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Extreme Heat Causes Problems for Growers in Western United States

Information provided by Hermann G. Thoennissen HTG International, LLC

Forecast highs Sunday in the West from the National Weather Service. (WeatherBell)
Forecast highs Sunday in the West from the National Weather Service. (WeatherBell)

About Hermann G. Thoennissen:

Hermann G. Thoennissen (affectionately known as Herman The German) is a friend of with deep expertise in orchard development & management, employee training, and business transition. With 35+ years of experience in various orchard management positions in WA State, 30+ years as the owner of HTG International LLC., 20+ years of experience in using aerial imagery in permanent crop production and also being an active team member of FarmCloud since 2012, Hermann provides consulting services to some of the largest organizations in the global Tree fruit and Ag industry in the western US and China. is happy to share exclusive access to Hermann's research notes on the effects of the fires in Washington State.


As many of you have experienced, Western United States is currently experiencing an extreme "heat dome" that has set recording breaking temperatures. In's local region of Walla Walla, WA temperatures are expected to reach 109° F (42° C) over the weekend and about 111° F (44° C) on Monday and Tuesday of next week.

For many of us this means staying inside as much as possible, visiting a local pool or lake, and praying that your AC doesn't stop working. For growers things tend to be immensely different. Below is some information provided by Hermann G. Thoennissen due to his expertise and experience in the agriculture industry over the years.

Thoughts from Herman G. Thoennissen:

"As we are approaching -according to weather channel forecast- at least seven days of 100F degree plus weather with possibility as high as 114F I want to summarize what I have learned over the years.

Here are my comments as to this event.

  • Pan-evaporation from heat and wind will be in excess of .5 inches per day. The potential exists to reach .7 inches on the days above 105F if we have wind of 5 mph or higher and humidity below 20%.

  • Most orchard irrigation systems are not built to supply this much water to all acres for that many days.

  • Therefore, it is important to get FC close to 100% by Saturday June 26.

  • Foliage- and fruit burn stems from radiation burn not from high temperature.

  • However, when internal fruit temps go above 107 to 110F structural cell damage starts to develop.

  • Overhead cooling will drop in-canopy temperature and thus, will reduce evaporation from the trees.

  • Very most likely overhead cooling will not get more than 50% of the applied water per acre into the ground.

  • Therefore, one must monitor FC below 16 inches and maintain as close to 80% FC (preferably above prior to heat) as the system allows.

  • Systems applying 50gpa or more should be fine when run continuously above 90F and if run intermittently below 90F.

  • System with application rates of under 45gpa and fine droplets -resulting in high evaporation- will most likely not be able to provide enough moisture during cooling to offset the evaporation. When this is the case lower soil layers dry out. With that the uptake of nutrients is severely restricted.

  • This early in the season cooling for fruit color is not a consideration.

  • Young leaves and most fruit have not developed a mature cuticula to deal with radiation. This is evidenced by the foliage sunburn one can observe from recent heat.

  • Protecting foliage and fruit from sunburn is the goal.

  • Applying physical/reflective protectants like Surround, Ecclipse or alike is one good tool. Raynox and similar materials are another option. Discuss with your fieldman, follow the labels.

  • Following the heat wave there will be quite the discussion of the pros and cons of various types of shade cloth."


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