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. It works in both Fahrenheit and Celsius, it covers every stage from clone to late flower, and it is free. Print it, tape it up, and use it as your quick gut check every time you walk the room.
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.
The One Thing a Printed Chart Cannot Do
Here is the honest limitation. A static chart calculates air VPD, which uses air temperature. The number that actually matters to the plant is leaf VPD, which uses leaf temperature.
Under strong LEDs, leaf temperature can run a couple of degrees cooler or warmer than the surrounding air depending on transpiration and airflow. That gap shifts the real target away from what the wall chart says. When the data shows plants underperforming inside a “correct” air-VPD range, leaf temperature is usually the missing piece.
A wall chart is still worth having as your fast reference. Just know that it is the air-VPD version, and leaf VPD is the more precise measurement once you are ready to chase it. That is a whole topic on its own.
Download the Printable Chart
The free PDF includes both a Fahrenheit chart and a Celsius chart, stage bands marked, sized to print clean on standard paper. Tape it inside the door where you do your walk-throughs.
Download the free Growgoyle 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.
Software That Runs Your Grow
The chart tells you where you should be. Growgoyle is the software that runs your grow and tells you where you actually are, batch by batch, without you logging it by hand.
METRC tracks your grow for the state. Growgoyle tracks it for you. It keeps your environment history, your batches, and your stage targets in one place so a drifting room shows up as data before it shows up as a problem in the tent.
You don’t need to wait for a new batch. Got a room in flower right now? That’s all you need.
Start your free 30-day trial and put the c
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