VPD CHART
Leaf deficit, not room air. Drag the chart or the sliders.
Vapor pressure deficit is the gap between how much water the air can hold and how much it is holding. The plant feels that gap at the leaf, not on the wall thermometer. A small gap means the leaf is not moving water. A wide gap pulls water out faster than the roots replace it.
Dialing it in is how you keep the stomata open. Clones want a tight range, about 0.4 to 0.8 kPa, so a cut does not dry out before it roots. Veg sits around 0.8 to 1.1. Flower sits around 1.1 to 1.4, dry enough to limit mold and still wet enough that the plant is not slamming its pores shut. The chart uses room temperature, humidity, and a leaf offset, because an LED room runs the leaf cooler than the air.
Set the stage, move the two sliders, and the number is the leaf VPD. Green is the band for that stage. Outside it, the plant is working against the room.
How to check it
You need two cheap tools. A thermometer-hygrometer for the room, and an infrared temp gun for the leaf. The wall unit tells you the air. The gun tells you what the leaf is actually doing. Those are different numbers.
The chart has three inputs.
- Room temperature. Air at canopy height, not the number on the driver or the floor. Fahrenheit on this chart.
- Humidity. Relative humidity at the same spot, as a percent. 60 means the air is holding 60 percent of what it can hold at that temperature.
- Leaf offset. How much cooler the leaf is than the air. LEDs usually run the leaf 1 to 3°F cooler. The chart starts at 2°F. If you measure the leaf, use the real gap instead of the guess.
- Hang the hygrometer at the top of the canopy, out of the direct beam. Let it sit 10 minutes.
- Read room temperature and humidity. Write both down. That is the first two inputs.
- Set the infrared gun to a high emissivity, about 0.95. A leaf is not a mirror.
- Shoot the top of a leaf, not the light, not the pot, not a wet spot. Hold the gun close, about a handspan, so the dot is only on the leaf.
- Subtract leaf temperature from room temperature. Room 78, leaf 76, offset is 2. If the leaf is warmer, the offset is negative.
- Pick the stage. Clone, veg, or flower. Move the sliders to your room numbers and set the offset.
- Read the leaf kPa. Green is the band for that stage. Under it, the room is too wet for the leaf. Over it, the leaf is dumping water faster than the roots replace it.
Check a few leaves, not one. A leaf in the beam runs warmer than a leaf in the shade. Take the reading when the lights have been on for an hour, not at lights-on. The number is a target for the room, not a diagnosis of the plant.