Shut Off your HAF at 86°F

As temperature rises, the air can become too dry for the plant. To preserve its water reserves, it stops transpiring. That has the side effect of stopping energy production as well.
And this occurs before the plant wilts.

Lowering ventilation reduces the drying effect by keeping the transpired humidity around the canopy.

We do that by shutting down the circulation fans (HAF).

The extra airflow they provide is harmful in this situation.

It might seem strange to close the ventilation when it's hot. However, plants handle heat waves better when they can transpire and regulate their temperature.

The determining factor is the percentage of humidity. The threshold at which humidity is too low for the tomatoes changes with the temperature.

Here is a table showing when you should close your circulation fans.
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