Climate Control in the Grow Room: VPD, Temperature and CO₂

Lighting gets the most attention in cannabis growing – yet the climate in the grow room is at least as important. Temperature, humidity, air circulation and CO₂ concentration directly influence how efficiently the plant photosynthesises, how well it absorbs nutrients, and how resistant it is to pests and fungi.

## Vapor Pressure Deficit (VPD): The Underestimated Concept

VPD is the decisive parameter for optimal plant growth – yet it is still neglected by many growers. VPD describes the difference between the water vapour pressure the air can absorb at a given temperature (saturation vapour pressure) and the water vapour pressure actually present.

Simply put: VPD measures how "thirsty" the air is. High VPD means the air is dry and aggressively draws moisture from the leaves. Low VPD means the air is nearly saturated and the plant can barely transpire.

Why VPD matters: Plants regulate their water balance through stomata (pores in leaves). When stomata are open, water evaporates while simultaneously absorbing CO₂ – a prerequisite for photosynthesis. Too-high VPD forces stomata to close (protection against drying out). Too-low VPD slows transpiration and thus nutrient uptake.

Optimal VPD values: - Seedling stage: 0.4–0.8 kPa - Vegetative phase: 0.8–1.2 kPa - Early flowering: 1.0–1.5 kPa - Late flowering: 1.5–2.0 kPa

VPD tables (depending on temperature and RH) are available online or as posters for the grow room. Many modern grow controllers calculate VPD automatically.

## Temperature: Phase-Based Recommendations

Cannabis grows across a wide temperature range but has clear preferences:

Vegetative phase (light period): 22–28 °C ideal. Below 18 °C, growth slows significantly. Above 30 °C, heat stress begins.

Flowering phase (light period): 20–26 °C ideal. Lower temperatures (20–22 °C) in late flowering can improve terpene production and resin quality. Temperatures above 28 °C in flowering destroy terpenes and can impair cannabinoid quality.

Dark period: Temperature should be 5–8 °C below the light period temperature – day temperature swings of 5–10 °C are normal and promote growth when controlled.

Root zone: Roots prefer slightly cooler conditions than leaves. 18–22 °C in the root zone is optimal. Wet, cold substrates significantly slow nutrient uptake.

## Relative Humidity (RH): Optimise Phase by Phase

Seedling stage: 65–80 % RH. Seedlings don't yet have a mature root system and absorb much moisture through their leaves. High humidity is advantageous here.

Vegetative phase: 50–70 % RH. The plant transpires actively and can utilise high humidity well.

Early flowering: 40–60 % RH. With increasing flower density, the risk of mould rises.

Late flowering: 35–45 % RH. In the last 2–3 weeks before harvest, low humidity is critical to prevent Botrytis (grey mould) in dense flower clusters. This is the most common harvest mistake: humidity too high during the ripening phase.

Equipment: A dehumidifier is often indispensable during flowering, especially in more humid climates or with large plant populations. Affordable dehumidifiers with 10–20 litres/day capacity are sufficient for smaller grow rooms.

## Air Circulation vs. Exhaust: Two Different Systems

Internal circulation: Fans that move air within the grow room. They strengthen stems (thigmomorphogenesis), distribute CO₂ evenly, prevent humid dead zones and reduce mould risk. At least one oscillating fan per grow room is essential.

Exhaust (external): Carbon filter + exhaust fan drawing air out of the grow room. Odour control, temperature regulation and CO₂ renewal. Rule of thumb: the air in the grow room should be exchanged 1–3 times per minute.

Calculating fan capacity: Room volume (m³) × 60 × factor (1–3) = required m³/hour. For a 2 m × 2 m × 2 m grow room (8 m³): 8 × 60 × 2 = 960 m³/h as a starting point.

## CO₂ Enrichment: Only for Advanced Setups

Plants need CO₂ for photosynthesis. Natural outdoor air contains approximately 420 ppm of CO₂. Through CO₂ enrichment, the concentration can be raised to 1,000–1,500 ppm, significantly accelerating plant growth.

Important prerequisites for CO₂ use: 1. Lighting must be powerful enough (at least 800–1,000 µmol/m²/s PPFD). With weaker lights, light energy – not CO₂ – limits growth. 2. Temperatures must be increased (26–30 °C), as plants photosynthesise more efficiently at higher temperature + CO₂. 3. The system must be sealed – with open ventilation, CO₂ dissipates immediately.

CO₂ sources: Cylinders with pressure regulator and controller, CO₂ generators (burning propane/butane), natural CO₂ bags (limited effect).

CO₂ conclusion: For most hobby and CSC setups, CO₂ enrichment is not necessary. Optimised climate, optimal nutrients and optimal lighting have a greater impact on yield than CO₂.

## Monitoring: Hygrometers and Controllers

Reliable measurement is the foundation of good climate management. A good digital thermo-hygrometer (combining thermometer and humidity gauge) with min/max recording costs €10–30 and is indispensable.

For larger setups, grow controllers that continuously measure temperature, RH and VPD and automatically control fans, dehumidifiers and heaters are recommended.