Reading the Air
Plants don’t read thermometers. They don’t respond to temperature alone, or humidity alone. They respond to the relationship between them — specifically, to the difference between the vapour pressure of water inside their leaves and the vapour pressure of the surrounding air. That difference has a name: Vapour Pressure Deficit, or VPD. It’s the metric that determines whether a plant is actively growing, conserving water, or quietly stressed. It drives transpiration, nutrient uptake, and stomatal behaviour. Commercial greenhouse operators have used VPD as their primary climate metric for decades. Most small-scale growers — market gardeners, hobby glasshouse operators, urban food producers — have never encountered it, because the equipment to measure and act on it has cost thousands of dollars and required specialist configuration.
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