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6.1 The Perfect Climate: Mastering Temp, Humidity, & VPD
Maintaining a stable and appropriate climate is crucial for plant health and growth. Three key factors—temperature, humidity, and Vapor Pressure Deficit (VPD)—work together to create the perfect environment.
Temps, Wetness and the Relationship of the two
Temperature (Temp):
- The temperature of your grow space directly impacts a plant's metabolic rate and ability to perform photosynthesis.
- Daytime (Lights On): Aim for 75-85°F (24-29°C) during the vegetative stage and 68-78°F (20-25°C) during the flowering stage.
- Nighttime (Lights Off): A slightly cooler temperature is ideal, but the drop should not be more than 10°F (5°C) from the daytime temperature.
Humidity (Relative Humidity - RH):
- This is the amount of water vapor in the air. A plant's ability to "breathe" and transpire (release water vapor) is highly dependent on humidity.
- Seedling Stage: 65-70% RH is ideal to help the young plant absorb moisture through its leaves while its roots are still developing.
- Vegetative Stage: 40-70% RH. The plant is growing quickly and needs to transpire efficiently.
- Flowering Stage: 40-50% RH. Lower humidity is crucial in the flowering stage to prevent mold and bud rot.
VPD (Vapor Pressure Deficit):
- This is a more advanced metric that measures the difference between the amount of moisture in the air and the amount of moisture the air can hold when it's fully saturated. A good VPD indicates that your plant is transpiring at an optimal rate.
- A high VPD means the air is very dry, and the plant will transpire (and therefore drink) faster.
- A low VPD means the air is very humid, and the plant will transpire slower.
- You can use a VPD chart to find the perfect temperature and humidity combination for each stage of growth.
The Law of the Dew Point
The dew point is a crucial, yet often overlooked, environmental factor.
It is the temperature at which the air becomes saturated with moisture and condensation begins to form.The key rule of thumb for any grower is to: ALWAYS keep your low temperature above the recorded Dew Point High for the same period.
The air temperature in a grow room can change much faster than the temperature of solid objects like leaves and buds. The problem occurs when the temperature of the air drops below the dew point. When this happens, surfaces which are still warm will condense water, and you will end up with water droplets between leaves and potentially inside the buds.
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