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Case File No. 27-EN01
Subject: Vapor Pressure Deficit (VPD)
Persons of Interest: Matty, The Hygrometer
Investigating Officer: Matty Ridge
Experiment 7C — The Humidity Lie
16:00 — Tent RH reads 65%. Looks fine, right? Wrong.
16:15 — Checked leaf temperature with IR gun: 24°C. Air temp is rising to 28°C due to a midday spike.
16:20 — RH is still 65%, but the plants are drooping slightly, metabolism slowing. The Relative Humidity (RH) is lying. VPD chart confirms the pressure deficit has dropped to 0.4 kPa.
Conclusion: Plants stop drinking when the air feels full. RH is for weather forecasts. VPD is for factory management. Never trust RH alone. It’s a liar.
To most growers, 60% RH is just a number. To a certified foreman like me, it’s a trap. RH is temperature-dependent, meaning it changes its meaning every time your light dims or your cooling kicks in. VPD (kPa) is the true metric that unlocks maximum CO₂ and nutrient uptake.
The Core Equation: VPD vs. The Transpiration Tax
Your plant is constantly pumping water from the roots to the leaves—this is transpiration. This pull is what moves nutrients and drives growth. The speed of that pump is controlled by VPD.
The formula compares the Vapor Pressure (VP) inside the leaf (which is nearly 100% RH) to the VP in the surrounding air. This difference is the Vapor Pressure Deficit:
VPD = VPsaturated – VPambient
The ambient air is constantly demanding moisture. This pressure deficit is what drives water out of the stomata. Think of a tiny, invisible vacuum cleaner pulling water from your leaves.
- Low VPD: The “vacuum” is weak. Transpiration stalls, CO₂ intake slows, and the risk of mold (botrytis) rises.
- High VPD: The “vacuum” is too strong. Plant slams stomata shut to conserve water, leading to stress and slow growth.
Foreman’s Insight:
“If your VPD’s off, your plants aren’t thirsty—they’re suffocating. It’s not humidity management, it’s oxygen management.”
The Command Chain: VPD Targets (kPa) & LST Integration
The absolute most critical data point you need is the Leaf Surface Temperature (LST). This is not up for debate. If you cannot measure LST, assume your leaves are 3–4°C cooler than the ambient air and adjust your RH accordingly.
MANDATE: The table below must be designed into a High-Res, Color-Coded Infographic for the article’s anchor visual. (Refer to Image 2: “VPD Command Chain Chart”)
| Growth Phase | Target VPD (kPa) | Primary Goal / Why |
|---|---|---|
| Propagation / Clones | 0.4 – 0.7 | Rooting, minimal stress, high RH |
| Vegetative Growth | 0.8 – 1.0 | Maximize water/nutrient uptake, rapid growth |
| Early/Mid Flower (Weeks 1-5) | 1.0 – 1.2 | Maintain high speed, manage stretch |
| Late Flower (Weeks 6-Harvest) | 1.2 – 1.5 | Harden density, concentrate oils, and prevent mold |
| Night Cycle | ≥ 0.5 kPa | Must prevent dew point (mold formation) |
The Field Report: Symptoms & Diagnosis
Each VPD imbalance is an operational failure you must diagnose immediately.
| Symptom | Cause (VPD) | Foreman’s Diagnosis |
|---|---|---|
| Drooping Leaves / Leaf Margin Curling | Too High (≥ 1.5 kPa) | Lower temperature and/or raise humidity fast. Check your LST is not too hot. |
| Mushy Growth / Mold / Mildew | Too Low (≤ 0.7 kPa) | Immediately raise temperature or dehumidify. Mandate strict nighttime VPD. |
| Slow Nutrient Uptake / Yellowing | Low VPD in Flower | Confirm VPD is ≥ 1.0 kPa to restart the transpiration pump for PK delivery. |
Advanced Protocols: The Final kPa Dial-In
Controlling VPD is the difference between pulling 0.8 g/W and hitting the 1.2 g/W target.
LST is Non-Negotiable:
You must account for Leaf Surface Temperature (LST). If you cannot measure it, assume your leaves are 3–4°C cooler than the ambient air and adjust your RH up by 5–10% to compensate. A small error in LST calculation is a massive error in VPD.
The Finishing Crunch (1.2–1.5 kPa):
This calculated, controlled water stress forces the plant to conserve, which concentrates sugars and essential oils, driving density, potency, and terpene production. It’s a high-stakes play that forces oil concentration.
Nighttime Safety Protocol:
The minimum VPD at night must be 0.5 kPa. Letting the air reach saturation (dew point) for even a few hours is a guaranteed way to introduce mold and fungi. Use mild heat or dehumidification to keep the air moving and the deficit open.
Matty’s Hulk-Out Warnings
- Do not rely on a cheap combination sensor. An inaccurate kPa reading is worse than no reading at all. Invest in quality sensors.
- Do not let your temperature swing wildly at transition (lights on/off). A rapid 5°C drop can cause an instantaneous VPD collapse—that’s how you lose a crop in under an hour.
- Do not chase the VPD number by violently changing the temperature. Focus on slowly adjusting the RH via humidifiers/dehumidifiers, as temperature spikes cause irreversible plant stress.
Case Closure Log
Metrics: Stabilised. VPD is maintained in the optimal band.
Air Exchange: Optimised. Transpiration is driving maximum nutrient uptake.
Final Verdict: VPD is the true command center for yield. You cannot out-fertilize a bad environment. Master the kPa and your grams per watt will follow.
Filed and signed,
Officer Ridge — Dept. of Bud & Order