Start from our stage chart and set the room so your measured leaves land on its VPD. The chart gives leaf VPD, leaf temperature, night room temperature, PPFD and CO₂ for every week. Your setpoints are the air temperature and RH that hit those targets in your room, with your lights, airflow, plant density and equipment. That's why another facility's settings rarely transfer straight over.
The chart targets
VPD, leaf and night numbers are the same in our LED and HPS guides; CO₂ is shown as LED / HPS where they differ. PPFD targets are in PPFD and DLI.
| Stage | Leaf VPD (kPa) | Leaf (°F) | Night room (°F) | CO₂ (ppm) |
|---|---|---|---|---|
| Veg week 1 | 0.7 | 81 | 81 | 450 |
| Veg week 2 | 0.8 | 82 | 82 | 550 |
| Veg week 3 | 0.9 | 83 | 82 | 650 |
| Veg week 4 | 0.9 | 83 | 82 | 700 |
| Flower week 1 | 0.9 | 83 | 77 | 1,000 |
| Flower week 2 | 0.9 | 83 | 77 | 1,075 / 1,050 |
| Flower week 3 | 1.0 | 82 | 77 | 1,150 / 1,100 |
| Flower week 4 | 1.1 | 82 | 72 | 1,250 / 1,200 |
| Flower week 5 | 1.1 | 82 | 72 | 1,300 / 1,200 |
| Flower week 6 | 1.2 | 81 | 71 | 1,300 / 1,200 |
| Flower week 7 | 1.3 | 80 | 70 | 1,200 / 1,125 |
| Flower week 8 | 1.4 | 78 | 68 | 1,000 |
| Flower week 9 | 1.5 | 75 | 65 | 800 |
CO₂ is optional; the same light and feed chart works without it. Clones run at 0.5 kPa.
Turn targets into setpoints
- Measure air temperature and RH at canopy height with a handheld reference (we use a Vaisala HM41) and leaf temperature with an IR thermometer (we've used a Fluke 64). Take readings in several spots.
- Put them into our VPD calculator and try small RH or temperature changes there before touching the equipment.
- You don't have to hit the leaf temperature exactly. Adjust humidity to land on the chart VPD at your measured leaf temperature.
- Set the controller, let the room settle, then measure the leaves again with the AC and dehumidifiers both on and off.
- If the controller uses a leaf offset, set it from your measured leaf-to-air difference. See why controllers and calculators disagree.
There's a worked example in VPD: Why Leaf Temperature Matters.
Night setpoints
The night number is room temperature. We aim to match the daytime VPD at night, which in a cooler room means lower RH. Never raise night humidity to chase a VPD number. Set dehumidifiers to cycle a little below target; a quick rebound that comes back down fast is fine. More in Why Humidity Spikes at Lights-Off.
Equipment that can hold it
- Separate AC and dehumidifiers. That's our preference, for serviceability and redundancy.
- Size cooling for the whole load: lights, dehumidifiers and everything else in the room.
- Don't go colder to fight humidity. A colder room can reduce what the dehumidifiers remove.
- Shared air means a shared environment. Rooms on the same AC, dehumidifiers and air share conditions too; check the layout before you stagger stages in them.
- Can't hold the setpoint? Fix sensor placement, cycling or capacity before trying more extreme settings.
Change one thing at a time
Make a small change, hold it long enough to see the effect and compare readings in more than one location. Watch water use: a higher-VPD setting deepens drybacks and a lower one keeps pots wet longer, so adjust irrigation with it (here's how). The CropSalt feed chart stays the same.
Need help applying this?
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