What Is VPD, and What Should It Be Each Week of Flower?
If you only watch relative humidity (RH), you're missing the number the plant actually feels. VPD tells you how hard the air is pulling water out of the leaf — and it's one of the most powerful levers in crop steering.
What VPD is (no formula needed)
VPD (Vapor Pressure Deficit) is the distance between the air's current moisture and the point where it's saturated, measured in kPa. It folds temperature and humidity into a single number, because a plant doesn't feel RH alone — it feels the water-pulling force that results from both together.
- VPD too low (very humid air) — the plant transpires little, takes up nutrients slowly, and risks mold and leaf disease.
- VPD too high (dry air) — the plant transpires fast, closes its stomata to protect itself, grows slowly, and stresses.
- VPD just right — stomata open, good gas exchange, steady water and nutrient uptake.
Target ranges by stage (general reference)
| Stage | Target VPD (kPa) | Why |
|---|---|---|
| Seedling / clone | 0.4 – 0.8 | Few roots yet — protect leaves from losing water too fast |
| Vegetative | 0.8 – 1.2 | Encourage transpiration and leaf growth |
| Early flower (wk 1–3) | 1.0 – 1.3 | Transition — raise the pull gradually |
| Mid flower (wk 4–6) | 1.2 – 1.5 | Flowers bulking, high transpiration demand |
| Late flower (wk 7+) | 1.3 – 1.6 | Lower humidity to prevent mold in dense flowers |
These ranges are starting points — adjust for your strain, growing system, and how your plants actually respond.
Reading it right: watch the trend, not one point
A single VPD reading tells you little. What matters is how many hours a day the plant sits inside the target zone. A farm with a pretty afternoon VPD but spikes at midday and dips overnight is creating stress invisible to a daily average. That's why continuous logging with a "target zone band" beats walking over to read a gauge once a day.
The GACP connection
Good news for farms eyeing certification: the same system that logs VPD continuously is the system that produces your GACP environmental records. You get better cultivation and inspection-ready documentation from a single effort. Read What overnight dryback reveals about irrigation to complete the crop-steering picture.