The vent is open — there's ventilation. But powdery mildew keeps coming back, tip burn on the lettuce won't clear, basil rots at the base. An open vent provides air exchange with the outside — but not air movement between plants. In a dense canopy where leaves touch each other, the air can be completely still around the clock even with an open window. Ventilation and circulation are two different tasks and are solved by different equipment.
Quick glossary: Ventilation — air exchange between the internal volume and the outside environment; removes excess humidity, heat, and CO₂, and supplies fresh air. Circulation — air movement inside the space without exchange with the outside; equalises temperature and humidity, strips the boundary moisture layer from the leaf surface, reduces condensation risk. Boundary layer — the thin layer of still air that always exists at the leaf surface in the absence of air movement; with a large boundary layer, transpiration and gas exchange slow down regardless of VPD in the main air volume.
Two Different Tasks: Air Exchange and Internal Movement
Exhaust ventilation — vents, fans that push air outside, supply-and-exhaust systems — solves the problem of moisture, temperature, and gas balance between the greenhouse and the outside. Open a vent — excess humidity and heat leave, CO₂ enters (or the reverse depending on conditions).
Circulation fans — horizontal fans directed between plant rows — solve a different problem: air movement within the canopy itself. They do not change the air composition in the room — they strip the boundary layer from leaves, equalise temperature and humidity between the upper zone and the canopy, and prevent moisture droplets from sitting on surfaces.
Without circulation in the canopy, exhaust ventilation pulls air through the upper zone — while the air between the leaves stands still. This is exactly why an open window does not resolve powdery mildew developing on lower tiers.
What Happens with Stagnant Air in the Canopy
Condensation and fungal diseases. With weak air movement the boundary layer at the leaf is saturated with moisture from transpiration. Any nighttime temperature drop — and that moisture condenses directly on the leaf. A ready substrate for Botrytis, powdery mildew, and bacterial necroses. The majority of disease outbreaks in protected growing begin in precisely this boundary layer during overnight cooling.
Accumulation of conidia and spores. Without air movement, spores released from infected plants are not diluted — they concentrate around the source and infect neighbouring plants far more effectively. Air movement does not eliminate the risk but reduces spore concentration around the plant.
Local elevation of temperature and humidity. In a dense canopy without circulation, temperature between leaves can be 3–5°C above the main volume — especially under LED panels with side heating. VPD in the canopy then differs from what the sensor under the ceiling shows, and the plant is actually in conditions far removed from the measured values.
Suppressed transpiration. A saturated boundary layer reduces the driving force for transpiration — so in stagnant air the plant transpires less than under normal air movement even at the same VPD in the main volume. The result is lower water flow and less calcium reaching growth points.
How to Organise Circulation: What and Where
Horizontal circulation fans are the primary tool. Direct them along rows or between racking so that air moves across the entire zone with no dead spots. Target air speed at leaf level: 0.3–0.5 m/s — perceptible movement without mechanical damage.
Quantity: one fan per 20–30 m² with correct placement is usually sufficient for basic circulation. In vertical systems or multi-tier racking — a dedicated fan for each tier or between tiers.
Placement: do not aim directly at plants — this causes mechanical stress and excessive leaf-edge drying. Direct airflow between rows or along aisles so that it wraps around the plants. Verify air movement at every point in the zone: a lit match or smoke device reveals where the air is still.
Operating schedule: circulation fans should run continuously — day and night. Stopping them at night during a temperature drop is the most dangerous moment, when condensation begins and circulation is needed most.
Ventilation and CO₂: Not Exhausting What You Enriched
When CO₂ enrichment is running in a closed loop, open ventilation expels the accumulated gas outdoors — making enrichment pointless. The logic: CO₂ enrichment in closed mode during active photosynthesis (daytime), ventilation for humidity and temperature control — in the evening or at night when CO₂ is no longer being consumed.
If daytime ventilation is needed for excess temperature or humidity — calculate whether the loss of CO₂ is justified. Sometimes cooling is a higher priority than enrichment.
Circulation fans can run continuously regardless — they do not expel CO₂ outdoors, they only move air within the space.
Three Mistakes That Cost the Most
Stopping circulation fans at night to give plants "rest." Night is the most dangerous moment: temperature drops, humidity rises, condensation forms. Precisely at night is when circulation is most needed. Fans that stop at night produce regular morning disease outbreaks.
Using only exhaust ventilation without circulation in the canopy. Vents and exhaust fans control the room's air balance — but do not touch the air between the leaves. Microgreen diseases and fungal outbreaks in the greenhouse with "open ventilation" are the typical result of absent circulation.
Directing airflow straight at plants for "better effect." A direct flow at 1+ m/s onto leaves causes mechanical stress, excessive drying of leaf margins, and deformation of young shoots. The flow should wrap around plants, not blast them directly.
How to Know Ventilation and Circulation Are Set Up Correctly
Morning inspection shows no condensation on leaves even after a nighttime temperature drop. Air movement is perceptible at any point in the plant canopy — check with your hand or a thread. Temperature and humidity in the canopy differ from the sensor reading in the main volume by no more than 2°C and 10% RH. Fungal outbreaks starting in the morning — absent.
For deeper understanding: VPD: Vapour Pressure Deficit and Why the Plant "Won't Drink" at Normal Humidity — explains how air movement in the canopy directly affects VPD at the leaf surface and why the sensor reading in the main volume can be normal while the plant is in entirely different conditions.