Vapour Pressure Deficit (VPD) describes the drying power of the air and how strongly it pulls water from a plant’s leaves. It is the difference between how much moisture the air can hold and how much it is actually holding, and it directly controls transpiration.
When VPD is in a moderate range, water moves smoothly from roots to leaves, supporting nutrient transport and stable growth. When VPD is too low, air is already humid and water movement slows, reducing transpiration and nutrient uptake. When VPD is too high, water loss from leaves can outpace root uptake, leading to stress even when moisture is available in the pot.
Indoors, VPD interacts strongly with light. Low light combined with low VPD often leads to slow dry-down and oxygen restriction, while higher VPD increases water demand and narrows the margin for root and watering errors.
In short, VPD explains how air conditions influence water movement through a plant, helping growers understand why humidity and temperature affect watering behaviour even when light and care routines haven’t changed.
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