How suitable is apple orchard netting as a sunburn control measure?

Written by Antoinette Avorgbedor, Intern at Washington State University’s Tree Fruit Research and Extension Center and the Center for Sustaining Agriculture and Natural Resources

More likely than not, you have passed large apple orchards in your travels around the Pacific Northwest area and observed nets spanning wide areas of apple trees. Sometimes the entire top and all the sides of orchards are enclosed. A 2017 survey conducted in Washington State to assess the extent of netting found that about 5% of the surveyed acres were under nets and an additional 7% was estimated to be added in 2018 (Mupambi et al. 2019). Intuitively, you think nets are supposed to keep pests and trespassers out. At least, that is what I thought when I first saw an apple orchard covered with netting. That happens to be only a secondary reason for which tree fruit growers invest in such extensive enclosing techniques. A whopping 98.3% of the growers surveyed indicated that sunburn reduction was one of their most important reasons for using netting (the survey allowed growers to choose multiple reasons). I couldn’t help but wonder: What does this growing popularity of shade netting mean for the future of apple sunburn control?

I’m working as an intern with Lee Kalcsits, Washington State University tree fruit physiologist. Dr. Kalcsits and his team have been extensively researching the impacts of netting on apple orchards. Their results indicate that the sunlight scattered by netting provides a more conducive environment for plant and fruit development than harsh, direct sunlight. In addition, leaf and fruit surface temperature, as well as soil temperatures, are lowered by netting. Water loss by evapotranspiration is also reduced when nets are used in orchards.

One might ask: how relevant is this information? From my perspective, these findings are potentially revolutionary to the improvement of sustainable water use for agriculture and the overall maximization of tree fruit production quality. Many apple growers spray water from overhead sprinklers to cool down fruit in the hotter months of summer, a practice known as evaporative cooling. Considering that an estimated 179,146 acres of apples were grown in Washington State in 2017 and an average 32,317 liters (0.026 ac-ft) were used for irrigation per acre daily, let’s do some grade school math. How much water was used in 2017 to reduce sunburn in Washington State if evaporative cooling consumed about 36% of the irrigation water? That’s right! A whole lot of water used annually! Of course, that is a very rough estimate, but there is ongoing research to accurately measure and quantify the amount of water expended during the season on evaporative cooling.

Water is an essential yet increasingly scarce resource globally, especially for agriculture. Efforts to conserve water in the Columbia River Basin could play a pivotal part in maintaining the longevity and productivity of the massive fruit growing industry in the Pacific Northwest. One can acknowledge the potential to substitute the extensive use of water in evaporative cooling with netting, although the amount of water saved has not yet been quantified. Moreover, there is a potential to substantially reduce irrigation volume and/or frequency, because netting reduces tree and soil water loss by evapotranspiration. “Netting”, Kalcsits says, “is a viable solution to replace the use of evaporative cooling to prevent sunburn in high-light apple-growing regions such as the Pacific Northwest.  It has added benefits of fruit protection from hail, wind, and enhanced growth and productivity.”

The additional benefits of netting are bonuses for some, but not all, scales of orchards. Retrofitting nets in an orchard is also an expensive investment with labor-intensive installation processes. The use of polyethylene materials comes at a cost to the environment; we need to think about how to dispose of voluminous amounts of netting material after their useful life ends. But bearing in mind that orchard netting can last ten or more years if proper care is taken, a cost-benefit analysis may reveal economic benefits overall.

Then again, such extrapolations are very dependent on individual growers’ needs and each case may vary vastly. Luckily, the technology of netting presents variety as well. The bottom line: there are various methods of netting structure installations, different net colors and shading percentages that provide a wide array of possibilities for different categories of apple growers to decide what works best for their type of orchards.

As temperatures rise in the future and the timing of available water shifts, fruit growers may experience an increasing need to conserve water for irrigation purposes. Netting could then become a desirable, if not absolutely necessary, method of reducing tree fruit loss due to sunburn, together with wind and hail damage. Further studies are being undertaken by Dr. Kalcsits and his team to quantify the water-savings potential of employing netting compared to evaporative cooling. Also, more researchers will be working to bridge the information gaps that currently exist with regards to the economic benefits. My hope, in the meantime, is that the advancement of netting in the apple growing industry can be a means to conserve our water and improve agriculture in the region.

Reference:

Mupambi, G., Layne, D.R., Kalcsits, L.A., Musacchi, S., Serra, S., Schmidt, T., Hanrahan, I., 2019. Use of Protective Netting in Washington State Apple Production. Washington State University Extension Publication No. TB60E, 1–20.

Also published at AgClimate.net