VPD by Growth Stage: Chart, Targets, and How to Hold Them (2026)
By LoopString
Target leaf VPD by growth stage: seedlings and clones 0.4–0.8 kPa, vegetative 0.8–1.2 kPa, early flower 1.0–1.2 kPa, flowering 1.0–1.5 kPa, and late flower 1.2–1.6 kPa. Vapor pressure deficit (VPD) is the single number that tells you whether your temperature and humidity are letting the plant transpire and feed properly — and unlike relative humidity, it already accounts for temperature. Below is the chart, the reasoning, and how to keep VPD inside the band as conditions shift through the day.
Want the number for your exact conditions? Use the stage-aware VPD calculator — it takes a leaf-temperature offset and tells you the control action, not just the kPa.
What is vapor pressure deficit (VPD)?
VPD is the difference between how much water vapor the air is holding and how much it could hold at saturation, measured in kilopascals (kPa). That gap is the force pulling moisture out of the leaf. When VPD is too low the air is nearly saturated, transpiration slows, and calcium and other nutrients stop moving — and mold risk climbs. When VPD is too high the air is parched, the plant closes its stomata to conserve water, and growth stalls. Keeping VPD in the right band keeps the plant breathing and feeding at full rate.
VPD chart by growth stage
These are practitioner-consensus leaf VPD bands, corroborated across multiple grower and vendor sources. Ramp gradually between stages rather than jumping, and treat the edges as guidance, not hard limits.
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Leaf VPD vs air VPD — the distinction that matters
Most cheap calculators compute air VPD: they plug air temperature into both sides of the equation. But transpiration happens at the leaf surface, and a leaf under LED or HID light while actively transpiring runs roughly 2–3°C (3–5°F) cooler than the surrounding air. Because saturation vapor pressure rises steeply with temperature, that few-degree difference moves the real VPD by 0.2–0.4 kPa. If you dial your room to an air-VPD target, the plant is actually experiencing something else. The fix is to measure or estimate a leaf-temperature offset and calculate leaf VPD — which is what the calculator does.
Why relative humidity alone is not enough
Growers who chase a fixed RH number drift out of range constantly, because the same RH is a different VPD at every temperature. 60% RH is about 1.17 kPa at 75°F but about 1.59 kPa at 85°F — the first is a healthy veg number, the second is dry enough to stall growth. Every time the lights heat the canopy or the outside weather shifts, a fixed-RH setpoint quietly moves your VPD. VPD is the target that already folds temperature and humidity together, which is why it is the number worth controlling to.
From a number to a control action
A reading only helps if it tells you what to change:
- VPD below the band (too humid): raise VPD — dehumidify, nudge temperature up, or increase airflow.
- VPD in the band: hold it — the work is keeping it there as the photoperiod and weather move it.
- VPD above the band (too dry): lower VPD — humidify or nudge temperature down.
The hard part is that VPD never sits still: it climbs through the lights-on period and drops at lights-off. Holding it by hand means watching a hygrometer all day and reacting. Holding it automatically means a sensor reading temperature and humidity continuously, a per-stage target band, and actuators — humidifier, dehumidifier, heater, fan — driven to stay inside it.
That is exactly what LoopString grow-room automation does: edge PID control on a Raspberry Pi drives the equipment to hold leaf VPD in the band, threshold alerts fire the moment it drifts out, and per-stage schedules move the target as the plant moves through its lifecycle — so the setpoint follows the grow instead of waiting for you to change it.
Quick reference: dialing in VPD
- Measure canopy air temperature and relative humidity.
- Estimate a leaf-temperature offset (start at 2–3°C / 3–5°F cooler under light).
- Calculate leaf VPD and compare it to the stage band above.
- If you are out of range, apply the control action — then automate it so it holds overnight and through stage changes.
Frequently asked questions
Target leaf VPD by stage: seedlings and clones 0.4–0.8 kPa, vegetative 0.8–1.2 kPa, early flower 1.0–1.2 kPa, flowering 1.0–1.5 kPa, and late flower 1.2–1.6 kPa. Ramp gradually between stages rather than jumping, and treat the edges as guidance, not hard limits. These are practitioner-consensus leaf VPD bands.