VPD Made Simple

Vapor Pressure Deficit describes how thirsty the air is for water. When air is dry for a given temperature, plants lose water fast; when it’s moist, leaves can stay open with less stress. Managing VPD is about letting stomata stay comfortably open—more CO₂ intake, more growth, less disease.

You don’t need to be an engineer. In veg, run slightly higher humidity with moderate temperatures so plants grow fast and stack nodes tightly. In late flower, reduce humidity to cut mold risk and tighten buds. The simplest upgrade is placing a good hygrometer at canopy height and moving air gently through the leaves.

Tents get “microclimates”—humid pockets inside bushes and dry edges near fans. Use one fan above and one below the canopy to circulate without blasting buds. This mixes the air column, prevents dew points from forming inside the plant, and keeps leaf surfaces at comfortable transpiration rates.

Dry margins and tacoing in an otherwise cool room suggests the air is too dry for that temperature. Soft, slightly droopy leaves with slow growth suggest the air is too moist or stagnant. Minor tweaks—fan angle, a small dehumidifier bump, or a tray of water—often fix it within a day.

Bud rot loves three things: high humidity, still air, and dense flowers. Break any one of the three and you slash risk. Defoliate modestly to open lanes, keep gentle cross-breezes, and avoid watering right before lights out.

At lights off, temperatures drop and relative humidity spikes. If morning RH is consistently high, schedule dehumidifiers to run an hour into lights off and the hour before lights on. This bookends the riskiest window.

Log temp and RH once in the morning and once at lights on. Note how plants looked. Over weeks, you’ll see that “feel” correlates to numbers. Your goal isn’t a perfect chart; it’s steady, comfortable ranges that keep stomata open and plants calm.

Reliable hygrometer at canopy height, small dehumidifier or humidifier sized to your space, oscillating fans, and a smart plug to schedule devices.