When it’s hot and humid, the vents run out of road
Location: -
Plant: Tomato
Area: -
Product used: Trutina Risk Reduction Report
We tend to trust the vents to hold the climate. During a recent hot-and-humid spell, they simply ran out of road and the data shows exactly where.
For most of the fortnight the vents ran 90–99% open (mean 94%) and the energy screen was pulled back. There was almost no control headroom left.
And yet the greenhouse VPD still swung from ~0.35 kPa before dawn to ~3.3 kPa in the afternoon. On 10 of 14 days it pushed past the 1.5 kPa stress line.
Here's the mechanism people underestimate: when the outside air is hot and humid, opening up further just pulls in more hot, humid air. You can't create VPD passively when there's no drier air to let in. A wide-open, passive greenhouse has no answer to that weather.
So the honest read isn't "the climate settings were wrong." The passive levers were saturated. What was still in the grower's hands were the other levers, irrigation and EC, the radiation load (a diffuse or shading screen), and active humidification / air movement where the house has it.
The takeaway I keep coming back to: know which of your levers still has travel left. In a hot-humid week, the vents may already be at the stop and planning around that beats fighting it.
How do you steer a passive greenhouse through hot-and-humid weather, when venting alone can't hold the VPD?
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