VPD (Vapor Pressure Deficit) Basics

What VPD actually is

Vapor Pressure Deficit is the difference between how much water vapor the air is currently holding and how much it could hold if saturated, measured in kilopascals (kPa). When VPD is low, the air is close to saturated and plants struggle to transpire. When VPD is high, the air is thirsty and pulls water out of leaves faster than roots can replace it.

There are two flavors worth knowing:

• Air VPD: calculated from air temperature and relative humidity alone.
• Leaf VPD (more accurate): uses leaf temperature instead of air temperature. Leaves under transpiration are typically 1–3 °C cooler than the surrounding air, which meaningfully changes the number ^[1] Strong evidence.

The underlying physics — that transpiration is driven by the vapor pressure gradient between the stomatal cavity and the bulk air — is well-established plant physiology, not cannabis folklore ^[1][2] Strong evidence.

Why growers use it

Relative humidity by itself is misleading. 60% RH at 20 °C and 60% RH at 28 °C describe very different conditions for a plant: the warmer air holds far more water, so the deficit — and the pull on the leaf — is much greater. VPD collapses temperature and humidity into a single number that better predicts how hard the plant is working to move water ^[1] Strong evidence.

In practice, dialing VPD into a sensible range correlates with:

• Steadier transpiration and nutrient uptake.
• Fewer humidity-driven problems (powdery mildew, botrytis at the low end; tip burn, wilting, and stomatal closure at the high end) ^[3] Strong evidence.
• More consistent results between rooms and runs.

What VPD does not do: it does not directly increase cannabinoid content, terpene production, or yield in a way that's been cleanly demonstrated in peer-reviewed cannabis research. Yield-gain claims you see on forums are Weak / limited at best. Treat VPD as an environmental hygiene tool, not a magic lever.

Target ranges (and why charts disagree)

Commonly cited targets for cannabis, drawn from horticultural practice rather than cannabis-specific clinical trials Weak / limited:

• Seedlings / clones: ~0.4–0.8 kPa (high humidity, low transpiration demand).
• Vegetative: ~0.8–1.2 kPa.
• Flowering: ~1.2–1.5 kPa.
• Late flower: some growers push to ~1.5–1.6 kPa to discourage mold; others stay lower.

These numbers are approximations borrowed from general greenhouse horticulture ^[2]. Different charts from different brands disagree by ±0.2 kPa, partly because they assume different leaf-to-air temperature offsets. The 'perfect VPD' debate is largely folklore — stability inside a reasonable range matters more than hitting a specific decimal.

How to do it: step-by-step

1. Measure accurately. Get a thermo-hygrometer with a documented accuracy spec (±2–3% RH is realistic for prosumer units). Place it at canopy height, in the airflow, shaded from direct lamp radiation. Calibrate humidity with a salt-test kit if you can.

2. Pick your stage's target range. Use the ranges above as a starting point, not gospel.

3. Calculate VPD. Use a calculator or chart. The simplified formula:

`SVP = 0.6108 × exp((17.27 × T) / (T + 237.3))` (saturation vapor pressure in kPa, T in °C)

`VPD = SVP × (1 − RH/100)`

For leaf VPD, compute SVP at leaf temperature and subtract the air's actual vapor pressure (SVP_air × RH/100).

4. Adjust one variable at a time. Raise temperature or lower humidity to raise VPD; do the opposite to lower it. Changing both at once makes it impossible to learn what your room actually does.

5. Log and observe. Record VPD, leaf appearance, and transpiration cues (substrate dry-back rate, leaf turgor, tip condition) for a couple of weeks. Trust the plant over the chart.

6. (Optional) Measure leaf temperature. A cheap IR thermometer pointed at a mid-canopy fan leaf gets you close enough to calculate true leaf VPD.

Common mistakes

• Trusting cheap hygrometers. Bargain-bin units can be 10%+ RH off. Your VPD number is only as good as the sensor.
• Ignoring leaf temperature. Under intense LED or HPS, leaves can run cooler (transpiring well) or hotter (stressed) than air. Air VPD alone hides this Strong evidence.
• Chasing decimals. Bouncing between 1.25 and 1.35 kPa is not a problem worth solving. Wild swings between 0.6 and 1.8 across a day are.
• Forgetting lights-off. VPD changes when temperature drops at night. Many rooms drift into mold territory in the dark phase.
• Treating VPD as a yield knob. It's a stress-management tool. The biggest yield levers are still genetics, light, CO₂, and irrigation Strong evidence.
• Assuming one sensor represents the whole room. Microclimates exist. Check corners, canopy edges, and dense leaf zones.

Related techniques

VPD pairs naturally with other environmental controls:

• CO₂ supplementation: higher CO₂ lets you run slightly higher temperatures and thus higher VPD without penalty, because stomatal behavior shifts ^[2] Strong evidence.
• Crop steering: irrigation strategies (generative vs vegetative steering) interact with VPD because both influence transpiration and turgor.
• Defoliation: changes canopy airflow and effective humidity around leaves.
• Dehumidification and HVAC sizing: a room that can't pull water out of the air during dark periods will never hit late-flower targets, regardless of your chart.

If you're new to environmental control, start with reliable measurement, then VPD, then CO₂. Skipping ahead is how people end up with expensive sensors and unhealthy plants.

Sources

How this page was made