🌱 What Is VPD?
Vapor Pressure Deficit (VPD) measures the difference between how much moisture the air holds and how much it could hold when fully saturated. Put simply:
✅ Low VPD = humid air, less evaporation from leaves.
✅ High VPD = dry air, more evaporation from leaves.
VPD is more accurate than just looking at humidity because it accounts for temperature. Warm air holds more moisture, so the same RH at different temperatures means very different conditions for your plants.
🌿 Why VPD Matters in Veg
The vegetative stage is when cannabis builds its foundation:
- Root expansion
- Thick stems
- Broad leaves
- Branch development
Perfect VPD makes all of this happen faster because it:
- Regulates transpiration (how water evaporates through leaf stomata).
- Balances nutrient flow (minerals move from roots to leaves via water movement).
- Maintains healthy gas exchange (CO₂ in, oxygen out).
If VPD is too low or too high, plants either suffocate in moisture or dry out too fast.
🌬️ Ideal VPD for Vegetative Growth
During veg, you want your plants to transpire actively to drive nutrient uptake but not so aggressively that they dry out.
✅ Ideal VPD Range for Veg: 0.8–1.2 kPa
This range gives you:
- Rapid photosynthesis
- Strong stem development
- Faster node spacing
- Less risk of fungal issues compared to seedling stage
🧠 The Science: What Happens Inside the Plant
VPD affects stomata — tiny pores on the underside of leaves:
- Low VPD (humid air) = stomata don’t open fully, slowing CO₂ intake and nutrient transport.
- High VPD (dry air) = stomata close to prevent water loss, slowing growth.
- Optimal VPD = stomata are partially open, exchanging gases and driving steady transpiration.
When VPD is dialed in, plants drink efficiently and grow faster.
🌡️ How Temperature and Humidity Create VPD
VPD depends on two things:
✅ Air Temperature
✅ Relative Humidity (RH)
Here’s an example of a perfect veg VPD target:
| Temp (°F) | RH (%) | VPD (kPa) |
|---|---|---|
| 75 | 65% | ~0.9 |
| 80 | 60% | ~1.0 |
| 85 | 55% | ~1.2 |
Tip: As temps rise, you need lower RH to maintain the same VPD.
🌱 How to Measure VPD
Most growers use:
- A reliable thermo-hygrometer to measure temp & RH
- A VPD chart or calculator (many free online tools)
Some modern grow controllers display live VPD readings so you can adjust your environment in real time.
🌿 Signs Your VPD Is Wrong
Too Low VPD (humid):
- Leaves look droopy or swollen
- Slower growth
- Greater risk of mold
Too High VPD (dry):
- Leaf edges curl up
- Plants wilt between watering
- Nutrient burn as plants drink more water but take in more nutrients too quickly
🌞 Benefits of Perfect VPD in Veg
When you keep VPD in the sweet spot:
✅ Faster leaf development
✅ Quicker root growth
✅ Thicker stems and branches
✅ Better nutrient uptake
✅ Lower disease risk
This sets the stage for a stronger transition into flower.
🛠️ How to Dial in VPD
Here are some simple strategies:
✅ Use a humidifier or dehumidifier to control RH.
✅ Adjust temps with ventilation or heaters.
✅ Keep airflow consistent.
✅ Measure daily — environment changes fast in small spaces.
Remember: warmer air holds more moisture. If you bump your temps, lower your RH to stay in the target range.
🌿 What About Clones and Small Veg Plants?
For clones or early veg, many growers prefer a slightly lower VPD (0.6–0.9 kPa) to reduce stress. Once roots are established, increase VPD to the 0.8–1.2 kPa range.
