VPD is a lane to steer by, not a number to chase.
What is VPD, and do I need to chase it?
GROWER ANSWER
VPD is how hard the air pulls water out of the leaf. Temperature and humidity set it together, so it's not a third thing to manage. You don't need to chase it, and you don't need a VPD meter to grow well. If you already watch temperature and humidity and read the plant, you're already steering it.
VPD is a lane to steer by, not a number to chase.
VPD in plain words
VPD stands for vapor pressure deficit: the drying power of the air. Warm, dry air pulls hard. Cool, humid air pulls gently. That pull moves water up from the roots and out through the leaves.
Too little pull: in very humid air she moves water slowly, and damp air invites mold.
Too much pull: in hot, dry air she can lose water faster than the roots replace it, so she curls, droops or closes up and growth slows.
Deficit or density? Vapor density is just how much water the air holds. The deficit is the gap between the leaf and the air, and that gap is the pull on the leaf.
It's measured against the leaf, not the room. Under LED the top leaves usually run 1 to 4°F cooler than the air, so the VPD at the leaf is a little lower than an air reading.
In winter, heating a cold room dries its air, so the pull climbs. See My tent is too dry this winter. What do I do?
Do I need a meter or a chart?
No. A thermometer and a humidity reading you trust, in the right spot, give you both ingredients. A controller that shows VPD does the math for you: handy, not required.
A chart is a map, not a test. The lane climbs with the stage: roughly 0.8 to 1.2 kPa in veg, up to about 1.2 to 1.5 kPa in late flower. Every stage's lane, with its humidity, is on What humidity should I run, and what if I can't hit it? Temperature and humidity work as a pair: when the room runs warm, run the humid end of the humidity lane, and the reverse.
A healthy plant a little outside the chart is not a problem. The number only matters when she says it does.
Read it in this order
Read the plant first. Leaves held up toward the light, good color, clean new growth and pots drying on a normal rhythm mean the room is working.
Prove the reading. Put the sensor at canopy height, shaded, out of the fan blast, away from wet pots and walls. Test the hygrometer: damp salt sealed in a container with it for 6 to 8 hours should read about 75%.
Check that the room holds steady. Look across a full day and night, lights on and off. A steady room a little off the chart beats one swinging all day.
Check the roots before the air. Lift the pot. A droop in a light pot points to thirst; in a heavy pot it usually means too wet. A VPD number can't tell you which.
Then move one lever, gently. Temperature is often the easier lever: warming the air also lowers its humidity, so a small change moves VPD without adding or pulling moisture. Change humidity 3 to 5 points or temperature about 2°F at a time, never both at once. Let it settle at least an hour, and judge it over a full day and night.
Leaf temperature is the tiebreaker. If the air reads steady and in its lane but she looks stressed, point an infrared thermometer at the top leaves. A leaf at or above the air temperature means her cooling has stalled.
What not to do
Don't go to war on the room at midnight. Cranking the dehumidifier, the AC and a humidifier at once to drag one decimal into the green makes the room swing.
Don't fix a happy plant because a chart says so.
Don't let a good VPD clear a humid flower room. Mold follows humidity that stays high for hours and water sitting on surfaces, whatever the VPD reads. In late flower, watch the humidity, most of all at lights off.
When it's something else
Drooping during the day: see How often should I water, and why is she drooping?
Tops tacoing or bleaching near the light: see Tacoing or bleached tops: is my light too close?
Steering by dryback: lifting the pot tells you wet or dry. Measuring dryback as a percentage takes a moisture sensor or a scale.
On our shelf in Worcester: Fans and the controllers that read temperature, humidity and VPD are in Climate, air & controls. Call (508) 459-9609 or get directions.
Keep going
Keeping the tent's air steady: see our Tent airflow shelf guide.
Watch the episode: VPD Too High? Why Chasing the Perfect Number Backfires on MyRemy Live.
Watch the episode: Why More Grow Data Won't Fix Your Plant: Diagnose in the Right Order on MyRemy Live.
Read the plant before the readout. The whole system is in the free Stability Playbook. Get the free Stability Playbook
More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.