Environment
Advanced
Climate Control: Temperature, Humidity, and CO₂
Environmental mastery is what separates adequate grows from extraordinary ones — temperature, humidity, and CO₂ each profoundly affect both plant health and final product quality.
TEMPERATURE
Cannabis grows optimally in the following temperature ranges:
Seedling stage: 20–25°C (68–77°F). New seedlings are sensitive to temperature extremes in both directions.
Vegetative stage: 22–28°C (72–82°F) during the light period, 18–24°C (65–75°F) during the dark period. A moderate day/night differential (4–8°C) is beneficial for overall plant health.
Flowering stage: 18–26°C (65–79°F) during lights-on. Keeping temperatures slightly lower in flower reduces terpene evaporation from developing buds, preserving aroma and flavour. Temperatures above 30°C during flowering cause airy, loose buds with bleached trichomes and reduced potency.
Final 2 weeks: Many experienced growers drop temperatures to 15–18°C at night during the final 2 weeks of flowering to stress anthocyanin production (triggering purple colouration in susceptible strains) and to maximise terpene preservation.
HUMIDITY (Relative Humidity / VPD)
Target humidity ranges by stage:
Seedling: 65–70% RH — seedlings absorb moisture through leaves before their root system is established.
Vegetative: 50–70% RH — higher humidity supports vigorous leaf growth.
Early flowering (weeks 1–4): 45–55% RH — reduce from veg levels.
Late flowering (weeks 5–harvest): 40–50% RH — critical to prevent botrytis in developing buds.
Final 2 weeks: 35–45% RH — lowest humidity of the cycle.
Vapour Pressure Deficit (VPD) — the mathematical relationship between temperature and relative humidity — is the more accurate metric for advanced growers, as it accounts for the interaction between the two variables. Target VPD of 0.8–1.2 kPa in vegetative growth and 1.0–1.5 kPa in flowering.
CO₂ ENRICHMENT
Ambient CO₂ concentration is approximately 400 ppm. Cannabis can utilise CO₂ up to approximately 1,500 ppm when all other variables are optimised — adequate lighting intensity, temperature, humidity, and nutrients. CO₂ enrichment to 1,000–1,500 ppm, when combined with high-intensity lighting (>600 μmol PPFD), can increase growth rates and yields by 20–40%.
CO₂ supplementation requires: a fully sealed environment (to prevent CO₂ escaping and CO₂ monitoring and control hardware), high-intensity lighting that can actually use the additional CO₂ (standard 400W HPS or equivalent is insufficient — high-output LEDs or 1,000W HPS systems are needed), and the experience to manage all other variables at the level required to benefit from CO₂. Not recommended for beginners.