Every grow room I have ever run had a chart taped to the wall next to the door. Not because I could not pull the number up on my phone, but because a chart on the wall gets looked at. A number in an app gets ignored until something goes wrong.
This is that chart, now with separate air VPD and leaf VPD pages. The free four-page PDF includes Fahrenheit and Celsius charts, stage bands, and clearly labeled leaf-temperature assumptions. Print the pages you use, tape them up, and make the room check automatic.
The leaf charts assume leaves are about 2°F (1°C) cooler than room air. Measure your canopy temperature and use the calculator when your actual offset differs.
What a VPD Chart Actually Shows You
VPD stands for vapor pressure deficit. In plain terms, it is the difference between how much moisture the air is holding and how much it could hold at that temperature. It is the real driver behind how fast your plants pull water and transpire.
A VPD chart takes two things you can read off any controller, air temperature and relative humidity, and turns them into a single VPD value measured in kilopascals (kPa). Instead of guessing whether 78F and 60% humidity is good or bad, you find the row and column and read the band.
The whole point is that temperature and humidity mean nothing on their own. 65% humidity is fine in veg and a problem in late flower. The chart bakes that relationship in so you do not have to do the math in your head.
How to Read the Axes
The chart is a grid. One axis is air temperature, the other is relative humidity.
- Find your current temperature along one edge.
- Find your current relative humidity along the other.
- The cell where they meet is your VPD, shown in kPa and colored by band.
That is it. Two readings, one answer. The color tells you if you are sitting where you want to be.
One habit worth building: read the chart at the same points every day, not just when you happen to glance at the controller. First thing when lights come on, and again a few hours in, tells you more than a single random check. Rooms drift on a schedule, and the chart only helps if you catch the drift.
What the Color Bands Mean
Most VPD charts, including this one, use three bands.
- Too dry (high VPD): the air is pulling water out of the plant faster than the roots can replace it. You will see taco-ing, curled leaf edges, and stress in flower.
- Ideal (target band): transpiration and CO2 uptake are in a healthy range for that stage. This is where you want to live.
- Too humid (low VPD): the plant cannot transpire fast enough. Growth slows and, more importantly, you are building the wet, stagnant conditions that botrytis and powdery mildew love.
The ideal band is not one fixed number. It shifts by growth stage, which is the part most charts get lazy about.
A quick note on the humid side, since that is where most rooms get burned. Sitting in the too-humid band for an afternoon will not hurt you. Sitting there every night while lights are off, week after week in flower, is how disease pressure builds quietly until you find it on a bud. Watch the band during your dark period, not just the middle of the day.
Target VPD Ranges by Stage
Younger plants have small root systems and undeveloped stomata, so they want a gentler deficit. As the plant matures you push VPD up to drive transpiration and keep humidity in check during flower.
| Stage | Target VPD (kPa) |
|---|---|
| Clones / seedlings | 0.4 to 0.8 |
| Early veg | 0.8 to 1.0 |
| Late veg | 1.0 to 1.2 |
| Early flower | 1.0 to 1.3 |
| Late flower | 1.2 to 1.6 |
These ranges line up with the controlled-environment research on cannabis and other high-value crops. Work by Zheng and colleagues at the University of Guelph on cannabis production environments, along with the broader greenhouse literature summarized by Llewellyn, supports keeping younger tissue in a lower deficit and raising it through the cycle. Rodriguez-Morrison’s light and environment studies reinforce how tightly climate and plant response are linked.
Treat the ranges as a starting point, not gospel. Your genetics, airflow, and light intensity all nudge the sweet spot. The chart gets you close, then your own data dials it in.
Air VPD and Leaf VPD Are Both Included
Air VPD uses room temperature and relative humidity. It is useful for a fast check because those readings are available on nearly every controller.
Leaf VPD uses the saturation pressure at the leaf surface, so canopy temperature matters. The PDF now includes leaf VPD charts calculated for leaves running about 2°F cooler than room air, or 1°C on the Celsius page. That is a practical wall-chart offset, not a universal correction.
Check representative upper-canopy leaves with an infrared thermometer. If your leaves are warmer, cooler, or changing through the light cycle, use the free VPD calculator with the measured leaf temperature. For the full explanation, read leaf VPD versus air VPD.
What Is in the Printable PDF
- Air VPD chart in Fahrenheit
- Leaf VPD chart in Fahrenheit, using a 2°F cooler-leaf offset
- Air VPD chart in Celsius
- Leaf VPD chart in Celsius, using a 1°C cooler-leaf offset
Each page is sized for standard US Letter paper and includes the same stage-band legend.
Download the printable air + leaf VPD chart (PDF)
From Chart to Real Numbers
A printed chart is great for a gut check. When you want the exact VPD for your current readings without hunting across a grid, the free VPD calculator gives you the number instantly and shows which stage band you land in.
Leave a Reply