Warm temperatures in high tunnels
High tunnels hold the heat for growing crops in spring and fall. But this time of year, they can get much hotter than plants prefer. Cool season crops like lettuce will not germinate when soil temperatures get above 80-degrees. Greens and brassicas will send up a flower stalk and become bitter. The high tunnel can even get too hot for warm season crops such as tomatoes and peppers.
Joe Hannan is an Extension horticulture field specialist at Iowa State University. He says they won’t set fruit if it’s above 90-degrees and you might also notice ripening disorders.
"You see it most common on tomato, where you get a yellow top on the tomato or you get a green top. And that’s due to those warm daytime temperatures and warm nighttime temperatures that doesn’t allow lycopene to form. Lycopene is what gives it that nice color," says Hannan. "If temperatures are really hot again during the daytime, during the nighttime, you can end up with fruit that kind of heat ripens and so it may not have the best flavor. But that a lot of times is dependent on what variety is being grown."
The building needs good airflow, so open the sidewalls and end walls as much as you can.
"I’ve even seen where people will take the sidewalls off and just leave the cover on the building. Another option is to look at adding shade cloth on top of the building. It has about a 30% reduction in light transmission, and that should help you cool down the building a couple of degrees," he says. "Roof top vents, if you have them, get them open or strongly think about putting them in."
Everyone has produce to sell in July and August, so Hannan says another option is to enjoy a little downtime and get changed over and geared up for fall production.
Learn more about managing high tunnels in the summer heat
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