VPD Growth Stage Charts

🌡️ Air Temperature vs. Leaf Temperature Important

There are two ways to express VPD, and conflating them is the most common source of confusion:

• Air-temperature VPD — calculated using the air temperature reading from a standard thermometer/hygrometer. This is what most consumer VPD charts display, and what all values on this page default to.
• Leaf-temperature VPD — uses an infrared thermometer to measure the actual leaf surface temperature. Because transpiring leaves are cooler than the surrounding air (typically 1–3 °C cooler under LED/HPS lighting), leaf-VPD is usually 0.1–0.3 kPa lower than air-VPD under normal conditions.

Leaf-VPD is more physiologically accurate — it represents what the stomata "experience" — but air-VPD is far easier to measure consistently. If your source quotes VPD ranges and doesn't specify, assume air-temperature VPD.

📐 The VPD Formula Info

VPD is calculated using the Tetens equation for saturation vapour pressure:

Where T is in °C and RH is relative humidity as a percentage. The heat map on this page computes air-VPD for each cell.

🌱 Leaf Temperature Offset Important

The typical 1–3 °C offset between air and leaf temperature is not fixed — it varies with:

• Light type & intensity: HPS runs hotter IR, so leaves are warmer (offset ~1 °C). LED emits less radiant heat, so leaves are cooler (offset ~2–3 °C).
• Air movement: Strong airflow reduces the boundary layer, bringing leaf temp closer to air temp.
• Transpiration rate: Rapidly transpiring plants cool themselves more, widening the offset.
• Plant health: Stressed or wilting plants transpire less, so leaf temp rises toward air temp.

For precision, use an infrared thermometer aimed at the upper canopy to measure leaf temperature directly.

📊 Growth Stage Ranges — Sources & Caveats Info

The VPD ranges shown are consensus values compiled from published cultivation guides (Pulse, AROYA, Crop Steering literature) and widely referenced by commercial cannabis cultivators. Key caveats:

• Genetics vary. Indica-dominant cultivars may tolerate slightly lower VPD in flower; sativas may push higher.
• Stage durations are approximate. A 14-week timeline assumes a typical photoperiod grow. Autoflowers compress this significantly.
• Late flower drying: Some growers deliberately push VPD above 1.6 kPa in the final 1–2 weeks to reduce bud moisture and enhance resin production. This is strain-dependent.

🌙 Lights-Off VPD — Why It's Different Important

During the dark period, several factors combine to lower VPD naturally:

• Temperature drops when lights go off — typically 2–5 °C below the daytime setpoint — reducing saturation vapour pressure and therefore VPD.
• Stomata largely close in the dark. The plant doesn't photosynthesise, so there is far less demand for transpiration-driven nutrient uptake.
• Humidity tends to rise as residual moisture in the medium continues evaporating while transpiration slows, pushing VPD lower still.

The primary concern at night shifts from driving transpiration to preventing condensation on leaf surfaces. Dew formation (VPD approaching 0) creates an ideal environment for botrytis and powdery mildew. Keeping VPD above 0.2 kPa (or 0.5 kPa in flower) with adequate airflow is the key night-time target.

The lights-off ranges shown on this page assume a typical 2–4 °C night-time temperature drop. If your room drops more than 5 °C, humidity control and dehumidification become critical.

⚠️ Danger Zones Important

• VPD < 0.4 kPa — The air is nearly saturated. Transpiration stalls, nutrient uptake drops, and the risk of powdery mildew, botrytis, and other fungal pathogens rises sharply.
• VPD > 1.6 kPa — The air is very dry relative to temperature. Stomata begin to close to conserve moisture, reducing CO₂ intake and photosynthesis. Persistent high VPD causes leaf curl, tip burn, and nutrient lockout.