lot higher wind load,” he said. “Soil conditions can affect trellises, materials, depth, anchors. Superficially, looking at a trellis there are nuances that should be taken into consideration.”

— Gary Pullano,  FGN senior correspondent

Photo: Gary Pullano