17 · Environment
Greenhouse and Indoor Climate Control
Climate control regulates the physical boundary conditions that dictate plant transpiration, stomatal conductance and latent heat exchange. Temperature and relative humidity interact according to psychrometric principles to establish vapour pressure deficit (VPD). When VPD is suppressed, transpiration ceases and calcium deficiencies develop; when excessive, stomata close to prevent desiccation, stopping photosynthesis.
Quick understanding
Plants do not care about relative humidity in isolation; they respond to Vapour Pressure Deficit (VPD), which is the drying power of the air. If the air is too dry (high VPD > 1.5 kPa), plants shut their leaf pores to conserve water and stop growing. If the air is too wet (low VPD < 0.4 kPa), plants cannot transpire, leading to calcium deficiency and fungal outbreaks.
On this page
Introduction
Controlled Environment Agriculture relies on manipulating the aerial microclimate surrounding plant canopies. While growers historically focused on dry-bulb temperature and relative humidity independently, modern environmental physics unites them via psychrometrics into Vapour Pressure Deficit (VPD).
VPD represents the difference between the saturation vapour pressure inside the humid substomatal cavity of the leaf (assumed to be 100% RH at leaf temperature) and the actual vapour pressure of the greenhouse air. It is the direct thermodynamic driving force pulling water and dissolved calcium from the roots through the xylem to the shoot tips.
Why VPD governs transpiration and disease
Transpiration serves two indispensable physiological roles: evaporative cooling of foliage under solar radiation, and mass-flow delivery of non-mobile mineral nutrients—predominantly calcium and boron. Under low VPD conditions (<0.4 kPa), the air cannot accept water vapour; transpiration stalls, root water uptake drops, and calcium cannot reach rapid-growing leaf margins.
Core concepts
Optimal VPD ranges by growth stage
| Growth Stage | Target VPD (kPa) | Target Temperature (°C) | Target RH (%) |
|---|---|---|---|
| Propagation / Rooting | 0.4 – 0.8 | 22 – 24 | 75 – 85 |
| Vegetative Canopy Growth | 0.8 – 1.1 | 20 – 24 | 65 – 75 |
| Generative / Fruiting | 1.0 – 1.4 | 19 – 23 | 60 – 70 |
| Critical Stress Thresholds | < 0.3 (Stagnant) or > 1.8 (Extreme) | Varies | Risk of tipburn / stomatal closure |
Sensible and latent heat exchange
In closed CEA systems, a growing crop acts as an evaporative humidifier. Over 90% of the water delivered by irrigation is converted into latent heat through transpiration. Environmental conditioning systems must remove this latent load through active mechanical dehumidification or controlled outdoor air exchange.
Engineering and HVAC practice
- Install aspirated sensor psychrometer boxes (with shielded fans) positioned at crop canopy height, not against perimeter walls.
- Utilize infrared leaf temperature sensors (IRT) to calculate actual leaf-to-air VPD rather than assuming leaf and air temperatures are identical.
- Deploy Horizontal Air Flow (HAF) fans arranged in a racetrack pattern to eliminate dead air pockets and break leaf boundary layers.
- Coordinate evaporative cooling pads and exhaust fans to ensure staging steps do not cause drastic temperature and humidity oscillations.
Advanced knowledge
Advanced greenhouse climate computers utilize enthalpy-based control algorithms. Enthalpy accounts for both sensible (temperature) and latent (moisture content) energy of the air. Instead of venting strictly based on dry-bulb temperature, the system compares indoor and outdoor air enthalpy to determine whether venting or heating/dehumidifying consumes fewer kilowatt-hours.
Key takeaways
- Vapour Pressure Deficit (VPD) is the true metric governing plant transpiration and stomatal opening.
- Target vegetative VPD is 0.8–1.1 kPa; target generative VPD is 1.0–1.4 kPa.
- Low VPD (<0.4 kPa) induces calcium deficiency (tipburn) and promotes fungal sporulation.
- Horizontal Air Flow (HAF) fans are critical to dismantle humid boundary layers around leaves.
- Over 90% of irrigation water becomes latent cooling load that must be dehumidified or vented.
Related knowledge
- Greenhouse Structures and Environmental EngineeringStructural frame engineering, cladding films, natural ventilation geometry, pad-and-fan and shading screens.
- Indoor Controlled Environment Agriculture (CEA)Closed-envelope building physics, HVAC cooling and dehumidification balances, airflow uniformity and facility biosecurity.
- Horticultural Lighting and PhotobiologyPAR spectrum, PPFD, Daily Light Integral (DLI), luminaire efficiency, thermal dissipation and supplemental photoperiod.
- Hydroponic Energy ManagementWhere the kilowatt-hours actually go, and why energy decides whether an indoor farm works.
- Plant Health and DiseaseReading symptoms, separating abiotic from biotic causes, and protecting the root zone.
Further reading
- American Society of Agricultural and Biological Engineers (ASABE)
Standards on greenhouse environmental control, heating, cooling and ventilation sizing.
- Wageningen University and Research
Next Generation Growing (NGG) manuals on humidity management and energy-efficient climate control.
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