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Prepare your high tunnel for a hot summer

After a record setting warm winter, NOAA is continuing to predict above average temperatures in their three month forecast as well as their July-August-September 2024 forecast. With so many new high tunnels in Minnesota we want to make sure that high tunnel growers are prepared for a hot summer. Shoulder season crops that are better suited to cool weather often struggle in June as temperatures heat up, and even heat tolerant crops like tomatoes can struggle when temperatures get too high. If you've ever worked in a high tunnel in August, you also know that it can be pretty miserable for people. There are a few key practice that can help you and your plants thrive in a high tunnel during hot summer weather.

What are some common issues that arise when high tunnels get too hot? 

  • Poor yields caused by flower abortion and pollen issues
  • Cool season crops that bolt or become bitter
  • Ripening disorders like blossom end rot, yellow shoulder, and internal white tissue in tomatoes
  • Sunscald, which is especially common in peppers

What strategies do we have to mitigate heat? 

Proper ventilation

The #1 design issue I see in high tunnels is not enough ventilation, specifically at the top of tunnels. High tunnel kits are not sold with proper vents, and so it's not surprising that most growers don't have enough ventilation in their tunnels. Most tunnels come with roll-up sides, but it's critical to have a place where hot air can escape from the top of your tunnel so that the air circulates properly, and cool air can be pulled in from the sides.

Diagram of air movement in tunnel. Natalie Hoidal. From: Choosing a high tunnel for your farm

This photo of a high tunnel at Fairhaven Farm is a great example of a tunnel with good ventilation. In addition to roll-up sides, they have a large door that can remain open and two big gable vents at the top of the tunnel. In addition to vents, add some fans to your tunnel to keep air circulating throughout. 

High tunnel with gable vents at Fairhaven Farm. Photo: Natalie Hoidal 


Shade cloth

Shade cloth is generally very underutilized in Minnesota high tunnels. I've probably visited 100 high tunnels at this point, and only three or four had shade cloth. Shade cloth can lower temperatures in a high tunnel by 6-9°F. Typically it's recommended to hang your shade cloth when air temperatures in the tunnel consistently reach around 80-85°F, and leave it up until fall. Most companies in the Midwest sell either 30% or 50% shade cloth, meaning 30% or 50% of incoming light is blocked, and costs range from $400-600 for a 30' x 96' high tunnel. This shade cloth can be used for many years if it is kept clean and stored properly (e.g. in a dry place away from mice). Based on shade cloth research in Iowa high tunnels, 30% shade cloth is a good bet for Minnesota farmers. 50% shade cloth may provide too much shading and reduce yields; 30%  reduced the incidence of sunscald in fruit and kept the tunnels cooler without compromising yields.

Photo: Ajay Nair, Iowa State University

Irrigation planning

Consistent irrigation is key to managing plant stress in hot conditions. High tunnel crops use more water than field crops since they tend to be larger, producing more biomass and fruit, and because they are growing in hotter conditions. One of the best ways to make sure you're watering consistently is by incorporating soil moisture sensing into your tunnel. The most common technology used by vegetable growers is measuring soil water tension with a tensiometer or electrical resistance sensors. These sensors are typically placed at two depths: 1/3 and 2/3 the depth of the crop root zone. This allows you to monitor water depletion throughout the crop root zone and calibrate the frequency and rate or irrigation. Read more about these tools here. 


Author: Natalie Hoidal

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