VPD (vapour pressure deficit) is the single most important environmental parameter for cannabis growers to understand. Get VPD right and your plants transpire efficiently, absorb nutrients rapidly, and yield at their genetic potential. Get it wrong and you'll fight slow growth, nutrient lockout, mold, and tip burn. This guide gives you the exact VPD targets for every cannabis growth stage, explains how to calculate it, and shows you how to fix it when it drifts.
VPD measures the drying power of the air — the difference between how much moisture the air could hold at a given temperature (saturation vapour pressure) and how much it actually holds right now. The higher the VPD, the more aggressively the air pulls moisture from cannabis leaves, driving transpiration.
When VPD is in the ideal range:
VPD is expressed in kilopascals (kPa). It is calculated from your air temperature and relative humidity (RH%).
Use this chart as your primary reference. All values are for leaf VPD (leaf temperature assumed ~1°C below air temperature, which is typical under grow lights).
| Growth Stage | Ideal VPD (kPa) | Temp (°F / °C) | RH% | Status |
|---|---|---|---|---|
| Seedling / Clone | 0.4 – 0.8 kPa | 75–80°F / 24–27°C | 65–75% | Low stress |
| Early Vegetative | 0.8 – 1.0 kPa | 72–78°F / 22–26°C | 55–65% | Optimal |
| Late Vegetative | 1.0 – 1.2 kPa | 72–78°F / 22–26°C | 50–60% | Optimal |
| Early Flower (Week 1–4) | 1.0 – 1.3 kPa | 68–76°F / 20–24°C | 45–55% | Optimal |
| Late Flower (Week 5+) | 1.2 – 1.5 kPa | 65–74°F / 18–23°C | 40–50% | Optimal |
| Flush / Pre-Harvest | 1.3 – 1.6 kPa | 65–72°F / 18–22°C | 35–45% | Controlled stress |
| VPD Too Low (any stage) | Below 0.4 kPa | — | Above 80% | Mold risk |
| VPD Too High (any stage) | Above 1.6 kPa | — | Below 35% | Heat/drought stress |
The formula for air VPD:
VPD = SVP × (1 − RH / 100)
Where SVP (saturation vapour pressure in kPa) = 0.6108 × e^(17.27 × T / (T + 237.3)) and T is temperature in °C.
Example: Grow room at 25°C (77°F) with 55% RH
Leaf VPD uses leaf surface temperature rather than air temperature. Leaf temp is typically 1–3°C below air temp under grow lights. GrowAI calculates both air VPD and leaf VPD automatically from your sensor readings.
When VPD exceeds 1.5–1.6 kPa, the air is aggressively dry relative to the plant's leaf temperature. Cannabis responds by:
When VPD falls below 0.4 kPa (high humidity, often above 70–80% RH in flower), the air is nearly saturated. Cannabis in this environment:
Cannabis doesn't photosynthesize at night, but transpiration continues at a reduced rate. Growers often let RH rise slightly during lights-off since stomata are largely closed — but never let RH exceed 60% in flower during the dark period. A common lights-off VPD target is 0.8–1.2 kPa (slightly lower than lights-on targets) to allow plant recovery without creating mold conditions.
GrowAI's day/night profiles let you set separate VPD alert thresholds for lights-on and lights-off periods automatically.
The three levers you control are temperature, humidity, and airflow. Understanding their relationship:
This is why you often need to adjust both temperature and humidity together to move VPD — raising humidity alone while keeping temperature high may not get you to target.
GrowAI calculates live VPD from your temperature and humidity sensors automatically — no manual formulas needed. Features include:
GrowAI calculates live VPD from your grow room sensors and alerts you the moment it drifts outside the ideal range for your current growth stage.
Get Early Access — Launching 4/20/2026Autoflowers follow the same VPD targets as photoperiod cannabis by stage. Start at 0.4–0.8 kPa in the seedling phase, move to 0.8–1.2 kPa during veg, and transition to 1.0–1.5 kPa during flowering. Since autoflowers move through stages faster, check VPD daily and adjust as the plant develops.
Yes. High VPD stress in late flower can reduce terpene expression as the plant conserves resources. Some growers intentionally drop VPD slightly in the final two weeks (0.8–1.2 kPa) to preserve terpene profiles. Others run a controlled late-flower stress at 1.4–1.6 kPa to encourage resin production. Monitor closely and don't exceed 1.6 kPa without active cooling.
Leaf VPD is more accurate for driving plant behaviour since it uses actual leaf surface temperature. However, leaf temperature requires a thermal sensor or reliable estimation. For most home growers, air VPD is a practical and effective target — just add 0.1–0.3 kPa to account for leaf surface being slightly cooler than air. GrowAI displays both.
Last updated: March 2026 | ← Back to all grow guides | GrowAI Home