Vapor Pressure Deficit (VPD) is a critical concept in cannabis cultivation, as it directly impacts plant transpiration, nutrient uptake, and overall growth. Here’s a breakdown of what VPD is and how it relates to growing cannabis:
What is VPD?
VPD measures the difference between the amount of moisture in the air and the air’s moisture-holding capacity at a given temperature. It combines humidity and temperature into one metric and is expressed in kilopascals (kPa).
• Low VPD: Indicates high relative humidity and low evaporation potential. Plants transpire less, which can lead to waterlogging and poor nutrient uptake.
• High VPD: Indicates low relative humidity and high evaporation potential. Plants transpire more, which can cause dehydration and stress.
Why is VPD Important for Cannabis?
1. Optimized Transpiration:
• Transpiration is how plants regulate water and nutrient uptake.
• Proper VPD ensures plants transpire at an optimal rate, promoting healthy growth.
2. Photosynthesis Efficiency:
• VPD affects stomata opening. When VPD is balanced, stomata remain open, allowing for efficient gas exchange (CO₂ intake and oxygen release).
3. Pest and Mold Prevention:
• Incorrect VPD (too low or too high) can create conditions favorable for pests, mold, and mildew.
4. Stress Reduction:
• A proper VPD minimizes stress, ensuring the plants allocate energy to growth and cannabinoid production rather than survival.
Ideal VPD for Cannabis
The ideal VPD depends on the growth stage:
Stage Temperature (°F) Relative Humidity (%) VPD Range (kPa)
Seedling/Clone 70–75 65–70 0.4–0.8
Vegetative 75–85 55–65 0.8–1.2
Flowering 70–80 40–50 1.2–1.6
How to Adjust VPD
1. Increase VPD:
• Lower relative humidity.
• Increase temperature.
2. Decrease VPD:
• Increase relative humidity.
• Lower temperature.
3. Tools to Monitor/Control:
• Use a hygrometer and thermometer to measure humidity and temperature.
• Employ humidifiers, dehumidifiers, and heaters/coolers to adjust conditions.
By maintaining optimal VPD throughout the cannabis lifecycle, you can significantly improve plant health, yield, and cannabinoid production.
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