VPD: Vapor Pressure Deficit
VPD measures how aggressively the air pulls moisture from plant leaves. It’s the single most important climate metric for greenhouse management — more useful than relative humidity alone because it accounts for both temperature and moisture.
The Scale
| VPD (kPa) | Condition | Plant Response |
|---|---|---|
| < 0.4 | Too humid | Stomata close, transpiration stops, disease risk |
| 0.4–0.8 | Propagation zone | Good for cuttings and seedlings |
| 0.8–1.2 | Ideal vegetative | Healthy transpiration, nutrient uptake |
| 1.2–1.6 | Late veg / early flower | Slightly stressed, can be beneficial |
| 1.6–2.0 | Moderate stress | Most crops show reduced growth |
| 2.0–3.0 | High stress | Leaf curl, wilting, stomatal closure |
| > 3.0 | Emergency | Tissue damage possible |
Why VPD Is Hard at 5,000 Feet
Longmont, Colorado sits at 4,979 feet — one of the driest inhabited regions in the continental US.
- Spring afternoon outdoor RH: 14–18%
- Summer afternoon outdoor RH: 30–55% (monsoon helps)
- Winter outdoor RH: 40–55%
When you heat dry outdoor air (even passively via solar gain), VPD skyrockets. The greenhouse regularly hits VPD 2.0+ on spring afternoons — even with active misting.
March has more VPD stress hours (12.5h >2.0 kPa) than August (8.3h) because March combines strong solar gain with the driest outdoor air of the year. August’s monsoon moisture naturally buffers VPD.
How Verdify Controls VPD
→ See Climate at 5,000 Feet for the full humidity system. → See State Machine for the trigger thresholds.