5.6 Environmental Control: Temperature, Humidity, & Airflow

Temperature directly affects a plant's metabolic rate and the speed of photosynthesis. Plants have an ideal temperature "sweet spot" where they perform best.

• Vegetative Stage:
  
  • Day: 75-85°F (24-29°C)

• Night: Maintain a 5-10°F drop (3-6°C) to simulate a natural environment and promote healthy growth.

• Flowering Stage:
  
  • Day: 68-78°F (20-25°C)

• Night: Maintain the same 5-10°F drop. Cooler temperatures in late flowering can enhance terpene and cannabinoid production.

• Dangers: Temperatures that are too high can cause heat stress, leading to leaf burn and stunted growth. Temperatures that are too low can slow down a plant's metabolism, hindering nutrient uptake and growth.

Humidity, or the amount of water vapor in the air, affects a plant's transpiration rate. An advanced way to think about this is through Vapor Pressure Deficit (VPD), which measures the difference between the moisture in the air and the moisture inside the plant.

• VPD Zones: 
  • Clones/Seedlings: High humidity (70-80% RH) is ideal to prevent new plants from drying out before roots are established.

• Vegetative Stage: Maintain moderate humidity (50-70% RH).

• Flowering Stage: Crucially, reduce humidity (40-50% RH) to prevent mold and mildew from forming on the dense buds.

• Dangers: High humidity in flowering is the leading cause of bud rot. Low humidity can cause plants to transpire too quickly, leading to nutrient lock-out and stress.

Proper airflow is essential for both plant health and the proper use of CO2, a key ingredient in photosynthesis.

• Exhaust & Intake: Your grow space needs a constant exchange of fresh air. An exhaust fan pulls stale, warm air out of the room, while an intake fan (or passive intake vents) draws fresh, CO2-rich air in. This ensures that your plants have a constant supply of CO2.

• Internal Circulation: Use oscillating fans to create a gentle breeze within the grow space. This serves two purposes:  

1. It strengthens the plant stems, making them more resilient.
2. It prevents stagnant air pockets, or "microclimates," from forming within the canopy, which can lead to mold and pests.

• CO2 Supplementation: For truly maximizing growth, advanced growers will add supplemental CO2 to their grow space during the light cycle. This is only effective when temperatures are high (80-85°F) and the light intensity is very high.