VPD Chart
Find your air temperature and relative humidity on the grid to read leaf VPD, coloured by growth-stage zone. Set a leaf-temperature offset, move the marker to your conditions, and print it for the grow.
Leaf VPD (kPa) by Temperature & Humidity
| °F \ RH% | 30 | 35 | 40 | 45 | 50 | 55 | 60 | 65 | 70 | 75 | 80 | 85 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 90 | 2.8 | 2.6 | 2.3 | 2.1 | 1.8 | 1.6 | 1.4 | 1.1 | 0.9 | 0.6 | 0.4 | 0.2 |
| 88 | 2.6 | 2.4 | 2.2 | 1.9 | 1.7 | 1.5 | 1.3 | 1.0 | 0.8 | 0.6 | 0.4 | 0.1 |
| 86 | 2.5 | 2.2 | 2.0 | 1.8 | 1.6 | 1.4 | 1.2 | 1.0 | 0.8 | 0.5 | 0.3 | 0.1 |
| 84 | 2.3 | 2.1 | 1.9 | 1.7 | 1.5 | 1.3 | 1.1 | 0.9 | 0.7 | 0.5 | 0.3 | 0.1 |
| 82 | 2.2 | 2.0 | 1.8 | 1.6 | 1.4 | 1.2 | 1.0 | 0.8 | 0.7 | 0.5 | 0.3 | 0.1 |
| 80 | 2.0 | 1.8 | 1.7 | 1.5 | 1.3 | 1.1 | 1.0 | 0.8 | 0.6 | 0.4 | 0.3 | 0.1 |
| 78 | 1.9 | 1.7 | 1.6 | 1.4 | 1.2 | 1.1 | 0.9 | 0.7 | 0.6 | 0.4 | 0.2 | 0.1 |
| 76 | 1.8 | 1.6 | 1.5 | 1.3 | 1.1 | 1.0 | 0.8 | 0.7 | 0.5 | 0.4 | 0.2 | 0.1 |
| 74 | 1.6 | 1.5 | 1.4 | 1.2 | 1.1 | 0.9 | 0.8 | 0.6 | 0.5 | 0.4 | 0.2 | 0.1 |
| 72 | 1.5 | 1.4 | 1.3 | 1.1 | 1.0 | 0.9 | 0.7 | 0.6 | 0.5 | 0.3 | 0.2 | 0.1 |
| 70 | 1.4 | 1.3 | 1.2 | 1.1 | 0.9 | 0.8 | 0.7 | 0.6 | 0.4 | 0.3 | 0.2 | 0.1 |
| 68 | 1.3 | 1.2 | 1.1 | 1.0 | 0.9 | 0.7 | 0.6 | 0.5 | 0.4 | 0.3 | 0.2 | 0.0 |
| 66 | 1.2 | 1.1 | 1.0 | 0.9 | 0.8 | 0.7 | 0.6 | 0.5 | 0.4 | 0.3 | 0.2 | 0.0 |
| 64 | 1.2 | 1.1 | 1.0 | 0.9 | 0.7 | 0.6 | 0.5 | 0.4 | 0.3 | 0.2 | 0.1 | 0.0 |
| 62 | 1.1 | 1.0 | 0.9 | 0.8 | 0.7 | 0.6 | 0.5 | 0.4 | 0.3 | 0.2 | 0.1 | 0.0 |
| 60 | 1.0 | 0.9 | 0.8 | 0.7 | 0.6 | 0.6 | 0.5 | 0.4 | 0.3 | 0.2 | 0.1 | 0.0 |
- Too dry — over-transpiration
- Late flower / ripening (1.2–1.6)
- Veg / flower (0.8–1.2)
- Seedling / clone (0.4–0.8)
- Too humid — disease risk (<0.4)
Colours group leaf VPD into coarse stage zones; exact per-stage target bands (e.g. flower 1.0–1.5 kPa) can straddle two colours. Treat band edges as guidance, not hard limits.
The outlined cell is the closest grid point to your input (78°F, 55% RH). Rows are air temperature, columns are relative humidity; values are leaf VPD in kPa at a 2.2°C leaf offset.
Target leaf VPD by growth stage
| Stage | Target leaf VPD | Why |
|---|---|---|
| Seedling / Clone | 0.4–0.8 kPa | High humidity while roots establish; light transpiration. |
| Vegetative | 0.8–1.2 kPa | Push transpiration and nutrient uptake as canopy fills. |
| Early Flower | 1.0–1.2 kPa | Transition band; ramp gradually, do not jump. |
| Flowering | 1.0–1.5 kPa | Steady, moderately dry air for dense flower. |
| Late Flower / Ripening | 1.2–1.6 kPa | Drier air lowers mold risk as buds mature. |
Key Takeaways
- A VPD chart maps every temperature-and-humidity combination to a leaf-VPD value, coloured by growth-stage zone — read your cell, then move toward the band for your stage.
- Leaf temperature matters: leaves run 2–3°C cooler than air under light, shifting real VPD by 0.2–0.4 kPa. This chart applies the offset you set.
- Stage bands: seedling 0.4–0.8, veg 0.8–1.2, early flower 1.0–1.2, flower 1.0–1.5, late flower 1.2–1.6 kPa.
- A printed chart is a static reference. Holding VPD in the band as lights and weather shift needs a sensor and actuators driven to the target.
How to read a VPD chart
Air temperature runs down the left axis and relative humidity across the top. The cell where they meet is your leaf VPD in kilopascals. The colours group those values into zones: blue is too humid (sluggish transpiration, disease pressure), the greens are the healthy veg-to-flower range, yellow is the drier late-flower band, and red is too dry (plants close their stomata and stall). To move toward your stage's band, warm the room or dehumidify to raise VPD, or cool and humidify to lower it.
Why leaf temperature changes the chart
Most printed VPD charts assume the leaf is the same temperature as the air. It is not: under LED or HID light a transpiring leaf sits roughly 2–3°C (3–5°F) cooler. Saturation vapor pressure climbs steeply with temperature, so that small difference moves real VPD by 0.2–0.4 kPa — enough to push a cell from "in band" to "too dry." Set the leaf-temperature offset above and the whole chart shifts to reflect the canopy, not the room.
From chart to control
A chart tells you where you are and which way to move. It cannot hold you there — VPD climbs through the photoperiod as lights heat the room and drops at lights-off. Holding it means a sensor reading temperature and humidity continuously, a per-stage target band, and actuators (humidifier, dehumidifier, heater, fan) driven to stay inside it, which is what LoopString grow-room automation does on a Raspberry Pi at the edge. Pair this chart with the VPD calculator for the exact number and the PPFD/DLI calculator to match your light.